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The process of 1,1,1,2-tetrafluoroethane (Freon-134a) hydrate dissociation was studied using high-resolution pulsed NMR technique. The diffusional-kinetic nature of the process, its kinetic parameters (rate constant and half-life time), and the characteristic feature that the residual hydrate is stabilized by the products of its dissociation were established experimentally. It was found that this effect abates significantly with rise of temperature and that the activation energy of freon hydrate dissociation is comparable to dissociation of hydrates which are relatively stable at atmospheric pressure. [ABSTRACT FROM AUTHOR]

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We recently reported the use of cyclopropanecarboxylic acid (CPCa) as a model additive that can readily react under the combined effect of flash heating and stress in steel tribocontacts to form tribopolymers, along with marked improvement in tribological performance. In this paper, we present results of how chemical structural modification of CPCa may impact on the formation of tribopolymers and hence friction and wear properties, both by experiments and molecular dynamics simulation. Four lubricant additives, viz., CPCa, cyclobutanecarboxylic acid (CBCa), cyclopropane-1,1-dicarboxylic acid (CPDCa), and cyclobutane-1,1-dicarboxylic acid (CBDCa) consisting of a metastable ring structure and one or two carboxyl groups dissolved in an ester base oil were studied. Friction and wear rate using these additives rank in the order of CPDCa < CBDCa < CPCa < CBCa. Raman spectroscopy analysis reveals that these additive molecules form tribopolymer films at the contact area. Molecular dynamics simulation shows that CPCa with the less stable cyclopropane ring fragments more readily than CBCa. Such fragmentation appears to be essential for subsequent tribopolymerization and formation of protective tribofilms. These simulations further demonstrate that having two carboxyl groups as in the case of CPDCa results in stronger binding of the additive molecules to the surface, thus increasing the residence time and hence facilitating mechanically or thermally induced dissociation and subsequent polymerization. The net result is that CPDCa gives the lowest friction and negligible wear under our testing conditions. [ABSTRACT FROM AUTHOR]

• Hydrate in clayey-silt cores exhibits stepwise dissociation characteristics. • Grain size and lithology significantly affect hydrate dissociation kinetics. • A dissociation kinetic model refining driving force and surface area is developed. Natural gas hydrate (NGH) in clayey silts might be the majority of its global reserve. A thorough understanding on the dissociation kinetics of NGH in clayey silts is crucial for its safe and efficient exploitation. In this study, the dissociation characteristics of methane hydrate in quartz silts, montmorillonites and South China Sea (SCS) sediments made cores in multi-step depressurization was experimentally studied and modeled. In quartz silt core, methane hydrate thoroughly dissociated once the pressured is lowered to the equilibrium pressure in bulk. In montmorillonite and SCS sediment cores, methane hydrate dissociation initiated at higher pressure than quartz silt. Capillary and clay induced water activity depressions are suggested to be the major causes. Stepwise methane hydrate dissociation was discovered in the montmorillonite and SCS sediment cores in multi-stage depressurization. In one step depressurization, hydrate in the fine grain cores with larger surface area dissociated faster. In montmorillonite and SCS sediment cores, lower water content leads to faster dissociation at the same pressure, which may be attributed to the higher driven force for dissociation (fugacity difference f e - f). A kinetic model for NGH dissociation in clayey silts was established by introducing fitted equivalent diameter and surface area correction factor of hydrate particles to the Kim-Bishnoi model. In the model, the hydrate equilibrium conditions in clayey silts were predicted by the water activity measurement (WAM) technique. With a maximum deviation of 9.81% in gas production, the model prediction and experimental results are in good agreement. The experimental findings and model reported in this work may be helpful to understand the NGH phase transition mechanism in clayey silt sediments. [ABSTRACT FROM AUTHOR]

