Search

We consider the dissolution of a chemically inert solid in a binary liquid mixture with a critical point of solution. When the mixture, acting as the solvent, has come to equilibrium with the solid, the state of the system is completely described by the temperature, pressure, and a concentration variable formed by dividing the molar amount of one solvent component by that of the other. Under conditions of fixed pressure, the principle of critical point isomorphism predicts that the slope of a van't Hoff plot of the solubility of the solid should diverge toward infinity as the temperature enters the critical region. The sign of the divergence is negative when the dissolution is endothermic, whereas it is positive when the dissolution is exothermic. In experiments where excess solid phenolphthalein dissolves in a binary mixture of nitrobenzene + dodecane, we have observed exothermic dissolution concurrently with a positive divergence of the van't Hoff slope. The data are insufficiently precise to compute an accurate numerical value for the exponent of the temperature power law expected to govern this divergence; nevertheless, on the basis of Widom scaling theory, we argue that the exponent should be equal to 0.326, which is identical to the value of the exponent that governs the temperature dependence of the shape of the liquid-liquid coexistence curve. Being entirely physical in nature, the anomalous solubility effect should be observable in the case of any chemically inert solid dissolving in any one of the more than 1000 liquid pairs known to have a critical point of solution. [ABSTRACT FROM AUTHOR]

Liquid mixtures, which have a phase diagram exhibiting a miscibility gap ending in a critical point of solution, have been used as solvents for chemical reactions. The reaction rate in the forward direction has often been observed to slow down as a function of temperature in the critical region. Theories based upon the Gibbs free energy of reaction as the driving force for chemical change have been invoked to explain this behavior. With the assumption that the reaction is proceeding under relaxation conditions, these theories expand the free energy in a Taylor series about the position of equilibrium. Since the free energy is zero at equilibrium, the leading term in the Taylor series is proportional to the first derivative of the free energy with respect to the extent of reaction. To analyze the critical behavior of this derivative, the theories exploit the principle of critical point isomorphism, which is thought to govern all critical phenomena. They find that the derivative goes to zero in the critical region, which accounts for the slowing down observed in the reaction rate. As has been pointed out, however, most experimental rate investigations have been carried out under irreversible conditions as opposed to relaxation conditions [Shen et al. J. Phys. Chem. A 2015, 119, 8784-8791]. Below, we consider a reaction governed by first order kinetics and invoke transition state theory to take into account the irreversible conditions. We express the apparent activation energy in terms of thermodynamic derivatives evaluated under standard conditions as well as the pseudoequilibrium conditions associated with the reactant and the activated complex. We show that these derivatives approach infinity in the critical region. The apparent activation energy follows this behavior, and its divergence accounts for the slowing down of the reaction rate. [ABSTRACT FROM AUTHOR]

Type-II and quasi type-II heterostructure nanocrystals are known to exhibit extended excited-state lifetimes compared to their single material counterparts because of reduced wave function overlap between the electron and hole. However, due to fast and efficient hole trapping and nonuniform morphologies, the photophysics of dot-in-rod heterostructures are more rich and complex than this simple picture. Using transient absorption spectroscopy, we observe that the behavior of electrons in the CdS "rod" or "bulb" regions of nonuniform ZnSe/CdS and CdSe/CdS dot-in-rods is similar regardless of the "dot" material, which supports previous work demonstrating that hole trapping and particle morphology drive electron dynamics. Furthermore, we show that the longest lived state in these dot-in-rods is not generated by the type-II or quasi type-II band alignment between the dot and the rod, but rather by electron-hole dissociation that occurs due to fast hole trapping in the CdS rod and electron localization to the bulb. We propose that specific variations in particle morphology and surface chemistry determine the mechanism and efficiency of charge separation and recombination in these nanostructures, and therefore impact their excited-state dynamics to a greater extent than the heterostructure energy level alignment alone. [ABSTRACT FROM AUTHOR]

Throughout the undergraduate curriculum, students utilize linearized forms of nonlinear equationsfrom the Clausius-Clapeyron equation in general chemistry to the Michaelis-Menten equation in biochemistry. Presenting the linearized forms of equations as a fait accompli may be a lost opportunity to empower students with understanding the general process of linearization as an analytical tool. This work describes a series of activities that can be implemented and spaced throughout the analytical chemistry curriculum. The activities are low-cost and chemical-free, so they can be implemented in a lecture setting. Distributed practice allows students to attain familiarity with linearization, and the practical guided application in class provides real experience with the otherwise abstract mathematical idea. [ABSTRACT FROM AUTHOR]

Binary liquid mixtures having a consolute point can be used as solvents for chemical reactions. When excess cerium(IV) oxide is brought into equilibrium with a mixture of isobutyric acid + water, and the concentration of cerium in the liquid phase is plotted in van't Hoff form, a straight line results for temperatures sufficiently in excess of the critical solution temperature. Within 1 K of the critical temperature, however, the concentration becomes substantially suppressed, and the van't Hoff slope diverges toward negative infinity. According to the phase rule, one mole fraction can be fixed. Given this restriction, the temperature behavior of the data is in exact agreement with the predictions of both the principle of critical point isomorphism and the Gibbs-Helmholtz equation. In addition, we have determined the concentration of lead in the liquid phase when crystalline lead(II) sulfate reacts with potassium iodide in isobutyric acid + water. When plotted in van't Hoff form, the data lie on a straight line for all temperatures including the critical region. The phase rule indicates that two mole fractions can be fixed. With this restriction, the data are in exact agreement with the principle of critical point isomorphism. [ABSTRACT FROM AUTHOR]

Nitrogen-substituted methylenecyclopropanes have been prepared by a strain-driven Overman rearrangement of cyclopropenylmethyl trichloroacetimidates. The reaction proceeds at room temperature and without the need of a transition-metal catalyst. Furthermore, it has been shown that C-3-substituted cyclopropenylmethyl trichloroacetimidates undergo a hydrolytic ring-opening reaction to form allenylcarbinols. [ABSTRACT FROM AUTHOR]

A combination of solid-state 13C NMR tensor data andDFT computational methods is utilized to predict the conformationin disordered methyl α-l-rhamnofuranoside. This previouslyuncharacterized solid is found to be crystalline and consists of atleast six distinct conformations that exchange on the kHz time scale.A total of 66 model structures were evaluated, and six were identifiedas being consistent with experimental 13C NMR data. Allfeasible structures have very similar carbon and oxygen positionsand differ most significantly in OH hydrogen orientations. A concertedrearrangement of OH hydrogens is proposed to account for the observeddynamic disorder. This rearrangement is accompanied by smaller changesin ring conformation and is slow enough to be observed on the NMRtime scale due to severe steric crowding among ring substituents.The relatively minor differences in non-hydrogen atom positions inthe final structures suggest that characterization of a complete crystalstructure by X-ray powder diffraction may be feasible. [ABSTRACT FROM AUTHOR]

