Search

Provides an overview of enzyme reactions which contribute to the character and acceptability of plant foods. A detailed discussion of polyphenoloxidase is also provided as an example of an enzyme which can markedly affect the character and acceptability of such foods. (JN)

• A derivative of a density with respect to a field diverges at the critical point. • This phenomenon includes all known binary liquid mixtures with a critical point. • Phase rule predicts isomorphism of physical and chemical critical point properties. The isobaric heat capacity and the coefficient of thermal expansion of a binary liquid mixture with a critical point of solution diverge as functions of temperature in the critical region. When the same liquids are used as solvents for chemical reactions, the van't Hoff plot of the extent of reaction diverges in the critical region. According to the phase rule, the physical and chemical properties are isomorphic in the sense that in each case the critical behavior is determined by three fixed thermodynamic intensive variables. [ABSTRACT FROM AUTHOR]

Attempts to derive a formula for the G value for a general plasma chemical reaction as a function of the electrical power absorbed, the fraction of molecules transformed and the flow rate of the gas entering the reactor. Theory of the G value, dose and dose rate in terms of macroscopic processes; G value for the plasma decomposition of ammonia; Discussion and conclusions.

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]

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]

Abstract: The paper addresses the effects of salinity and temperature on the chemistry of important parameters affecting coagulation pretreatment including the ion product of water, acid-base chemistry, dissolved metal speciation, and precipitation reactions for aluminum and iron coagulants. The ion product of seawater is greater than for freshwaters and affects chemical hydrolysis and metal-hydroxide solubility reactions. Inorganic carbon is the main cause of seawater alkalinity and buffer intensity but borate also contributes. Buffer intensity is an important parameter in assessing coagulation pH adjustment. Mineral particles are relatively unstable in seawater from electrical double layer compression, and when present these particles are easily coagulated. Algal-particle stability is affected by steric effects and algal motility. Dissolved natural organic matter from algae and humic substances causes fouling of RO membranes and pretreatment removal is essential. Aluminum coagulants are not recommended, and not used, because they are too soluble in seawater. Ferric coagulants are preferred and used. The equilibrium solubility of Fe with amorphous ferric hydroxide in seawater is low over a wide range of pH and temperature conditions. Ferric chloride dosing guidelines are presented for various raw seawater quality characteristics. The effect of pH on coagulant dose and the role of buffer intensity are addressed. A dual coagulation strategy is recommended for treating seawater with moderate to high concentrations of algae or seawater with humic matter. This involves a low and constant dose with high charge-density cationic polymers using Fe as the main coagulant where it is varied in response to raw water quality changes. [Copyright &y& Elsevier]

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]

The potential for reduction of 99TcO4-(aq) to poorly soluble 99TcO2 · nH2O(s) by biogenic sediment-associated Fe(II) was investigated with three Fe(III)-oxide containing subsurface materials and the dissimilatory metal-reducing subsurface bacterium Shewanella putrefaciens CN32. Two of the subsurface materials from the U.S. Department of Energy’s Hanford and Oak Ridge sites contained significant amounts of Mn(III,IV) oxides and net bioreduction of Fe(III) to Fe(II) was not observed until essentially all of the hydroxylamine HCl-extractable Mn was reduced. In anoxic, unreduced sediment or where Mn oxide bioreduction was incomplete, exogenous biogenic TcO2 · nH2O(s) was slowly oxidized over a period of weeks. Subsurface materials that were bioreduced to varying degrees and then pasteurized to eliminate biological activity, reduced TcO4-(aq) at rates that generally increased with increasing concentrations of 0.5 N HCl-extractable Fe(II). Two of the sediments showed a common relationship between extractable Fe(II) concentration (in mM) and the first-order reduction rate (in h-1), whereas the third demonstrated a markedly different trend. A combination of chemical extractions and 57Fe Mössbauer spectroscopy were used to characterize the Fe(III) and Fe(II) phases. There was little evidence of the formation of secondary Fe(II) biominerals as a result of bioreduction, suggesting that the reactive forms of Fe(II) were predominantly surface complexes of different forms. The reduction rates of Tc(VII)O4- were slowest in the sediment that contained plentiful layer silicates (illite, vermiculite, and smectite), suggesting that Fe(II) sorption complexes on these phases were least reactive toward pertechnetate. These results suggest that the in situ microbial reduction of sediment-associated Fe(III), either naturally or via redox manipulation, may be effective at immobilizing TcO4-(aq) associated with groundwater contaminant plumes. [Copyright &y& Elsevier]