A new pyclen-3,9-diacetate derivative ligand (H23,9-OPC2A) was synthesized possessing an etheric O-atom opposite to the pyridine ring, to improve the dissociation kinetics of its Mn(II) complex (pyclen = 3,6,9,15-tetraazabicyclo(9.3.1)pentadeca-1(15),11,13-triene). The new ligand is less basic than the N-containing analogue (H23,9-PC2A) due to the non-protonable O-atom. In spite of its lower basicity, the conditional stability of the [Mn(3,9-OPC2A)] (pMn = −log(Mn(II)), cL = cMn(II) = 0.01 mM. pH = 7.4) remains unaffected (pMn = 8.69), compared to the [Mn(3,9-PC2A)] (pMn = 8.64). The [Mn(3,9-OPC2A)] possesses one water molecule, having a lower exchange rate with bulk solvents (kex298 = 5.3 ± 0.4 × 107 s−1) than [Mn(3,9-PC2A)] (kex298 = 1.26 × 108 s−1). These mild differences are rationalized by density-functional theory (DFT) calculations. The acid assisted dissociation of [Mn(3,9-OPC2A)] is considerably slower (k1 = 2.81 ± 0.07 M−1 s−1) than that of the complexes of diacetates or bisamides of various 12-membered macrocycles and the parent H23,9-PC2A. The [Mn(3,9-OPC2A)] is inert in rat/human serum as confirmed by 52Mn labeling (nM range), as well as by relaxometry (mM range). However, a 600-fold excess of EDTA (pH = 7.4) or a mixture of essential metal ions, propagated some transchelation/transmetalation in 7 days. The H23,9-OPC2A is labeled efficiently with 52Mn at elevated temperatures, yet at 37 °C the parent H23,9-PC2A performs slightly better. Ultimately, the H23,9-OPC2A shows advantageous features for further ligand designs for bifunctional chelators.

• The activation energy of CO 2 hydrate at negative temperatures was determined. • The calculation method was developed to determine the "true size" of the particles. • An efficient way to utilize harmful gases is to create the large monolithic samples. The dissociation of gas hydrate in the form of pressed granules of different diameters has been investigated. Kinetic equations to describe dissociation of gas hydrates at negative temperatures have been obtained for the first time. The activation energy for CO 2 hydrate is higher than for methane hydrate, and the intrinsic decomposition rate constant value, on the contrary, is almost 15 times lower for CO 2 hydrate. The article proposes methods to improve the efficiency of gas hydrate storage and utilization of carbon dioxide. The pressed granules were obtained from gas hydrate particles of the same size (0.5 mm). The activation energy was found to be practically independent of the diameter of the pressed granules and the thickness of the powder layer, but to be determined by the minimum ("effective") size of particles, forming the granules or a layer. The "effective" size refers to the diameter of uniform particles for which coalescence, pressing and any other packaging are excluded. It is shown for the first time that the value of the pre-exponential factor depends on the radius of the granule R (layer thickness h) if the radius (layer thickness h) exceeds the characteristic critical size R cr (h cr). The R cr value is determined by the thermal and diffusion resistance of the layer and is different for different gas hydrates. [ABSTRACT FROM AUTHOR]

Variable temperature electrospray ionization (ESI) is coupled with mass spectrometry techniques in order to investigate structural transitions of monoclonal antibody immunoglobulin G (IgG) in a 100-mM ammonium acetate (pH 7.0) solution from 26 to 70 °C. At 26 °C, the mass spectrum for intact IgG shows six charge states + 22 to + 26. Upon increasing the solution temperature, the fraction of low-charge states decreases and new, higher-charge state ions are observed. Upon analysis, it appears that heating the solution aids in desolvation of the intact IgG precursor. Above ~ 50 °C, a cleavage event between the light and heavy chains is observed. An analysis of the kinetics for these processes at different temperatures yields transition state thermochemistry of ΔH‡ = 95 ± 10 kJ mol−1, ΔS‡ = 8 ± 1 J mol−1 K−1, and ΔG‡ = 92 ± 11 kJ mol−1. The mechanism for light chain dissociation appears to involve disulfide bond scrambling that ultimately results in a non-native Cys199–Cys217 disulfide bond in the light chain product. Above ~ 70 °C, we are unable to produce a stable ESI signal. The loss of signal is ascribed to aggregation that is primarily associated with the remaining portion of the antibody after having lost the light chain. [ABSTRACT FROM AUTHOR]

We report ion mobility spectrometry and mass spectrometry studies of the non-enzymatic step-by-step degradation of substance P (subP), an 11-residue neuropeptide, with the sequence Arg1-Pro2-Lys3-Pro4-Gln5-Gln6-Phe7-Phe8-Gly9-Leu10-Met11-NH2, in ethanol. At elevated solution temperatures (55 to 75 °C), several reactions are observed, including a protonation event, i.e., [subP+2H]2+ + H+ → [subP+3H]3+, that appears to be regulated by a configurational change and two sequential bond cleavages (the Pro2-Lys3 peptide bond is cleaved to form the smaller nonapeptide Lys3-Met11-NH2 [subP(3–11)], and subsequently, subP(3–11) is cleaved at the Pro4-Gln5 peptide bond to yield the heptapeptide Gln5-Met11-NH2 [subP(5–11)]). Each of the product peptides [subP(3–11) and subP(5–11)] is accompanied by a complementary diketopiperazine (DKP): cyclo-Arg1-Pro2 (cRP) for the first cleavage, and cyclo-Lys3-Pro4 (cKP) for the second. Insight about the mechanism of degradation is obtained by comparing kinetics calculations of trial model mechanisms with experimental data. The best model of our experimental data indicates that the initial cleavage of subP is regulated by a conformational change, likely a trans→cis isomerization of the Arg1-Pro2 peptide bond. The subP(3–11) product has a long lifetime (t1/2 ~ 30 h at 55 °C) and appears to transition through several structural intermediates prior to dissociation, suggesting that subP(3–11) is initially formed with a Lys3-trans-Pro4 peptide bond configuration and that slow trans→cis isomerization regulates the second bond cleavage event as well. From these data and our model mechanisms, we obtain transition state thermochemistry ranging from ΔH‡ = 41 to 85 kJ mol−1 and ΔS‡ = − 43 to – 157 J mol−1 K−1 for each step in the reaction. [ABSTRACT FROM AUTHOR]