The biological function of a protein is intimately related to its three-dimensional molecular structure. Although X-ray diffraction from single crystals can be employed to solve for the molecular structure, use of this method is often impeded by the slow rate of precipitation of crystals in the pH-buffered, water-based, electrolyte solutions which ordinarily serve as growth media. By taking into account the interionic electrostatic forces that affect protein solubility, nucleation, growth, and Ostwald ripening, we find that the following sequence of growth solution procedures should be effective in producing crystals of any water-soluble protein, which dissolves endothermically. The protein should be dissolved at room temperature in a growth solution, and then the temperature should be lowered to the cold room temperature at 4 °C to establish the supersaturation. To control nucleation, establish a measurable crystallization rate, and limit the number of crystals competing for the dissolved protein, the salt concentration should be minimal, and the pH should be different from the pI. As the rate of decay of the supersaturation approaches zero, Ostwald ripening will commence. If the salt concentration and temperature are maintained as above, and the value of the pH is chosen to be intermediate between the two most widely spaced but numerically adjacent pKa values of any of the ionizable amino acid residues along the protein chain, the number of crystals will decrease and the average crystal size will increase. By taking into account hydrodynamic convection in a growth solution in a gravitational field, we construct a figure of merit, M, that when evaluated using terrestrial measurements, can be used to discriminate between proteins that should benefit from crystallization in microgravity and those that should receive no benefit. The threshold value for the onset of benefits appears to be M ≥ 0.004. Finally, we discriminate between the magnetic field requirements appropriate for the complete levitation of a crystal growth solution in a gravitational field and those appropriate for the suppression of natural convection alone. We formulate a strategy based in physics and physical chemistry for choosing the pH, salt concentration, and temperature to control the effects of the hydrodynamics and interionic electrostatic forces that determine the nucleation, growth, and ripening of the crystals of water-soluble proteins which dissolve endothermically. We propose a hydrodynamic figure of merit for identifying proteins which might benefit from crystallization in microgravity. [ABSTRACT FROM AUTHOR]

The oxidation of pyrroles usually leads to uncontrolled polymerization and decomposition. To overcome this problem, the controlled oxidation of substituted pyrroles with Dess-Martin periodinane is reported. This strategy yields a range of 5-aroyloxypyrrolinones. [ABSTRACT FROM AUTHOR]

Fish consumption advisories instruct vulnerable consumers to avoid high mercury fish and to limit total fish intake to reduce neurotoxic risk. Consumption data from the U.S. suggest that nontarget consumers also respond to such advice. These consumers reduce exposure to mercury and other toxicants at the cost of reduction in cardioprotective fatty acids. We present a probabilistic model to assess these risk trade-offs. We use NHANES consumption data to simulate exposure to contaminants and nutrients in fish, employ dose-response relationships to convert exposure to health end points, and monetize them using benefit transfer. Our results suggest that newborns gained on average 0.033 IQ points from their mothers' compliance with the prominent FDA/EPA advisory. The welfare gain for a birth cohort is estimated at $386 million. This gain could be fully offset by increments in cardiovascular risk if 0.6% of consumers aged 40 and older reduced fish intake by one monthly meal until they reached the age of 60 or if 0.1% of them permanently reduced fish intake. [ABSTRACT FROM AUTHOR]

Water oxidation is a critical step in water splitting to make hydrogen fuel. We report the solution synthesis, structural/compositional characterization, and oxygen evolution reaction (OER) electrocatalytic properties of ∼2-3 nm thick films of NiOx, CoOx, NiyCo1-yOx, Ni0.9Fe0.1Ox, IrOx, MnOx, and FeOx. The thin-film geometry enables the use of quartz crystal microgravimetry, voltammetry, and steady-state Tafel measurements to study the electrocatalytic activity and electrochemical properties of the oxides. Ni0.9Fe0.1Ox was found to be the most active water oxidation catalyst in basic media, passing 10 mA cm-2 at an overpotential of 336 mV with a Tafel slope of 30 mV dec-1 with oxygen evolution reaction (OER) activity roughly an order of magnitude higher than IrOx control films and similar to the best known OER catalysts in basic media. The high activity is attributed to the in situ formation of layered Ni0.9Fe0.1OOH oxyhydroxide species with nearly every Ni atom electrochemically active. In contrast to previous reports that showed synergy between Co and Ni oxides for OER catalysis, NiyCo1-yOx thin films showed decreasing activity relative to the pure NiOx films with increasing Co content. This finding is explained by the suppressed in situ formation of the active layered oxyhydroxide with increasing Co. The high OER activity and simple synthesis make these Ni-based catalyst thin films useful for incorporating with semiconductor photoelectrodes for direct solar-driven water splitting or in high-surface-area electrodes for water electrolysis. [ABSTRACT FROM AUTHOR]

A remotely controlled autonomous method for long-term high-frequency sampling of environmental waters in remote locations is described. The method which preserves sample integrity of dissolved trace metals and major ions for month-long periods employs a gravitational filtration system (GFS) that separates dissolved and particulate phases as samples are collected. The key elements of GFS are (1) a modified "air-outlet" filter holder to maximize filtration rate and thus minimize filtration artifacts; and (2) the direct delivery of filtrate to dedicated bottle sets for specific analytes. Depth and screen filter types were evaluated with depth filters showing best performance. GFS performance is validated using ground, stream, and estuary waters. Over 30 days of storage, samples with GFS treatment had average recoveries of 95 ± 19% and 105 ± 7% of Fe and Mn, respectively; without GFS treatment, average recoveries were only 16% and 18%. Dissolved major cations K, Mg, and Na were stable independent of collection methodology, whereas Ca in some groundwater samples decreased up to 42% without GFS due to CaCO3 precipitation. In-field performance of GFS equipped autosamplers is demonstrated using ground and streamwater samples collected at the Angelo Coast Range Reserve, California from October 3 to November 4 2011. [ABSTRACT FROM AUTHOR]

We examine three possible explanations for the millisecondrelaxationtime of the dynamic surface tension of water: the diffusion of surfactantcontaminants from the aqueous phase to the surface, the reorientationof surface water molecules’ dipole moments, and the buildupof a charged surface layer of hydroxide ions. The relaxation timeexpected for hydroxide is by far the closest to the measured time.Our model for the surface layer agrees with static equilibrium experimentsand, as we show here, predicts the relaxation time. The results stronglyimply that the equilibrium surface of water is highly charged by aflow of hydroxide to the surface. The model predicts that neitherdiffusion nor autolysis dominates and shows that both processes areneeded to describe the relaxation of the surface tension. We finda salt and pH dependence of the relaxation time and propose furtherexperiments. [ABSTRACT FROM AUTHOR]

The microbial reduction of Fe(III) and U(VI) was investigated in shallow aquifer sediments collected from subsurface flood deposits near the Hanford Reach of the Columbia River in Washington State. Increases in 0.5 N HCI-extractable Fe(II) were observed in incubated sediments and 57Fe Mossbauer spectroscopy revealed that Fe(III) associated with phyllosilicates and pyroxene was reduced to Fe(II). Aqueous uranium(VI) concentrations decreased in subsurface sediments incubated in sulfate-containing synthetic groundwater with the rate and extent being greater in sediment amended with organic carbon. X-ray absorption spectroscopy of bioreduced sediments indicated that 67-77% of the U signal was U(VI), probably as an adsorbed species associated with a new or modified reactive mineral phase. Phylotypes within the Deltaproteobacteria were more conamon in Hanford sediments incubated with U(VI) than without, and in U(VI)-free incubations, members of the Clostridiales were dominant with sulfate-reducing phylotypes more common in the sulfate-amended sediments. These results demonstrate the potential for anaerobic reduction of phyllosilicate Pe(ll1) and sulfate in Hanford unconfined aquifer sediments and biotransformations involving reduction and adsorption leading to decreased aqueous U concentrations. [ABSTRACT FROM AUTHOR]