Examines the tandem nitroaldol-dehydration reactions of phenylsulfonylnitromethane. Utilization of lithium diisopropylamide; Implication of unsaturated nitrosulfones for the formation of bis(alpha-nitrosulfones); Reaction of propanal with the dilithium salt of phenylsulfonylnitromethane.

Examines the solid-state 13C chemical shift tensors in terpenes using nuclear magnetic resonance (NMR) analyses. Evidence of mixed polymorphic forms in solid verbenol; Structural predictions from computed and experimental NMR shift tensors; Characterization of the minor polymorphic form of verbenol.

The article informs that the ability of anti-lymphocytic globulin to suppress a variety of experimentally induced autoimmune diseases in a number of species is well documented. Blocks of kidney were fixed in 10% formal-saline and embedded in paraffin. Four-micron sections were stained with haematoxylin and eosin, the periodic acid-Schiff reaction and the methanamine silver method. Serial dilutions were performed using the Takatsky microtitrator. Each analysis included normal serum and standard antiserum controls as well as unsubsidized sheep erythrocyte controls.

A straightforward synthesis of 2-phenylimidazo[1,2-a]pyridine is described. The reaction is designed to demonstrate to students the preparation of a bridged N-heterocycle, in which the heteroatom occupies a bridgehead position. The product is obtained in moderate to high yield and is highly crystalline. The compound can be purified either by direct recrystallization or silica gel column chromatography. Students characterize the compound by melting point, MS, IR spectroscopy, and NMR spectroscopy. Spectroscopic analysis reveals features that are potentially suitable for more in-depth discussions. [ABSTRACT FROM AUTHOR]

The synthesis of heterocyclic-fused imidazoles was achieved by flash vacuum pyrolysis (FVP) of N-heterocyclic isoxazol-5(2H)-ones via an iminocarbene intermediate. Unlike iminocarbenes generated from triazoles, no structural rearrangements were observed during the current synthesis method. We also demonstrated that less volatile isoxazol-5(2H)-one derivatives yield the corresponding imidazoles by condensed phase pyrolysis. [ABSTRACT FROM AUTHOR]

Two epoxide-containing peptidomimetics of the isopeptide, glutamyl-γ-glutamate, have been synthesized via a route that should be generally applicable to the synthesis of isopeptide analogues in which an oxirane replaces the scissile peptide bond~ Enzymes that catalyze the hydrolysis of peptides and isopeptides are often susceptible to inactivation by electrophilic substrate analogues. In this research, an epoxide was installed as an electrophilic replacement of the scissile isopeptide bond. The C-terminal glutamyl mimic was accessed by the stereospecific synthesis of suitably substituted cyclopentenes, 8 and 10, as surrogates for either the L- or D- enantiomer. The enantiomeric cyclopentenes were further elaborated to incorporate an appended sulfone that was reacted with a suitably protected glutamyl-γ-semialdehyde in a Julia-Kocienski olefination reaction. This olefination afforded predominantly the desired E-olefin isosteres of L-glutamyl-γ-D-glutamate and L-glutamyl-γ-L-glutamatc, following which peracid-mediated epoxidation and deprotection provided çhe epoxide-containing peptidomimetics, 4 and 5. [ABSTRACT FROM AUTHOR]

Abstract: Flexible racemic syntheses of the phthalide-containing antibiotics CJ-13,015, CJ-13,102, CJ-13,103, CJ-13,104 and CJ-13,108 that inhibit Helicobacter pylori have been carried out in a convergent fashion by Wittig coupling of a phthalide-containing aldehyde fragment with an appropriate phosphorous ylide. [Copyright &y& Elsevier]