Nitro-substituted 2,3,7,7,8,12,18-hexamethyl-5-phenyl-13,17-diethylporphins with nitro groups in positions 10 and 20 of the porphyrin core and in the para-position of the benzene ring have been synthesized. The structure of the obtained compounds has been confirmed by H1 NMR, UV, and IR spectroscopy as well as mass spectrometry. The kinetics of dissociation of zinc, copper, cobalt, and manganese complexes of the obtained porphyrins in proton-donor media has been studied. The stability of the porphyrin metal complexes has been related to the electronic effects of the substituents in the tetrapyrrole cycle and spatial distortion of the latter. [ABSTRACT FROM AUTHOR]

We report two macrocyclic ligands based on a 1,7-diaza-12-crown-4 platform functionalized with acetate (tO2DO2A2−) or piperidineacetamide (tO2DO2AMPip) pendant arms and a detailed characterization of the corresponding Mn(II) complexes. The X−ray structure of [Mn(tO2DO2A)(H2O)]·2H2O shows that the metal ion is coordinated by six donor atoms of the macrocyclic ligand and one water molecule, to result in seven-coordination. The Cu(II) analogue presents a distorted octahedral coordination environment. The protonation constants of the ligands and the stability constants of the complexes formed with Mn(II) and other biologically relevant metal ions (Mg(II), Ca(II), Cu(II) and Zn(II)) were determined using potentiometric titrations (I = 0.15 M NaCl, T = 25 °C). The conditional stabilities of Mn(II) complexes at pH 7.4 are comparable to those reported for the cyclen-based tDO2A2− ligand. The dissociation of the Mn(II) chelates were investigated by evaluating the rate constants of metal exchange reactions with Cu(II) under acidic conditions (I = 0.15 M NaCl, T = 25 °C). Dissociation of the [Mn(tO2DO2A)(H2O)] complex occurs through both proton− and metal−assisted pathways, while the [Mn(tO2DO2AMPip)(H2O)] analogue dissociates through spontaneous and proton-assisted mechanisms. The Mn(II) complex of tO2DO2A2− is remarkably inert with respect to its dissociation, while the amide analogue is significantly more labile. The presence of a water molecule coordinated to Mn(II) imparts relatively high relaxivities to the complexes. The parameters determining this key property were investigated using 17O NMR (Nuclear Magnetic Resonance) transverse relaxation rates and 1H nuclear magnetic relaxation dispersion (NMRD) profiles.

Graphical abstract Abstract This study integrates X-ray radiography and 3D computed tomography (CT) to measure xenon hydrate dissociation kinetics in a porous medium (sand) through depressurization at the quasi-isothermal condition. Before dissociation, we use X-ray CT to measure hydrate volume and surface area in the sand and find [hydrate volume]2/3 is a linear approximation of hydrate surface area regardless of differences in hydrate specific area. During dissociation, we use X-ray radiography to quantify the evolutions of hydrate volume and dissociation rate. We find [hydrate volume]2/3 (i.e., hydrate surface area) is a linear metric of hydrate dissociation rate. Additionally we observe two circumstances when dissociate rate is not linear with [hydrate volume]2/3. First, above a critical hydrate saturation of 35–45%, hydrate dissociation rate initially stays constant and then decreases with [volume]2/3. Second, the dissociation rates of patchy hydrates in large pores initially stay constant and are significantly slower than hydrates in small pores with the same sizes. Since methane hydrates in nature are patchy and heterogeneously distributed, the new knowledge will facilitate modeling of hydrates production with a simple metric of dissociation rate (i.e., [volume]2/3) and constraints of hydrate saturation and pore size. [ABSTRACT FROM AUTHOR]