A series of ruthenium complexes having the general form [Ru(bpy)3-n(CN-Me-bpy)n](PF6)2 (where bpy = 2,2′-bipyridine, CN-Me-bpy = 4,4′-dicyano-5,5′-dimethyl-2,2′-bipyridine, and n = 1-3 for complexes 1-3, respectively) have been synthesized and characterized using a variety of steady-state and nanosecond time-resolved spectroscopies. Electrochemical measurements indicate that the CN-Me-bpy ligand is significantly easier to reduce than the unsubstituted bipyridine (on the order of ~500 mV), implying that the lowest energy 3MLCT (metal-to-ligand charge transfer) state will be associated with the CN-Me-bpy ligand(s) in all three compounds. Comparison of the Huang—Rhys factors derived from spectral fitting analyses of the steady state emission spectra of complexes 1-3 suggests all three compounds are characterized by excited-state geometries that are less distorted relative to their ground states as compared to [Ru(bpy)3](PF6)2; the effect of the more nested ground- and excited-state potentials is reflected in the unusually high radiative quantum yields (13% (1), 27% (2), and 40% (3)) and long 3MLCT-state room-temperature lifetimes (1.6 μs, 2.6 μs, and 3.5 μs, respectively) for these compounds. Coupling of the p* system into the CN groups is confirmed by nanosecond step-scan IR spectra which reveal a ~40 cm-1 bathochromic shift of the CN stretching frequency, indicative of a weaker CN bond in the 3MLCT excited state relative to the ground state. The fact that the shift is the same for complexes 1-3 is evidence that, in all three complexes, the long-lived excited state is localized on a single CN-Me-bpy ligand rather than being delocalized over multiple ligands. [ABSTRACT FROM AUTHOR]

The fate of pertechnetate (99Tc(VII)O4-) during bioreduction was investigated in the presence of 2-line ferrihydrite (Fh) and various dissimilatory metal reducing bacteria (DMRB) (Geobacter, Anaeromyxobacter, Shewanella) in comparison with TcO4- bioreduction in the absence of Fh. In the presence of Fh, Tc was present primarily as a fine-grained Tc(IV)/Fe precipitate that was distinct from the Tc(IV)O2∙nH2O solids produced by direct biological Tc(VII) reduction. Aqueous Tc concentrations (<0.2 μm) in the bioreduced Fh suspensions (1.7 to 3.2 × 10-9 mol L-1) were over 1 order of magnitude lower than when TcO4- was biologically reduced in the absence of Fh (4.0 × 10-8 to 1.0 × 10-7 mol L-1). EXAFS analyses of the bioreduced Fh-Tc products were consistent with variable chain length Tc-O octahedra bonded to Fe-O octahedra associated with the surface of the residual or secondary Fe(III) oxide. In contrast, biogenic TcO2∙nH2O had significantly more Tc-Tc second neighbors and a distinct long-range order consistent with small particle polymers of TcO2. In Fe-rich subsurface sediments, the reduction of Tc(VII) by Fe(II) may predominate over direct microbial pathways, potentially leading to lower concentrations of aqueous 99Tc(IV). [ABSTRACT FROM AUTHOR]

This paper reports the preparation of microgels with an interpenetrating polymer network (IPN) structure using microfluidic synthesis. Microgels with a narrow size distribution were generated from ionically cross-linked alginate and chemically cross-linked poly(N-isopropylacrylamide). The mechanical properties of the microgels, such as Young’s modulus and the characteristic relaxation time, were studied using atomic force microscopy (AFM). The lower limits of the elasticity found in this study are within the range of the elasticity reported for neutrophils, making these microgels a promising model system for studies of cell flow through constrained geometries. [ABSTRACT FROM AUTHOR]

We developed a probabilistic model to characterize the plausible distribution of health and economic benefits that would accrue to the U.S. population following reduction of methyl mercury (MeHg) exposure. MeHg, a known human developmental neurotoxicant, may increase fatal heart attack risks. Model parameters reflect current understanding of the relationships between MeHg intake, health risks, and societal valuation of these risks. The expected monetary value of the annual health benefits generated by a 10% reduction in U.S. population exposure to MeHg for one year is $860 million; 80% of this is associated with reductions in fatal heart attacks and the remainder with IQ gains. The plausible distribution of the benefits is quite broad with 5th and 95th percentile estimates of approximately $50 million and $3.5 billion, respectively. The largest source of uncertainty is whether epidemiological associations between MeHg exposure and fatal heart attacks reflect causality. The next largest sources of uncertainty concern the slope of the relationship between maternal MeHg exposure and reduced intelligence among children and whether this relationship exhibits a threshold. Our analysis suggests that the possible causal relationship between MeHg exposure and fatal heart attacks should be better characterized, using additional epidemiological studies and formally elicited expert judgment. [ABSTRACT FROM AUTHOR]

Analyses combining X-ray powder diffraction (XRD) and solid-state NMR (SSNMR) data can now provide crystal structures in challenging powders that are inaccessible by traditional methods. The flavonoid catechin is an ideal candidate for these methods, as it has eluded crystallographic characterization despite extensive study. Catechin was first described nearly two centuries ago, and its powders exhibit numerous levels of hydration. Here, synchrotron XRD data provide all heavy-atom positions in (+)-catechin 4.5-hydrate and establish the space group as C2. SSNMR data (13C tensor and 1H/13C correlation) complete the conformation by providing catechin's five OH hydrogen orientations. Since 1903, this phase has been erroneously identified as a 4.0 hydrate, but XRD and density data establish that this discrepancy is due to the facile loss of the water molecule located at a Wyckoff special position in the unit cell. A final improvement to heavy-atom positions is provided by a geometry optimization of bond lengths and valence angles with XRD torsion angles held constant. The structural enhancement in this final structure is confirmed by the significantly improved fit of computed 13C tensors to experimental data. [ABSTRACT FROM AUTHOR]

The synthesis, structure, and photophysical properties of a new family of trinuclear CuRe2 chromophore-quencher complexes having the general form [Cu(pyacac)2(Re(bpy')(CO)3)2](OTf)2 (where pyacac = 3-(4-pyridyl)-acetylacetonate and bpy' = 4,4'-5,5'-tetramethyl-2,2'-bipyridine (tmb, 1), 4,4'-dimethyl- 2,2'-bipyridine (dmb, 2), 2,2'-bipyridine (bpy, 3), 4,4'-dichloro-2,2'-bipyridine (dclb, 4), and 4,4'-diethylester- 2,2'-bipyridine (deeb, 5)) are reported. Time-resolved emission data acquired in room-temperature CH2CI2 solutions revealed excited-state lifetimes of 14.9 ± 0.7, 8.1 * 0.4, 8.2 ± 0.4, 5.6 ± 0.3, and 5.0 ± 0.3 ns for complexes 1-5, respectively. The emission in each case is assigned to decay of the Re'-based 3MLCT excited state; the lifetimes are all significantly less than the corresponding BeRe2 model complexes, which were also prepared and characterized. Electron transfer was found to be significantly endothermic for all five CuRe2 complexes: this fact, coupled with the ca. 10 Å donor-acceptor distance and favorable spectral overlap between the 3MLCT emission profile and absorptions of the Cu" center, implicates dipolar energy transfer as the dominant quenching pathway in these compounds. Gaussian decorivolution of the ground- state absorption spectrum of Cu(phacac)2 (phacac = 3-phenylacetylacetonate) allowed for a differential analysis of the spectral overlap between the donor emission spectra and the two observed ligand-field transitions of the Cu" ion. Energy transfer was found to occur preferentially to the lower energy ligand-field band due primarily to more favorable dipole orientation as measured by the κ2 term from Färster theory. These results, supported by time-dependent DFT calculations on Cu(phacac)2, indicate enhanced dipolar coupling to the dxz→dxy, transition of the Cu" center and thus represent an orbitally specific energy-transfer process occurring in this system. [ABSTRACT FROM AUTHOR]

The quantification of the mechanical stability of lipid bilayers is important in establishing composition−structure−property relations and sheds light on our understanding of the functions of biological membranes. Here, we designed an experiment to directly probe and quantify the nanomechanical stability and rigidity of the ceramide-enriched platforms that play a distinctive role in a variety of cellular processes. Our force mapping results have demonstrated that the ceramide-enriched domains require both methyl β-cyclodextrin (MbCD) and chloroform treatments to weaken their highly ordered organization, suggesting a lipid packing that is different from that in typical gel states. Our results also show the expulsion of cholesterol from the sphingolipid/cholesterol-enriched domains as a result of ceramide incorporation. This work provides quantitative information on the nanomechanical stability and rigidity of coexisting phase-segregated lipid bilayers with the presence of ceramide-enriched platforms, indicating that that generation of ceramide in cells drastically alters the structural organization and the mechanical property of biological membranes. [ABSTRACT FROM AUTHOR]