Solution-phase photoinduced low-spin to high-spin conversion in the FeII polypyridyl complex [Fe(tren(py)3)]2+ (where tren(py)3 is tris(2-pyridylmethyliminoethyl)amine) has been studied via picosecond soft X-ray spectroscopy. Following ¹A1 → 1MLCT (metal-to-ligand charge transfer) excitation at 560 nm, changes in the iron L2- and L3-edges were observed concomitant with formation of the transient high-spin 5T2 state. Charge-transfer multiplet calculations coupled with data acquired on low-spin and high-spin model complexes revealed a reduction in ligand field splitting of ∼1 eV in the high-spin state relative to the singlet ground state. A significant reduction in orbital overlap between the central Fe-3d and the ligand N-2p orbitals was directly observed, consistent with the expected ca. 0.2 Å increase in Fe-N bond length upon formation of the high-spin state. The overall occupancy of the Fe-3d orbitals remains constant upon spin crossover, suggesting that the reduction in s-donation is compensated by significant attenuation of π-back-bonding in the metal-ligand interactions. These results demonstrate the feasibility and unique potential of time-resolved soft X-ray absorption spectroscopy to study ultrafast reactions in the liquid phase by directly probing the valence orbitals of first-row metals as well as lighter elements during the course of photochemical transformations. [ABSTRACT FROM AUTHOR]

Sodium boltwoodite (NaUO2SiO3OH·1.5 H2O) was used to assess the kinetics of microbial reduction of solid-phase U(VI) by a dissimilatory metal-reducing bacterium (DMRB), Shewanella oneidensis strain MR-1. The bioreduction kinetics was studied with Na-boltwoodite in suspension or within alginate beads in a nongrowth medium with lactate as electron donor at pH 6.8 buffered with PIPES. Concentrations of U(VI)tot and cell number were varied to evaluate the coupling of U(VI) dissolution, diffusion, and microbial activity. Microscopic and spectroscopic analyses with transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and laser-induced fluorescence spectroscopy (LIFS) collectively indicated that solid-phase U(VI) was first dissolved and diffused out of grain interiors before it was reduced on bacterial surfaces and/or within the periplasm. The kinetics of solid-phase U(VI) bioreduction was well described by a coupled model of bicarbonate-promoted dissolution of Na-boltwoodite, intragrain uranyl diffusion, and Monod type bioreduction kinetics with respect to dissolved U(VI) concentration. The results demonstrated that microbial reduction of solid-phase U(VI) is controlled by coupled biological, chemical, and physical processes. [ABSTRACT FROM AUTHOR]

Amide-linkedspiropyran—anthraquinone (SP—AQ) conjugates were shown to mediate ZnTPPS4- photosensitized transmembrane reduction of occluded Co(bpy)33+ within unilamellar phosphatidylcholine vesicles by external EDTA. Overall quantum yields for these reactions were dependent upon the isomeric state of the dye; specifically, 30–35% photoconversion of the closed-ring spiropyran (SP) moiety to the open-ring merocyanine (MC) form caused the quantum yield to decrease by 6-fold in the simple conjugate and 3-fold for an analogue containing a lipophilic 4-dodecylphenoxy substituent on the anthraquinone moiety. Transient spectroscopic and fluorescence quenching measurements revealed that two factors contributed to these photoisomerization-induced changes in quantum yields: increased efficiencies of fluorescence quenching of ¹ZnTPPS4- by the merocyanine group and lowered transmembrane diffusion rates of the merocyanine-containing redox carriers. Transient spectrophotometry also revealed the sequential formation and decay of two reaction intermediates, identified as 3ZnTPpS4 and a species with the optical properties of a semiquinone radical. Kinetic profiles for Co(bpy)33+ reduction under continuous photolysis in the presence and absence of added ionophores indicated that transmembrane redox mediated by SP-AQ was electroneutral, but reaction by the other quinone-containing mediators was electrogenic. The minimal reaction mechanism suggested from the combined studies is oxidative quenching of vesicle-bound ³ZnTPPS4- by the anthraquinone unit, followed by either H+/e- cotransport by transmembrane diffusion of SP-AQH or, for the other redox mediators, semiquinone anion-quinone electron exchange leading to net transmembrane electron transfer, with subsequent one-electron reduction of the internal Co(bpy)33+. Thermal one-electron reduction of Co(bpy)33+ by EDTA is energetically unfavorable; the photosensitized reaction therefore occurs with partial conversion of photonic energy to chemical and transmembrane electrochemical potentials. [ABSTRACT FROM AUTHOR]