After the serious nuclear accident at the Fukushima Daiichi Nuclear Power Plant caused by the Great East Japan Earthquake in 2011, the development of feasible, safe, and highly sensitive analytical methods (in terms of low levels of radiation exposure and radioactive waste generation) for radioactive samples, especially actinide (An) ions, represents an important challenge. Here we propose a methodology for selecting appropriate emissive probes for An ions with very low consumption and emission of radioactivity by capillary electrophoresis–laser-induced fluorescence detection (CE-LIF), using a small chemical library of probes with eight different chelating moieties. It was found that the emissive probe L1, which possesses the tetradentate chelating moiety 1,10-phenanthroline-2,9-dicarboxylic acid (PDA), was suitable for detecting uranyl ions. The detection limit for the uranyl–L1 complex using CE-LIF combined with dynamic ternary complexation and on-capillary concentration techniques was determined to be 2.9 × 10 −12  M (0.7 ppt). No interference from the large excess of matrix metal ions was observed. This method was successfully applied to real radioactive liquid samples collected from nuclear facilities, including the Fukushima Daiichi Nuclear Power Plant. This strategy not only permitted the development of a safe and rapid analytical method but also provided insight into the coordination chemistry of An ion complexes. Specifically, the PDA structure provided substantial kinetic inertness to its uranyl complex; the formation of a ternary complex between uranyl–L1 and carbonate was revealed; and unusual interactions were observed between the π-electron systems of uranyl and the phenanthroline ring, which stabilized the uranyl–PDA interaction. [ABSTRACT FROM AUTHOR]

• CO 2 hydrate dissociation behaviour in individual capsules was investigated. • A novel two-stage numerical model was developed to simulate dissociation of encapsulated hydrate. • Parametric study was conducted for dissociation process of encapsulated hydrate. • Surface-to-volume ratio and the dissociation driving force are the two main factors influencing the dissociation kinetics. Gas hydrates have a wide implementation in the capture, storage, transport and utilisation of a range of gases, in which the dissociation kinetics is of great importance to gas transport and recovery efficiency. Encapsulation has been proven an effective technique to enhance gas–liquid mass transfer and thus improve gas hydrate formation kinetics. However, the dissociation behaviour of gas hydrate in individual capsules remains unknown. In this work, the dissociation kinetics of encapsulated CO 2 -TBAB semi-clathrate hydrates in different shapes are experimentally investigated under various operating conditions, and, for the first time, a two-stage numerical model is developed which integrates mass transfer, heat transfer and the intrinsic gas hydrate reaction kinetics to simulate the gas hydrate dissociation process. The effects of temperature, pressure, capsule volume and capsule geometry on gas hydrate dissociation kinetics are investigated. The results reveal that the proposed model is capable of capturing two distinct experimentally-observed dissociation stages, a rapid and subsequently slow stage according to the measured dissociation rate. Surface-to-volume ratio of the capsule and the dissociation driving force are the two main factors influencing the dissociation kinetics. The ring-shaped capsule exhibits the most efficient dissociation process due to its high surface-to-volume ratio and long transition time to the slow dissociation stage. This work enhances the understanding of gas hydrate dissociation behaviour in individual capsules and guides the manipulation of dissociation for efficient hydrate-based gas transport and recovery. [ABSTRACT FROM AUTHOR]

Purpose: Comparison of the dissociation kinetics of rapid-acting insulins lispro, aspart, glulisine and human insulin under physiologically relevant conditions. Methods: Dissociation kinetics after dilution were monitored directly in terms of the average molecular mass using combined static and dynamic light scattering. Changes in tertiary structure were detected by near-UV circular dichroism. Results: Glulisine forms compact hexamers in formulation even in the absence of Zn. Upon severe dilution, these rapidly dissociate into monomers in less than 10 s. In contrast, in formulations of lispro and aspart, the presence of Zn and phenolic compounds is essential for formation of compact R6 hexamers. These slowly dissociate in times ranging from seconds to one hour depending on the concentration of phenolic additives. The disadvantage of the long dissociation times of lispro and aspart can be diminished by a rapid depletion of the concentration of phenolic additives independent of the insulin dilution. This is especially important in conditions similar to those after subcutaneous injection, where only minor dilution of the insulins occurs. Conclusion: Knowledge of the diverging dissociation mechanisms of lispro and aspart compared to glulisine will be helpful for optimizing formulation conditions of rapid-acting insulins. [ABSTRACT FROM AUTHOR]