Rate constants for the gas-phase reactions of OH radical, NO3radical, and ozone with allyl alcohol (AAL) and allyl isocyanate (AIC) have been measured using relative rate methods at atmospheric pressure in purified air. The experimental Arrhenius expression obtained for the reaction of the OH radical with AAL is (1.68 ± 0.89) × 10−12× exp(1100/T) cm3molecule−1s−1, for T= 282−315 K; the Arrhenius expression for the reaction of OH radical with AIC is (1.94 ± 1.04) × 10−14× exp(2207/T) cm3molecule−1s−1, for T= 282−317 K, where the indicated errors are one least-squares standard deviation. All OH radical reaction rate constants have been measured relative to k(OH + α-pinene) and k(OH + 1,3,5-trimethylbenzene). The rate constant for the gas-phase reaction of OH radical with allyl alcohol-d6isotopomer (AAL-d6) has been measured at T= 298 K, and the value is 5.10 × 10−11cm3molecule−1s−1. The kinetic isotope effect is small, with k(AAL)/k(AAL-d6) = 1.32. Rate constants for the gas-phase reactions of NO3radical with AAL [relative to k(NO3+methacrolein)] and O3[relative to k(O3+ β-pinene)] have been measured, and the values are 7.7 × 10−15cm3molecule−1s−1at T= 298 K and 1.6 × 10−17cm3molecule−1s−1at T= 296 K, respectively. Rate constants for the gas-phase reactions of NO3radical and O3with AIC have been measured, and the values are 9.4 × 10−16cm3molecule−1s−1at T= 299 K and 5.54 × 10−18cm3molecule−1s−1at T= 299 K, respectively. Multireference ab initio calculations at the MRMP2/6-311G(d,p) level have been carried out for reactions of OH radical with AAL and AIC. Results indicate that prereactive hydrogen bonded complexes form in the entrance channels for these reactions. [ABSTRACT FROM AUTHOR]

Human exposure to contaminated soils drives clean up criteria at many urban brownfields. Current risk assessment guidelines assume that humans ingest some fraction of soil smaller than 4 mm but have no estimates of what fraction of soil is - ingested by humans. Here, we evaluated soil adherence to human hands for 13 agricultural soils from Saskatchewan, Canada and 17 different soils from a brownfield located in lqaluit, Nunavut, Canada. In addition, we estimated average particle size adhering to human hands for residents of a northern urban setting. Further, we estimated how metal concentrations differed between the adhered and bulk (<4 mm) fraction of soil. The average particle size for adhered agricultural soils was 34 μm, adhered brownfiald soils was 105 μm, and particles adhered to human residents was 36 μm. Metals ware significantly enriched in these adhered fractions with an average enrichment [(adheredbulk)/bulk] in metal concentration of 184% (113% median) fur 24 different elements. Enrichment was greater for key toxicological elements of concern such as chromium (140%), copper (140%), nickel (130%), lead (110%), and zinc (130%) and was highest for silver (810%), mercury (630%), selenium (500%), and arsenic (420%). Enrichment were positively correlated with carbonate complexation constants (but not bulk solubility products) and suggests that the dominant mechanism controlling metal enrichment in these samples is a precipitation of carbonate surfaces that subsequently adsorb metals. Our results suggest that metals of toxicological concern are selectively enriched in the fraction of soil that humans incidentally ingest Investigators should likely process soil samples through a 45 μm sieve before estimating the risk associated with contaminated soils to humans. The chemical mechanisms resulting in metal enrichment likely differ between sites but at our site were linked to surface complexation with carbonates. [ABSTRACT FROM AUTHOR]

Exploring the fine structures and physicochemical properties of physiologically relevant membranes is crucial to understanding biological membrane functions including membrane mechanical stability. We report a direct correlation of the self-organized structures exhibited in phase-segregated supported lipid bilayers consisting of dioleoylphosphatidylcholine/egg sphingomyelin/cholesterol (DEC) in the absence and presence of ceramide (DEC-Ceramide) with their nanomechanical properties using AFM imaging and high-resolution force mapping. Direct incorporation of ceramide into phase-segregated supported lipid bilayers formed ceramide-enriched domains, where the height topography was found to be imaging setpoint dependent. In contrast, liquid ordered domains in both DEC and DEC-Ceramide presented similar heights regardless of AFM imaging settings. Owing to its capability for simultaneous determination of the topology and interaction forces, AFM-based force mapping was used in our study to directly correlate the structures and mechanical responses of different coexisting phases. The intrinsic breakthrough forces, regarded as fingerprints of bilayer stability, along with elastic moduli, adhesion forces, and indentation of the different phases in the bilayers were systematically determined on the nanometer scale, and the results were presented as two-dimensional visual maps using a self-developed code for force curves batch analysis. The mechanical stability and compactness were increased in both liquid ordered domains and fluid disordered phases of DEC-Ceramide, attributed to the influence of ceramide in the organization of the bilayer, as well as to the displacement of cholesterol as a result of the generation of ceramide-enriched domains. The use of AFM force mapping in studying phase segregation of multicomponent lipid membrane systems is a valuable complement to other biophysical techniques such as imaging and spectroscopy, as it provides unprecedented insight into lipid membrane mechanical properties and functions. [ABSTRACT FROM AUTHOR]

The time dependence of the dynamic mobility and the ultrasonic attenuation of octane and decane oil-in-water emulsions stabilized by sodium dodecyl sulfate (SDS) was measured. The emulsions grew to larger droplets due to Ostwald ripening. The growth rate measured by attenuation depends on the surfactant concentration and the polydispersity of the emulsion. At surfactant concentrations below the critical micelle concentration (cmc) of SDS, the growth was linear with time and the rate was dependent on the polydispersity of the drops; the rate was several times faster than that predicted on the basis of a diffusion growth mechanism. Above the cmc, however, as the droplets grew in size there was a point at which the rate of growth increased, which corresponds to the droplet size at which depletion forces due to the surfactant micelles become significant. Under these conditions both the electroacoustic dynamic mobility and the acoustic attenuation spectra displayed characteristics of flocs: a large decrease in the phase lag at higher frequencies in the dynamic mobility spectrum and a decrease in the attenuation coefficient at low-megahertz frequencies with an increase at higher frequencies. This depletion flocculation enhancement in ripening rates in the presence of SDS micelles provides another, alternative, and self-consistent mechanism for the effect of surfactant micelles on Ostwald ripening. [ABSTRACT FROM AUTHOR]