Single electron reduction of the 1,2,4-trioxane heterocycle of artemisinin (1) forms primary and secondary carbon-centered radicals. The complex structure of 1 does not lend itself to a satisfactory dissection of the electronic and steric effects that influence the formation and subsequent reaction of these carbon-centered free radicals. To help demarcate these effects, we characterized the reactions of achiral dispiro-1,2,4-trioxolane 4 and dispiro-1,2,4-trioxanes 5-7 with ferrous bromide and 4-oxoTEMPO. Our results suggest a small preference for attack of Fe(II) on the nonketal peroxide oxygen atom of 1. For 4, but not for 5 and 6, there was a strong preference for attack of Fe(II) on the less hindered peroxide bond oxygen atom. The steric hindrance afforded by a spiroadamantane in a five-membered trioxolane is evidently much greater than that for a corresponding six-membered trioxane. Unlike 1, 5-7 fragment by entropically favored β-scission pathways forming relatively stable α-oxa carbon-centered radicals. These data suggest that formation of either primary or secondary carbon-centered radicals is a necessary but insufficient criterion for antimalarial activity of 1 and synthetic peroxides. [ABSTRACT FROM AUTHOR]

Ca2+-calmodulin-dependent protein kinase II (CAMkinase II) is a ubiquitous Ser/Thr-directed protein kinase that is expressed from a family of four genes (α, β, γ, and δ) in mammalian cells. We have documented the three-dimensional structures and the biophysical and enzymatic properties of the four gene products. Biophysical analyses showed that each isoform assembles into oligomeric forms and their three-dimensional structures at 21-25 Å revealed that all four isoforms were dodecamers with similar but highly unusual architecture. A gear-shaped core comprising the association domain has the catalytic domains tethered on appendages, six of which extend from both ends of the core. At this level of resolution, we can discern no isoform-dependent differences in ultrastructure of the holoenzymes. Enzymatic analyses showed that the isoforms were similar in their Km for ATP and the peptide substrate syntide, but showed significant differences in their interactions with Ca2+-calmodulin as assessed by binding, substrate phosphorylation, and autophosphorylation. Interestingly, the rank order of CaM binding affinity (γ > β > δ > α) does not directly correlate with the rank order of their CaM dependence for autophosphorylation (β > γ > δ > α). Simulations utilizing this data revealed that the measured differences in CaM binding affinities play a minor role in the autophosphorylation of the enzyme, which is largely dictated by the rate of autophosphorylation for each isoform. [ABSTRACT FROM AUTHOR]

Tyrosine phosphorylation, regulated by protein tyrosine phosphatases (PTPs) and kinases (PTKs), is important in signaling pathways underlying tumorigenesis. A mutational analysis of the tyrosine phosphatase gene superfamily in human cancers identified 83 somatic mutations in six PTPs (PTPRF, PTPRC, PTPRT, PTPN3, PTPNT3, PTPN14), affecting 26% of colorectal cancers and a smaller fraction of lung, breast, and gastric cancers. Fifteen mutations were nonsense, frameshift, or splice-site alterations predicted to result in truncated proteins lacking phosphatase activity. Five missense mutations in the most commonly altered PTP (PTPRT) were biochemically examined and found to reduce phosphatase activity. Expression of wild-type but not a mutant PTPRT in human cancer cells inhibited cell growth. These observations suggest that the mutated tyrosine phosphatases are tumor suppressor genes, regulating cellular pathways that may be amenable to therapeutic intervention. [ABSTRACT FROM AUTHOR]

The article analyzes the high-affinity binding and direct electron transfer using Shewanella oneidensis cytochrome OmCA. Researchers have shown that purified OmcA binds and densely covers the surface of hematiteand reduces Fe (III) with maximal velocity. It corresponds to an electron flux that approaches observed fluxes in the most efficient bioreactors.