Nickel(II) complex, [Ni( L)] of the ligand 1,3-dimethyl-5-( p-Cl-phenylazo)-6-aminouracil ( HL) was isolated in the form of single crystals and characterized by single-crystal X-ray diffraction. The complex possesses square planar geometry with molecular formula CHClNNiO. The atomic arrangements about the Ni(II) are in trans-square planar fashion where the Ni(II) ion lies on an inversion centre. It crystallizes in the orthorhombic system, space group Pbca, with lattice parameters: a = 11.9282(7) Å, b = 11.2222(6) Å, c = 19.9242(12) Å and α = β = γ = 90°. The complex is stable in neutral to basic buffer solution region and dissociates to its anionic hydrazone form followed by the hydrazone in acidic buffer solution. The dissociation process was studied kinetically and is described as 'acid-limiting' or 'saturation kinetics', in which the rate becomes independent of the hydrogen ion concentration at high acid concentration. The process involves rapid protonation pre-equilibrium (constant, K) followed by rate-determining dissociation (constant, k) step and these constants are found to be 3.41 × 10 M and 8.37 × 10 Ms, respectively. The interaction of the complex with different anions like F, Cl, Br, AcO, NO or HSO was studied and found quite different results. With fluoride ions the complex using its -NH- fragments, forms H-bonded association complex whereas in presence of HSO the complex like in acidic buffer solution, dissociates to form (k = 1.28 Ms) the hydrazone form of the ligand. Graphical Abstract: The square planar nickel(II) complex of the ligand 1,3-dimethyl-5-( p-Cl-phenylazo)-6-aminouracil forms association complex with F ions and dissociates to form the hydrazone form of the ligand in acidic buffer solution and in presence of HSO . [ABSTRACT FROM AUTHOR]

Capillary electrophoresis was used to estimate the solvolytic dissociation rate (k d) of metal complexes of deferasirox (DFX, H 3 L), a drug used to treat iron overload. Inert CoIIIL 2 3− did not dissociate. The estimated k d value for FeIIIL 2 3− was (2.7 ± 0.3) × 10−4 s−1 (298 K, pH 7.4). The k d values of other complexes (AlIIIL 2 3−, NiIIL 2 4−, and MnIIL−) were in the range 10−3–10−4 s−1. In contrast, ZnIIL− and CuIIL− were too labile to allow k d estimation. The fact that the half-life of FeIIIL 2 3− (43.3 min) is shorter than the blood half-life of DFX (8–16 h) implies that the blood concentration of DFX should be high enough to prevent dissociation of FeIIIL 2 3−. The possibility of a safer iron-chelation therapy that avoids excretion of other essential metal ions such as ZnII is discussed, highlighting the importance of selectivity in terms of kinetic stability. The CER technique enabled estimation of the solvolytic dissociation rates of metal complexes of deferasirox, a chelator used in the treatment of iron overload. A safer chelation therapy that does not disrupt ZnII and CuII homeostasis is proposed on the basis of the kinetic selectivity of the complexation. [Display omitted] • Dissociation rates (k d) of metal–DFX (H 3 L) complexes were determined by CER. • The k d value for FeL 2 3− was (2.7 ± 0.3) × 10−4 s−1 (298 K, pH 7.4). • The k d values of AlL 2 3−, NiL 2 4−, and MnL− were leveled in the range 10−3–10−4 s−1. • Complexes ZnL− and CuL− were too labile to allow k d estimation. • A safer chelation therapy avoiding excretion of essential Zn and Cu is proposed. [ABSTRACT FROM AUTHOR]

High epitope-specific sensitivity of CD8+ T cells is required for optimal immune protection against intracellular pathogens as well as certain malignancies. The quality of antigen recognition of CD8+ T cells is usually described as 'avidity' to its cognate peptide MHCI complex. T cell avidity is mainly dependent on the structural qualities of the T cell receptor (TCR), as convincingly demonstrated by recombinant TCR re-expression experiments. Based on reversible MHCI multimer staining and koff-rate measurements of monomeric peptide MHCI complexes, we recently established a microscopic assay for determining the structural avidity of individual CD8+ T cells. Here we demonstrate that this assay can be adapted for rapid flow-cytometric avidity screening of epitope-specific T cell populations. Furthermore, we show that-in combination with conventional nonreversible MHCI multimer staining-even very small epitope-specific CD8+ T cell populations can be analyzed directly ex vivo without the need for previous TCR cloning or T cell sorting. This simplified approach provides highly accurate mean TCR-ligand koff-rate values for poly- or oligoclonal T cell populations and is ideally suited for high-throughput applications in basic research as well as clinical settings. © 2016 International Society for Advancement of Cytometry [ABSTRACT FROM AUTHOR]