The syntheses, physical, and photophysical properties of a family of complexes having the general formula [M2(L)(mcb)(Ru(4,4′-(X)2-bpy)2)](PF6)3 (where M = MnII or ZnII, X = CH3 or CF3, mcb is 4′-methyl-4-carboxy-2,2′-bipyridine, and L is a Schiff base macrocycle derived from 2,6-diformyl-4-methylphenol and bis(2-aminoethyl)-N-methylamine) are described. The isostructural molecules all consist of dinuclear metal cores covalently linked to a RuII polypyridyl complex. Photoexcitation of [Mn2(L)(mcb)(Ru((CF3)2-bpy)2)]-(PF6)3 (4) in deoxygenated CH2Cl2 solution results in emission characteristic of the ³MLCT excited state of the RuII chromophore but with a lifetime (τobs = 5.0 ± 0.1 ns) and radiative quantum yield (Φr ≈ 7 × 10-4) that are significantly attenuated relative to the ZnII model complex [Zn2(L)(mcb)(Ru((CF3)2-bpy)2)](PF6)3 (6) (τobs = 730 ± 30 ns and Φr = 0.024, respectively). Quenching of the ³MLCT excited state is even more extensive in the case of [Mn2(L)(mcb)(Ru((CH3)2-bpy)2)](PF6)3 (3), whose measured lifetime (τobs = 45 ± 5 ps) is >104 shorter than the corresponding model complex [Zn2(L)(mcb)(Ru((CH3)2-bpy)2)](PF6)3 (5) (τobs = 1.31 ± 0.05 µs). Time-resolved absorption measurements on both Mn-containing complexes at room-temperature revealed kinetics that were independent of probe wavelength; no spectroscopic signatures for electron-transfer photoproducts were observed. Time-resolved emission data for complex 4 acquired in CH2Cl2 solution over a range of 200-300 K could be fit to an expression of the form knr = k0 + A·exp{-ΔE7kBT} with k0 = 1.065 ± 0.05 × 107 s-1, A = 3.7 ± 0.5 × 1010 s-1, and ΔE= 1230 ± 30 cm-1. Assuming an electron-transfer mechanism, the variable-temperature data on complex 4 would require a reorganization energy of λ 0.4-0.5 eV which is too small to be associated with charge separation in this system. This result coupled with the lack of enhanced emission at temperatures below the glass-to-fluid transition of the solvent and the absence of visible absorption features associated with the Mn2II core allows for a definitive assignment of Dexter transfer as the dominant excited-state reaction pathway. A similar conclusion was reached for complex 3 based in part on the smaller driving force for electron transfer (ΔG0ET = -0.1 eV), the increase in probability of Dexter transfer due to the closer proximity of the donor excited state to the dimanganese acceptor, and a lack of emission from the compound upon formation of an optical glass at 80 K. Electronic coupling constants for Dexter transfer were determined to be ~10 cm-1 and ~0.15 cm-1 in complexes 3 and 4, respectively, indicating that the change in spatial localization of the excited state from the bridge (complex 3) to the periphery of the chromophore (complex 4) results in a decrease in electronic coupling to the dimanganese core of nearly 2 orders of magnitude.… [ABSTRACT FROM AUTHOR]

The electrical conduction of metal−molecule−metal junctions formed between Au-supported self-assembled monolayers of structurally different 1-hexanethiol, 1-decanethiol, and ferrocenyl-1-undecanethiol and a Pt-coated atomic force microscope (AFM) tip has been measured under different compression forces using conducting-probe AFM. The observed junction resistance had two distinct power law scaling changes with the compression force. Different scaling regions were assigned to the change in the contact area, tunneling distance, number of conduction pathways, and structure of the film under compression. [ABSTRACT FROM AUTHOR]

18O-isotope-labeling studies have led to the conclusion that there exist two major pathways for water oxidation catalyzed by dimeric ruthenium ions of the general type cis,cis-[L2RuIII(OH2)]2O4+. We have proposed that both pathways involve concerted addition of H and OH fragments derived from H2O to the complexes in their four-electron-oxidized states, i.e., [L2RuV(O)]2O4+, ultimately generating bound peroxy intermediates that decay with the evolution of O2. The pathways differ primarily in the site of addition of the OH fragment, which is either a ruthenyl O atom or a bipyridine ligand. In the former case, water addition is thought to give rise to a critical intermediate whose structure is L2RuIV(OH)ORuIV(OOH)L24+; the structures of intermediates involved in the other pathway are less well defined but may involve bipyridine OH adducts of the type L2RuV(O)ORuIV(OH)(L•OH)L4+, which could react further to generate unstable dioxetanes or similar endoperoxides. Published experimental and theoretical support for these pathways is reviewed within the broader context of water oxidation catalysis and related reactions reported for other diruthenium and group 8 monomeric diimine-based catalysts. New experiments that are designed to probe the issue of bipyridine ligand "noninnocence" in catalysis are described. Specifically, the relative contributions of the two pathways have been shown to correlate with substituent effects in 4,4′- and 5,5′-substituted bipyridine complexes in a manner consistent with the formation of a reactive OH-adduct intermediate in one of the pathways, and the formation of OH-bipyridine adducts during catalytic turnover has been directly confirmed by optical spectroscopy. Finally, a photosensitized system for catalyzed water oxidation has been developed that allows assessment of the catalytic efficiencies of the complex ions under neutral and alkaline conditions; these studies show that the ions are far better catalysts than had previously been assumed based upon reported catalytic parameters obtained with strong oxidants in acidic media. [ABSTRACT FROM AUTHOR]

We present a method for the synthesis of soluble and processable 3,4-alkylenedioxypyrrole (XDOP)-based conjugated polymers via an iododecarboxylation−deiodination polymerization methodology. Polymerization to the respected PXDOP derivatives of suitably high molecular weight (3.8−14.2 kDa vs polystyrene as measured by GPC) was achieved by either heating the neat monomers for a few hours or allowing them to remain at room temperature for several days. A family of four polymers was synthesized and characterized optically and electrochemically. Polymer solutions were transparent in the visible in their neutral states, red in their partially oxidized states, and grayish-green in their heavily oxidized states. Electrochemical measurements of cast films show that all redox processes occur at low potentials (ca.0 V vs the Fc/Fc), similar to electrodeposited films. Polymers spray-cast as films onto ITO exhibited high band gaps above 3.0 eV along with stable UV and near-IR electrochromism in organic solvents with almost no change in the visible. An amphiphilic polymer functionalized with oligoethoxy substituents exhibited enhanced electrochemistry in aqueous electrolyte and high-contrast electrochromism with respect to the nonpolar functionalized polymers in the visible and near-IR. [ABSTRACT FROM AUTHOR]

Field evidence from underground storage tank sites where leaded gasoline leaked indicates the lead scavengers 1,2- dibromoethane (ethylene dibromide, or EDB) and 1,2- dichloroethane (1,2-DCA) may be present in groundwater at levels that pose unacceptable risk. These compounds are seldom tested for at UST sites. Although dehalogenation of EDB and 1,2- DCA is well established, the effect of fuel hydrocarbons on their biodegradability under anaerobic conditions is poorly understood. Microcosms (2 L glass bottles) were prepared with soil and groundwater from a UST site in Clemson, South Carolina, using samples collected from the source (containing residual fuel) and less contaminated downgradient areas. Anaerobic biodegradation of EDB occurred in microcosms simulating natural attenuation, but was more extensive and predictable in treatments biostimulated with lactate. In the downgradient biostimulated microcosms, EDB decreased below its maximum contaminant level (MCL) (0.05 μg/L) at a first order rate of 9.4 ± 0.2 yr-1. The pathway for EDB dehalogenation proceeded mainly by dihaloelimination to ethene in the source microcosms, while sequential hydrogenolysis to bromoethane and ethane was predominant in the downgradient treatments. Biodegradation of EDB in the source microcosms was confirmed by carbon specific isotope analysis, with a δ13C enrichment factor of -5.6‰. The highest levels of EDB removal occurred in microcosms that produced the highest amounts of methane. Extensive biodegradation of benzene, ethylbenzene, toluene and ortho-xylene was also observed in the source and downgradient area microcosms. In contrast, biodegradation of 1,2-DCA proceeded at a considerably slower rate than EDB, with no response to lactate additions. The slower biodegradation rates for 1,2-DCA agree with field observations and indicate that even if EDB is removed to below its MCL 1,2-DCA may persist. [ABSTRACT FROM AUTHOR]