Prior to the exponential decrease of activity of a uricase from Candida sp. during storage at 37 °C, there was a plateau period of about 4 days at pH 7.4, 12 days at pH 9.2, and about 22 days in the presence of 30 μM oxonate at pH 7.4 or 9.2, but no degradation of polypeptides and no activity of resolved homodimers. To reveal determinants of the plateau period, a dissociation model involving a serial of conformation intermediates of homotetramer were proposed for kinetic analysis of the thermoinactivation process. In the dissociation model, the roles of interior noncovalent interactions essential for homotetramer integrity were reflected by an equivalent number of the artificial weakest noncovalent interaction; to avoid covariance among parameters, the rate constant for disrupting the artificial weakest noncovalent interaction was fixed at the minimum for physical significance of other parameters; among thermoinactivation curves simulated by numerical integration with different sets of parameters, the one for least-squares fitting to an experimental one gave the solution. Results found that the equivalent number of the artificial weakest noncovalent interaction primarily determined the plateau period; kinetics rather than thermodynamics for homotetramer dissociation determined the thermoinactivation process. These findings facilitated designing thermostable uricase mutants. [ABSTRACT FROM AUTHOR]

Background The IgE-binding DNA aptamer 17.4 is known to inhibit the interaction of IgE with the high-affinity IgE Fc receptor FcεRI. While this and other aptamers have been widely used and studied, there has been relatively little investigation of the kinetics and energetics of their interactions with their targets, by either single-molecule or ensemble methods. Methods The dissociation kinetics of the D17.4/IgE complex and the effects of temperature and ionic strength were studied using fluorescence anisotropy and single-molecule spectroscopy, and activation parameters calculated. Results The dissociation of D17.4/IgE complex showed a strong dependence on temperature and salt concentration. The k off of D17.4/IgE complex was calculated to be (2.92 ± 0.18) × 10 −3 s −1 at 50 mM NaCl, and (1.44 ± 0.02) × 10 −2 s −1 at 300 mM NaCl, both in 1 mM MgCl 2 and 25 °C. The dissociation activation energy for the D17.4/IgE complex, E a , was 16.0 ± 1.9 kcal mol −1 at 50 mM NaCl and 1 mM MgCl 2 . Interestingly, we found that the C19A mutant of D17.4 with stabilized stem structure showed slower dissociation kinetics compared to D17.4. Single-molecule observations of surface-immobilized D17.4/IgE showed much faster dissociation kinetics, and heterogeneity not observable by ensemble techniques. Conclusions The increasing k off value with increasing salt concentration is attributed to the electrostatic interactions between D17.4/IgE. We found that both the changes in activation enthalpy and activation entropy are insignificant with increasing NaCl concentration. The slower dissociation of the mutant C19A/IgE complex is likely due to the enhanced stability of the aptamer. General significance The activation parameters obtained by applying transition state analysis to kinetic data can provide details on mechanisms of molecular recognition and have applications in drug design. Single-molecule dissociation kinetics showed greater kinetic complexity than was observed in the ensemble in-solution systems, potentially reflecting conformational heterogeneity of the aptamer. This article is part of a Special Issue entitled: Physiological Enzymology and Protein Functions. [ABSTRACT FROM AUTHOR]

In an attempt to better understand the molecular mechanism of action of the antipsychotic lead LASSBio-579 and of its main metabolite LQFM 037, the aim of this work was to evaluate their intrinsic activity and binding kinetics at the dopamine D 2 receptor. In transfected HEK cells expressing the D 2L receptor under an inducible promoter, LASSBio-579 and LQFM 037, but not clozapine, behaved as weak partial agonists in [ 35 S]-GTPγS binding assays performed in optimized conditions previously shown to evidence the partial agonist profile of aripiprazole. Besides, data obtained in radioligand competition assays on rat striatal membranes suggested a rapid association to and dissociation from the D 2 -like receptors. Using the kinetic rate index based on the strategy of the dual-point competition association assay, we showed that our compounds share a similar kinetic profile with clozapine, distinct from the typical antipsychotic haloperidol. These two characteristics could contribute to the atypical-like profile observed after administration of LASSBio-579 to rodents, in models of positive and negative symptoms of schizophrenia. [ABSTRACT FROM AUTHOR]