The synthesis of suitably protected p-aminophenyl H-phosphinic acids and esters from the corresponding para-substituted aryl halides has been accomplished via the Pd-catalyzed cross-coupling reaction of anilinium hypophosphite, either in the absence or presence of a tetraalkyl orthosilicate, to provide the free H-phosphinic acid or the corresponding ester, respectively. Subsequent conjugate addition of either a PIII species or phosphorus anion, generated in situ from either the free H-phosphinic acid or ester, to a 2-methylene glutaric acid ester provided the aryl phosphinic acid analogue of p-aminobenzoyl glutamic acid. Alkylation of these suitably protected p-aminophenyl phosphinic acid esters with a 6-(bromomethyl)-pteridine or the corresponding (bromomethyl)pyridopyrmidine, followed by hydrolytic removal of protecting groups, provided the target aryl phosphinic acid analogues of folic acid and related antifolates. Alternatively, for the synthesis of the folate or 5-deazafolate analogues on a slightly larger scale, reductive amination with either N²-acetyl or N²-pivaloyl-6-formylpterin or the corresponding formylpyridopyrmidine and the same suitably protected p-aminophenyl phosphinic acid esters, followed by removal of protecting groups, is preferred. In the course of this research, it was observed that the nucleophilicity of both the aniline nitrogen and various PIII species derived from p-aminophenyl phosphinic acid derivatives is significantly reduced compared to that of the unsubstituted counterpart. [ABSTRACT FROM AUTHOR]

The synthesis, physical, and spectroscopic properties of a series of metal complexes bridged by the redox-active chloranilate ligand are described. Compounds containing the (CAcat,cat)4- ligand, where (CAcat,cat)4- represents the fully reduced aromatic form of chloranilate, have been prepared by two different routes from H2CA and H4CA starting materials; the corresponding (CAsq,cat)3- analogue was obtained by one-electron oxidation with decamethylferrocenium tetrafluoroborate. Homo- and heterobimetallic complexes containing CrIII and GaIII with chloranilate have been prepared, yielding the following six complexes: [Ga2(tren)2(CAcat,cat)](BPH4)2 (1), [Ga2(tren)2(CAsq,cat)}-(BPH4)2(BF4) (2), [GaCr(tren)2(CAcat,cat)](BPH4)2 (3), [GaCr(tren)2(CAsq,cat)](BPH4)2(BF4) (4), [Cr2(tren)2(CAcat,cat)] (BPH4)2 (5), and [Cr2(tren)2(CAsq,cat)](BPH4)2(BF4) (6) (where tren is tris(2-aminoethyl)amine). Single-crystal X-ray structures have been obtained for complexes 1, 3, and 5; nearly identical C-C bond distances within the quinoidal ligand confirm the aromatic character of the bridge in each case. Complex 2 exhibits a temperature-independent magnetic moment of μeff = 1.64 ± 0.04 μB in the solid state between 4 and 350 K, consistent with the CAsq,cat formulation of the ligand and an S = 1/2 ground state for complex 2. Complex 3 exhibits a value of μeff = 3.44 ± 0.09 μB that is also temperature-independent, indicating an S = 3/2 ground state. Complexes 4-6 are all influenced by Heisenberg spin exchange. The temperature-independent behavior of complexes 4 and 6 indicate the presence of strong antiferromagnetic exchange between the CrIII and the (sq,cat) bridging radical yielding well-isolated ground states of S = 1 and 5/2 for 4 and 6, respectively. In contrast, complex 5 exhibits a weak intramolecular antiferromagnetic exchange interaction between the two CrIII centers (J = -2 cm-1 for ... = -2JŜ1 · Ŝ2) via superexchange through the diamagnetic CAcat,cat bridge. The absorption spectra of the CAsq,cat-containing complexes exhibit a number of sharp, relatively intense features in fluid solution. Group theoretical arguments coupled with a qualitative ligand-field analysis including the effects of Heisenberg spin exchange suggest that several of the observed transitions are a consequence of exchange interactions in both the ground- and excited-state manifolds of the compounds. [ABSTRACT FROM AUTHOR]

The rate coefficient has been measured under pseudo-first-order conditions for the Cl CH3association reaction at T202, 250, and 298 K and P0.3−2.0 Torr helium using the technique of discharge-flow mass spectrometry with low-energy (12-eV) electron-impact ionization and collision-free sampling. Cl and CH3were generated rapidly and simultaneously by reaction of F with HCl and CH4, respectively. Fluorine atoms were produced by microwave discharge in an approximately 1% mixture of F2in He. The decay of CH3was monitored under pseudo-first-order conditions with the Cl-atom concentration in large excess over the CH3concentration (Cl0/CH309−67). Small corrections were made for both axial and radial diffusion and minor secondary chemistry. The rate coefficient was found to be in the falloff regime over the range of pressures studied. For example, at T202 K, the rate coefficient increases from 8.4 × 10-12at P0.30 Torr He to 1.8 × 10-11at P2.00 Torr He, both in units of cm3molecule-1s-1. A combination of ab initio quantum chemistry, variational transition-state theory, and master-equation simulations was employed in developing a theoretical model for the temperature and pressure dependence of the rate coefficient. Reasonable empirical representations of energy transfer and of the effect of spin−orbit interactions yield a temperature- and pressure-dependent rate coefficient that is in excellent agreement with the present experimental results. The high-pressure limiting rate coefficient from the RRKM calculations is k26.0 × 10-11cm3molecule-1s-1, independent of temperature in the range from 200 to 300 K. [ABSTRACT FROM AUTHOR]

Two newly synthesized pyrenylcyclam dendrons (1and 2) exhibit a new emission band centered at 450 nm when coordinated with copper triflate. Observed fluorescence shifts induced by coordinative metalation indicate an unusual intramolecular charge transfer from a pyrenyl excited state to the coordinated metal ion that competes with pyrene excimer formation. This interaction likely proceeds by photoexcitation of a -complex of the appended arene, followed by intramolecular charge transfer within the dendritic 1:1 cyclam/metal complex, effecting reduction of the bound Cu(II) metal ion. The appended dendritic groups not only decrease the equilibrium binding constant with Cu(II) but also participate in a new excited-state pathway as an alternative to energy-dissipative excimer formation. [ABSTRACT FROM AUTHOR]

A method for selecting high-probability structures from numerous computer-generated crystal structures is described. This procedure evaluates structures by comparing computed NMR shifts for each predicted structure to experimental solid-state NMR data. Four carbohydrates are evaluated:  methyl -d-galactopyranoside, methyl -d-glucopyranoside, methyl -d-mannopyranoside, and methyl -d-xylopyranoside. In these cases, 81.8% of the structures retained as probable fits by lattice energy comparisons are eliminated by the NMR criterion using tensor principal values. This analysis also ranks the correct structure as the best-fit for methyl -d-glucopyranoside and usually places the correct structure among the top five in other cases. Isotropic shift comparisons are less successful in selecting structure. The NMR analysis is sufficiently sensitive to identify a 30° error in one torsion angle of the purported correct structure of methyl -d-galactopyranoside. In this case, it is found that none of the 164 computer-generated structures match experimental data. The substances investigated experience only weak electrostatic fields; therefore, the NMR analysis chooses primarily by molecular conformation rather than lattice structure. NMR data thus provide a valuable independent selection criterion. The presence of strong electrostatic fields in polar samples can alter the results given here, and likely changes in the selection process are discussed. [ABSTRACT FROM AUTHOR]

Calmodulin is a central mediator of calcium-dependent signal transduction pathways and regulates the activity of a large number of diverse targets. Calcium-dependent interactions of calmodulin with regulated proteins are of generally high affinity but of quite variable thermodynamic origins. Here we investigate the influence of the binding of the calmodulin-binding domain of calmodulin kinase I on the fast internal dynamics of calcium-saturated calmodulin. NMR relaxation was used to probe motion on the backbone (viewed through the backbone amide NH group) and the side chains (viewed through methyl groups). The distribution of the amplitudes of side chain dynamics is trimodal. The microscopic details of side chain motion are compared with those of a thermodynamically and structurally similar complex of calmodulin with the calmodulin-binding domain of the smooth muscle myosin light chain kinase. While there are no significant differences in backbone dynamics and no net change in methyl- bearing side chain dynamics, a large redistribution of the amplitude of methyl dynamics is observed between the two complexes. The variation in dynamics was largely localized to the heterogeneously dynamic target-binding interface, suggesting that differential dynamics of the binding surface plays a functional role in the high-affinity binding interactions of calmodulin. These results begin to reveal a fundamental role for residual protein entropy in molecular recognition by calmodulin. [ABSTRACT FROM AUTHOR]

Using dilatometry, we examined the crystallization kinetics of the jack bean storage protein, canavalin. We found that the kinetic rate law is first order in the supersaturation. At pH values above the pI = 5.2, where the protein is negatively charged, the apparent rate constant, k, increases as the pH approaches the pI, decreases with increasing temperature, and increases with increasing salt concentration. The rate constant is essentially independent of the initial protein concentration. Precisely the same patterns of behavior were observed by Kim et al. [Mol.Phys.2003, 101, 2677−2686] in dilatometer experiments involving the crystallization of lysozyme even though lysozyme molecules are positively charged under crystal growth conditions. By combining dimensional analysis with a simple theory of protein crystal nucleation and growth, we show that k ≈ ω/cs, where ω is the rate coefficient that controls the rate of advance of the crystal facets, and cs is the solubility of the protein in the growth solution. We determined the dependence of ω upon temperature and salt concentration by analyzing the observations of Forsythe and Pusey [J. Cryst. Growth1994, 139, 89−94] of the rate of advance of the facets of growing lysozyme crystals. To the extent that our data overlap that of Forsythe and Pusey, we are able to show for both canavalin and lysozyme that the dependence of k, ω, and cs upon these variables is consistent with k ≈ ω/cs. Using dilatometry, we have examined the crystallization kinetics of canavalin and lysozyme. In both cases, the kinetic rate law is first order in the supersaturation. The apparent rate constant in both cases decreases with increasing temperature, while it increases with increasing salt concentration and with any change in the pH that carries the protein closer to its isoelectric point. [ABSTRACT FROM AUTHOR]

Synchrotron X-ray powder diffraction and solid-state 13C NMR shift tensor data are combined to provide a unique path to structure in microcrystalline organic solids. Analysis is demonstrated on ambuic acid powder, a widely occurring natural product, to provide the complete crystal structure. The NMR data verify phase purity, specify one molecule per asymmetric unit, and provide an initial structural model including relative stereochemistry and molecular conformation. A refinement of X-ray data from the initial model establishes that ambuic acid crystallizes in the P21 space group with unit cell parameters a = 15.5047(7), b = 4.3904(2), and c = 14.1933(4) Å and β = 110.3134(3)°. This combined analysis yields structural improvements at two dihedral angles over prior NMR predictions with differences of 103° and 37° found. Only minor differences of ±5.5°, on average, are observed at all remaining dihedral angles. Predicted hydroxyl hydrogen-bonding orientations also fit NMR predictions within ±6.9°. This refinement corrects chemical shift assignments at two carbons and reduces the NMR error by ∼16%. This work demonstrates that the combination of long-range order information from synchrotron powder diffraction data together with the accurate shorter range structure given by solid-state NMR measurements is a powerful tool for studying challenging organic solids. [ABSTRACT FROM AUTHOR]

Radical addition of H3PO2 to N-/C-protected vinyl glycine led to the corresponding H-phosphinic acid in excellent yield. The non-nucleophilic H-phosphinic acid was converted to a nucleophilic pm species, RP(OTMS)2, which was used in two approaches to the target phosphinic acid containing pseudopeptide. New methodology was developed that led to excellent yields in the reaction of RP(OTMS)2 with unactivated electrophiles, including an acyclic homoallylic bromide. However, en route to the target pseudopeptide, Arbuzov reaction of RP(OTMS)2 with a cyclic homoallylic bromide, (R)-3-(bromomethyl)-cyclopent-1-ene, led to a rearranged allylic phosphinic acid rather than the desired homoallylic derivative, a putative glutarate surrogate. Conjugate addition of RP(OTMS)2 to a-methylene glutarate containing a chiral auxiliary resulted in only modest diastereoselectivity. Purification by flash chromatography provided protected derivatives of both diastereomers of the pseudopeptide. Following global deprotection, coupling of (S)-H-Glu-γ-[Ψ(P(O)(OH)(CH2))]-(S)-Glu-OH and (S)-H-Glu-γ-[Ψ(P(O)(OH)(CH2))]-(R)-Glu-OH to (4-amino-4-deoxy-10-methyl)pteroyl azide led to the target compounds for biochemical study as inhibitors of the ATP-dependent ligase, folylpoly-γ-glutamate synthetase. [ABSTRACT FROM AUTHOR]

Femtosecond time-resolved absorption spectroscopy has been used to elucidate the excited- state dynamics associated with formation of the ²E excited state in a CrIII transition metal complex. Cr- (acac)3 (where acac is the deprotonated monoanion of acetylacetone) exhibits monophasic decay kinetics with τ = 1.1 ± 0.1 Ps following excitation into the lowest-energy ligand-field absorption band; the time constant is found to be independent of both excitation and probe wavelength across the entire 4A2 → 4T2absorption envelope. The lack of a significant shift in the excited-state absorption spectrum combined with the observed spectral narrowing is consistent with an assignment of this process as vibrational cooling (kvib) in the ²E state. The data on Cr(acac)3 indicate that intersystem crossing associated with the 4T2→ ²E conversion occurs at a rate kISC > 1013 S-1 and furthermore competes effectively with vibrational relaxation in the initially formed 4T2 state. Excitation into the higher energy 4LMCT state (&lemda;ex = 336 nm) gives rise to biphasic kinetics with τ1 = 50 ± 20 fs and τ2 2 = 1.2 ± 0.2 Ps. The slower component is again assigned to vibrational cooling in the ²E state, whereas the subpicosecond process is attributed to conversion from the charge-transfer to the ligand-field manifold. In addition to detailing a process central to the photophysics of CrIII, these results reinforce the notion that the conventional picture of excited-state dynamics in which kvib > kIC > kISC does not generally apply when describing excited-state formation in transition metal complexes. [ABSTRACT FROM AUTHOR]

ThermoFluor (a miniaturized high-throughput protein stability assay) was used to analyze the linkage between protein thermal stability and ligand binding. Equilibrium binding ligands increase protein thermal stability by an amount proportional to the concentration and affinity of the ligand. Binding constants (Kb) were measured by examining the systematic effect of ligand concentration on protein stability. The precise ligand effects depend on the thermodynamics of protein stability: in particular, the unfolding enthalpy. An extension of current theoretical treatments was developed for tight binding inhibitors, where ligand effect on Tm can also reveal binding stoichiometry. A thermodynamic analysis of carbonic anhydrase by differential scanning calorimetry (DSC) enabled a dissection of the Gibbs free energy of stability into enthalpic and entropic components. Under certain conditions, thermal stability increased by over 30 °C; the heat capacity of protein unfolding was estimated from the dependence of calorimetric enthalpy on Tm. The binding affinity of six sulfonamide inhibitors to two isozymes (human type 1 and bovine type 2) was analyzed by both ThermoFluor and isothermal titration calorimetry (ITC), resulting in a good correlation in the rank ordering of ligand affinity. This combined investigation by ThermoFluor, ITC, and DSC provides a detailed picture of the linkage between ligand binding and protein stability. The systematic effect of ligands on stability is shown to be a general tool to measure affinity. [ABSTRACT FROM AUTHOR]