Electron transfer dissociation of peptide ions with the diazirine-containing residue photomethionine (M*) results in side-chain dissociations by loss of CHN radicals in addition to standard backbone cleavages. The side-chain dissociations are particularly prominent upon activation of long-lived, charge-reduced, cation radicals (GM*GGR + 2H). Investigation of these cation radicals by near-UV photodissociation and collisional activation revealed different fragmentation products and mechanisms resulting from these ion activation modes. The dissociations observed for photomethionine were dramatically different from those previously reported for the lower homologue photoleucine; here, a difference by a single methylene group in the side chain had a large effect on the chemistries of the cation radicals upon ETD and further activation. ETD intermediates and products were probed by tandem 355-nm UV photodissociation-collision induced dissociation and found to contain chromophores that resulted from electron attachment to the diazirine ring. The nature of the newly formed chromophores and ion energetics and kinetics were investigated by electron structure calculations combining ab initio and density functional theory methods and Rice-Ramsperger-Kassel-Marcus (RRKM) theory. The dramatic difference between the dissociations of L* and M* containing peptide cation radicals is explained by electronic effects that play a role in stabilizing critical reaction intermediates and steer the dissociations into kinetically favored reaction channels. In addition, a new alternating UVPD-ETD-UVPD MS experiment is introduced and utilized for ion structure elucidation. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Imaging photoelectron photoion coincidence (iPEPICO) spectroscopy has been used to determine 0 K appearance energies for the unimolecular dissociation reactions of several energy selected straight chain alkyl iodide cations 1-C n H 2 n +1 I + → C n H 2 n +1 + + I , ( n = 3–7). The 0 K appearance energy of iodine atom loss, yielding in fact the 2-alkyl radical cation up to n = 6, was determined to be 9.836 ± 0.010, 9.752 ± 0.010, 9.721 ± 0.010, 9.684 ± 0.010 and 9.688 ± 0.015 eV in 1-C 3 H 7 I, 1-C 4 H 9 I, 1-C 5 H 11 I, 1-C 6 H 13 I, and 1-C 7 H 15 I, respectively. In 1-iodohexane and the smaller molecules, these correspond to the transition state along the 1-iodoalkane cation → 2-iodoalkane cation reaction path, and can be used in conjunction with isodesmic reaction energies to determine the reverse barriers to dissociative photoionization. The small kinetic shift is indicative of little H tunneling during isomerization. Directly computed reverse barriers show that run-of-the-mill computational approaches are of limited use when applied to open shell systems containing period 5 elements. Hindered rotors were found to play a minor role in the internal energy distribution and the dissociation rate constants. [ABSTRACT FROM AUTHOR]

The dissociation kinetics of selected trivalent lanthanide (Ln3+, Ln=La, Pr, Eu, Er, Lu) complexes of the macrocyclic ligand H2ODO2A (1-oxa-4,7,10-triazacyclododecane-4,10-diacetic acid), LnODO2A+, were studied in the [H+] range (0.1–2.4) × 10−4 M in the temperature range 15–45 °C. Excess Cu2+ ions were used as the scavenger for the ligand in acetate–acetic acid buffer medium. The dissociation reactions are independent of [Cu2+] and follow the rate law kobs = kd + kAC[Acetate] + K′klim[H+]/(1 + K′[H+]), where kd, kAC, and klim are the respective dissociation rate constants for the [H+]-independent, acetate-assisted, and the [H+]-dependent limiting pathways; K′ is the equilibrium constant for the protonation reaction LnODO2A+ + H+ ↔ LnODO2AH2+. The dissociation rates of LnODO2A+ complexes are all faster than those of the corresponding LnDO2A+ complexes (DO2A2− is the fully deprotonated dianion of the ligand H2DO2A, 1,4,7,10-tetrazacyclo-dodecane-1,7-diacetic acid), consistent with the notion that LnODO2A+ complexes are kinetically more labile and thermodynamically less stable than the corresponding LnDO2A+ complexes, and H2ODO2A is not pre-organized for Ln3+ ion complexation but H2DO2A is. The dissociation rates of LnODO2A+ complexes (ODO2A2− is the deprotonated dianion of the ligand H2ODO2A, 1-oxa-4,7,10-triazacyclododecane-4,10-diacetic acid) are all faster than those of the corresponding LnDO2A+ complexes (DO2A2− is the deprotonated dianion of the ligand H2DO2A, 1,4,7,10-tetraazacyclododecane-1,7- diacetic acid), consistent with the fact that LnODO2A+ complexes are kinetically more labile and thermodynamically less stable than the corresponding LnDO2A+ complexes, and H2ODO2A is not pre-organized for Ln3+ ion complexation but H2DO2A is. [ABSTRACT FROM AUTHOR]