A simple method is presented for patterning of protein antigens at a gold surface for use in surface plasmon resonance (SPR) imaging experiments. Microfluidic devices fabricated from poly(dimethylsiloxane) were used to flow various fluids over a gold substrate in spatially defined channels. This technique was used to pattern the surface chemistry of the gold as well as to adsorb antigens from solution to the modified substrates. The resulting antigen arrays were probed with complementary antibodies in order to demonstrate the effectiveness of the patterning for antibody capture experiments. SPR imaging was used to aid in the opthnization of array fabrication and to observe the interactions of unlabeled antibodies with these microarrays. This work presents a means of fabricating microarrays with controlled surface density of antigens. SPR imaging provides both quantitative and qualitative evaluation of antibody binding in a label free format [ABSTRACT FROM AUTHOR]

A fullerene derivative 10 with a terminal thiol group dissolves easily in common organic solvents and forms a densely packed self-assembled monolayer on gold surfaces. The functionalization of C60 is based on the 1,3-dipolar cycloaddition of the azomethine ylide generated in situ from the corresponding aldehyde and N-methylglycine. The monolayers were characterized by grazing angle reflectance FTIR spectroscopy, scan tunneling microscopy, and cyclic voltammetry. The cyclic voltammogram of a SAM of 10 showed two well-resolved reversible cathodic waves corresponding to the first two one-electron reductions of the fullerene fragment. [ABSTRACT FROM AUTHOR]

Electrochemical and spectroelectrochemical properties of five cobalt(III) acetate complexes [Co[supIII],[sub3](μ[sub3]-O)(CH[sub3]- CO[sub2])[sub5](OR)(py)[sub3]][PF[sub6]] are described, where py = pyridine and R = OCCH[sub3] (A), H (B), CH[sub3] (C), CH[sub2]CH=CH[sub2] (D), and CH[sub2]C[sub6]H[sub5] (E). Each is reduced irreversibly as observed by cyclic voltammetry at room temperature and at -40 °C in acetonitrile at scan rates up to 20 V s[sup-1], but oxidized reversibly to a mixed-valence Co(III)[sub2]Co(IV) species at ∼ 1.23 V vs the ferrocenium/ferrocene couple. Controlled potential coulometry confirmed a one-electron-oxidation process. Spectroelectrochemical oxidation of A at 5 °C showed isosbestic points in the electronic absorption spectrum that showed the oxidized complex to be stable in solution for at least 1 h. [ABSTRACT FROM AUTHOR]

The synthesis and characterization of nanoparticle-cored dendrimers (NCDs), consisting of a metal core capped by arylpolyethers terminated with ester or carboxylate groups, are reported. These NCDs, comprising nanometer-sized gold clusters at the core and organic dendrons radially connected to the gold core by gold-sulfur bonds, were analyzed by TEM, TGA, UV, IR, and NMR spectroscopies. The density of the branching units connected to the core decreased from 1.90/nm[SUP2] for a first-generation NCD (Au-G1 (CO[SUB2]Me)) to 0.80/nm[SUB2] for a fourth-generation NCD (Au-G4(CO[SUB2]Me)). Although the ester-terminated NCDs were stable and resisted aggregation, they were easily hydrolyzed to the corresponding watersoluble sodium salts. Aqueous solutions of (Au-Gn(CO[SUB2]Na)) exhibited micellar properties. Since these NCDs possess a relatively unpassivated metal core and an organic aryl ether shell with micellar and dendritic properties, they are expected to have important potential applications in catalysis. [ABSTRACT FROM AUTHOR]

Biogeochemical processes within riverbed hyporheic zones (HZ) can potentially impact the fate and transport of contaminants. We evaluated a modified freeze core technique for the collection of intact cobble-bed samples from the Columbia River HZ along a stretch of the Hanford Reach in Washington State and investigated microbiological and geochemical parameters of corresponding frozen and unfrozen samples. During three sampling periods (March, May, and November 2000), relatively high numbers of viable aerobic heterotrophic bacteria were recovered from both unfrozen (10[sup6]-10[sup7] cfu/g) and frozen samples (10sup5]-10[sup6] cfu/g). Relatively large populations of sulfate-, nitrate-, and iron-reducing bacteria were present, and significant concentrations of acid-volatile sulfide were measured in some samples, indicating that anoxic regions exist within this zone. Cr(Vl), a priority groundwater pollutant on adjacent U.S. Department of Energy lands, was probably removed from solution in HZ samples by a combination of microbial activity and chemical reduction, presumably via products of anaerobic microbial metabolism. These results suggest that biogeochemical processes in the Columbia River HZ may contribute to the natural attenuation of Cr(Vl). Although freezing modestly diminished recovery of viable bacteria, freeze core techniques proved reliable for the collection of intact hyporheic sediments. [ABSTRACT FROM AUTHOR]

Relative stereochemistry is predicted for ambuic acid using a novel solid-state NMR approach. This NMR technique entails a comparison of measured shift tensor principal values with computed values for all diastereomers, allowing the selection of a best-fit structure. The proposed method extends previous solution NMR structural data by simultaneously modeling with high statistical probability hydrogen-bonding arrangements and molecular conformation at two positions. A dimeric structure is proposed for ambuic acid based on the initial poor fit of the carboxyl carbon tensors to a monomeric model. The dimer model, consisting of hydrogen bonding between pairs of neighboring carboxyl groups, reduces the root mean square error at the carboxy tensor by a factor of 2.7. Lattice details are thus also described by the proposed approach. The structural characterization method presented is of general applicability and may be especially useful for characterizing difficult to crystallize or hydrogen-poor materials. [ABSTRACT FROM AUTHOR]

The synthesis and characterization of a group of new dendrimers—namely, nanoparticie-cored dendrimers (NCDs)—are described. These materials were obtained by the reduction of hydrogen tetrachloroaurate phase-transferred into toluene in the presence of Fréchet-type polyaryl ether dendritic disulfide wedges of generation 1-5. These materials, possessing nanometer-sized gold clusters at the core and dendritic wedges radially connected to the core by Au-S bonds, were analyzed by TEM and TGA, and by UV, IR, and NMR spectroscopies. The number of branching units connected to the core decreased with the generation of the dendritic wedge, and this number changed from 2.18/nm² for Au- G-2 to 0.27/nm² for Au-G-5. This result suggests that, in the higher-generation NCDs, a large fraction of the surface area of the metal cluster is not passivated and is therefore available for catalytic activity. [ABSTRACT FROM AUTHOR]

The rapid kinetics of bacterial U(VI) reduction and low solubility of uraninite (UO[sub 2,cr]) make this process an attractive option for removing uranium from groundwater. Nevertheless, conditions that may promote or inhibit U(VI) reduction are not well-defined. Recent descriptions of Ca-UO[sub 2]CO[sub 3] complexes indicate that these species may dominate the aqueous speciation of U(VI) in many environments. We monitored the bacterial reduction of U(VI) in bicarbonatebuffered solution in the presence and absence of Ca. XAFS measurements confirmed the presence of a CaU(VI)-CO[sub 3] complex in the initial solutions containing calcium. Calcium, at millimolar concentrations (0.45-5 mM), caused a significant decrease in the rate and extent of bacterial U(VI) reduction. Both facultative (Shewanella putrefaciens strain CN32) and obligate (Desulfovibrio desulfuricans, Geobacter sulfurreducens) anaerobic bacteria were affected by the presence of calcium. Reduction of U(VI) ceased when the calculated system E[sub h] reached -0.046 ± 0.001 V, based on the Ca[sub 2]UO[sub 2](CO[sub 3])[sub 3] → UO[sub 2,cr] couple. The results are consistent with the hypothesis that U is a less energetically favorable electron acceptor when the CaUO[sub 2]-CO[sub 3] complexes are present. The results do not support Ca inhibition caused by direct interactions with the cells or with the electron donor as the reduction of fumarate or Tc(VII)O[sub 4, sup -] under identical conditions was unaffected by the presence of Ca. [ABSTRACT FROM AUTHOR]