The dissociation kinetics of selected trivalent lanthanide (Ln3+, Ln=La, Pr, Eu, Er, Lu) complexes of the macrocyclic ligand H2ODO2A (1-oxa-4,7,10-triazacyclododecane-4,10-diacetic acid), LnODO2A+, were studied in the [H+] range (0.1–2.4) × 10−4 M in the temperature range 15–45 °C. Excess Cu2+ions were used as the scavenger for the ligand in acetate–acetic acid buffer medium. The dissociation reactions are independent of [Cu2+] and follow the rate lawkobs = kd + kAC[Acetate] + K′klim[H+]/(1 + K′[H+]), wherekd,kAC, andklimare the respective dissociation rate constants for the [H+]-independent, acetate-assisted, and the [H+]-dependent limiting pathways; K′ is the equilibrium constant for the protonation reaction LnODO2A++ H+LnODO2AH2+. The dissociation rates of LnODO2A+complexes are all faster than those of the corresponding LnDO2A+complexes (DO2A2−is the fully deprotonated dianion of the ligand H2DO2A, 1,4,7,10-tetrazacyclo-dodecane-1,7-diacetic acid), consistent with the notion that LnODO2A+complexes are kinetically more labile and thermodynamically less stable than the corresponding LnDO2A+complexes, and H2ODO2A is not pre-organized for Ln3+ion complexation but H2DO2A is. The dissociation rates of LnODO2A+complexes (ODO2A2−is the deprotonated dianion of the ligand H2ODO2A, 1-oxa-4,7,10-triazacyclododecane-4,10-diacetic acid) are all faster than those of the corresponding LnDO2A+complexes (DO2A2−is the deprotonated dianion of the ligand H2DO2A, 1,4,7,10-tetraazacyclododecane-1,7- diacetic acid), consistent with the fact that LnODO2A+complexes are kinetically more labile and thermodynamically less stable than the corresponding LnDO2A+complexes, and H2ODO2A is not pre-organized for Ln3+ion complexation but H2DO2A is. [ABSTRACT FROM PUBLISHER]

Abstract: We consider a simple model for the geminate electron–hole separation process in organic photovoltaic cells, in order to illustrate the influence of dimensionality of conducting channels on the efficiency of the process. The Miller–Abrahams expression for the transition rates between nearest neighbor sites was used for simulating random walks of the electron in the Coulomb field of the hole. The non-equilibrium kinetic Monte Carlo simulation results qualitatively confirm the equilibrium estimations, although quantitatively the efficiency of the higher dimensional systems is less pronounced. The lifetime of the electron prior to recombination is approximately equal to the lifetime prior to dissociation. Their values indicate that electrons perform long stochastic walks before they are captured by the collector or recombined. The non-equilibrium free energy considerably differs from the equilibrium one. The efficiency of the separation process decreases with increasing the distance to the collector, and this decrease is considerably less pronounced for the three dimensional system. The simulation results are in good agreement with the extension of the continuum Onsager theory that accounts for the finite recombination rate at nonzero reaction radius and non-exponential kinetics of the charge separation process. [Copyright &y& Elsevier]

Context: Pterocarpus marsupium (PM) (Leguminosae), Eugenia jambolana (EJ) (Myrtaceae) and Gymnema sylvestre (GS) (Asclepiadaceae) are the most important medicinal plants in the Indian system of traditional medicine for the treatment of hyperglycemia. Objectives: Dipeptidyl peptidase-4 (DPP-4) inhibitors are the emerging class of anti-diabetic agents. However, only few compounds are commercially available. Therefore, in the present study we tried to explore the naturally occurring PM, EJ and GS semi-standardized extracts for their potential DPP-4 inhibition in vitro and in vivo. Materials and methods: DPP-4 inhibition was evaluated by in vitro inhibitory assay, and enzyme kinetics were calculated using one-phase exponential decay equation. Glucose load (2 g/kg) was administered to control and diabetic rats 30 min following extract administration (100, 200 and 400 mg/kg) orally once, and blood samples were withdrawn at 0, 0.5, 1, 1.5, 2 and 3 h to measure plasma active glucagon-like peptide-1 (GLP-1) levels. Results: PM and EJ inhibit DPP-4 potently with IC50 values of 273.73 ± 2.96 and 278.94 ± 6.73 µg/mL, respectively, compared to GS (773.22 ± 9.21 µg/mL). PM, EJ and GS exhibit long duration of action with enzyme inhibitory half-lives of 462.3, 317.2 and 153.8 min, respectively. Extracts significantly increase GLP-1 levels compared to negative control groups and peak GLP-1 level was observed at 2 h for PM and EJ, whereas for GS it was at 1.5 h Discussion and conclusion: Taken together, results suggest the extracts may have potent DPP-4 inhibitory action, and their hypoglycemic action attributed through an increase in plasma active GLP-1 levels. [ABSTRACT FROM AUTHOR]