Kinetic anomalies in chymotryptic hydrolyses of p-nitrophenyl acetate and N-benzoyl-L-alanine methyl ester

Kinetic and thermodynamic parameters were evaluated for the acylation and the deacylation steps in the hydrolysis of p-nitrophenyl acetate by alpha-chymotrypsin at pH 7.8 and at temperatures between 15 and 35 degrees C by the use of stopped-flow and ordinary ultraviolet spectrophotometers. In contrast to the temperature dependencies of k2 and Ks reported in the literature (P.A. Adams and E.R. Swart, Biochem. J., 161, 83 (1977], no kinetic anomaly was observed in either of the steps, but reasonable straight lines were obtained in both Arrhenius and van't Hoff plots. On the other hand, in the chymotryptic hydrolysis of N-benzoyl-L-alanine methyl ester a sharp kinetic anomaly was found. The discrepancy in the case of p-nitrophenyl acetate is discussed in connection with a possible conformational change of the enzyme, an alteration of the rate-limiting step or differences in the experimental procedures. The cause of the anomaly observed in the case of N-benzoyl-L-alanine methyl ester is also discussed in detail.     

Computer simulations of enantioselective ester hydrolyses catalyzed by Pseudomonas cepacia lipase

On the basis of the X-ray crystal structure of the lipase from Pseudomonas cepacia (PcL)-an enzyme representative for a whole family of Pseudomonas lipases (lipase PS, SAM-2, AK 10, and others with a high degree of homology with PcL)-a computational study was performed to rationalize both the enantioselectivity and substrate specificity (tolerance) displayed by this lipase in the enantioselective hydrolysis of racemic esters 1a-12a from various secondary aromatic alcohols. The major goal of this project was the development of a binding model for PcL which is able to rationalize the experimental findings to predict "a priori the enantioselective behavior of PcL toward a wider range of substrates. A two-step modeling procedure, namely, docking experiments followed by construction of tetrahedral intermediates, was used for the simulation of the involved enzyme-substrate recognition/hydrolysis processes. The study of the recognition process (docking experiments) led to unambiguous identification of the binding geometry for the two enantiomeric series of substrates, but did not suggest a definitive interpretation of the behavior of PcL. Taking into consideration the stereoelectronic requirements of the enzymatic hydrolysis reaction, both the enantioselectivity and tolerance of the enzyme were then explained through the study of the tetrahedral intermediates, in turn constructed from the calculated docking geometries of 1a-12a.     

Influence of ester and amide-containing surfactant headgroups on selectivity in micellar electrokinetic chromatography

The selectivity differences between six anionic surfactants in micellar electrokinetic chromatography (MEKC) are presented and the structural influence of the surfactant head-group is investigated. It was determined that the surfactant structure can have a significant impact on retention and selectivity. Linear solvation energy relationships (LSERs) are used to study the role of solute size, polarity/polarizability, and hydrogen bonding characteristics in determining retention and selectivity. While both the solute size and hydrogen bond accepting ability were found to be the most important factors in solute retention, the hydrogen bonding characteristic of the solutes have the largest influence on selectivity differences between surfactants.     

Silica gel supported phosphonium salts as micellar and phase transfer catalysts

Phosphonium salts immobilized on silica gel have been found to be both micellar and phase-transfer catalysts. They are able to exchange their parent anions with the anions present in an aqueous solution and promote, in catalytic amounts, the decarboxylation of 6-nitrobenzisoxazole-3-carboxylate to give 2-cyano-5-nitrophenolate, a test for micellar catalysis.     

Soft nanosystems based on hydroxypiperidinium surfactants as adjuvants and micellar catalysts

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A comparison of monocyclic and bicyclic phospholanes as acyl-transfer catalysts

The synthesis and evaluation of chiral phosphines 11, 15a, 19a, 24a, and 28a as nucleophilic catalysts for anhydride activation and kinetic resolution of alcohols is described. The relative reactivity follows the order 11a > 11b > 15a > 1 in the monocyclic series, and 24a > 19a > 28ain the bicyclic series, with an overall rate advantage of ca. 2 orders of magnitude for the bicyclic phospholanes over the monocyclic analogues. The increased reactivity of the bicyclic phospholanes for the acylation of alcohols is attributed to conformational effects and ground-state destabilization in a highly associative mechanism. Kinetic resolution data demonstrate promising enantioselectivities for 24a.     

Application of sucrose fatty acid ester to reverse micellar extraction of lysozyme

Reverse micellar extraction of lysozyme has been carried out using an organic solution containing a mixture of monoester and polyester of sucrose fatty acid ester. The forward extraction of lysozyme from the feed aqueous phase to the reverse micellar organic phase of the mixture of monoester and polyester of sucrose fatty acid ester at pH 7.2 was strongly dependent upon the weight fraction of monoester, while any amount of lysozyme was not extracted only by using monoester or polyester. The forward extraction ratio dramatically increased with an increase in the concentration of fatty acid ester, and was high around neutral pH and at low ionic strength. The backward extraction of lysozyme from the reverse micellar organic phase to the recovery aqueous phase exhibited high efficiency at acidic pH value or at high ionic strength. The addition of sucrose into the recovery aqueous phase promoted the backward extraction ratio, and caused the activity of lysozyme recovered from the reverse micellar phase to be retained perfectly.     

Influence of ester chain length,enzyme, and physical parameters on lipase-catalysed hydrolyses of meso-oxiranedimethanol esters. Part 2

A range of meso-oxiranedimethanol diesters were prepared and subjected to hydrolysis by a selection of microbial and porcine-derived lipases. The best chain length was then subjected to a wider range of lipases. The physical reaction parameters were further investigated for the best enzyme/substrate combination. The results revealed the effects of ester chain length, choice of enzyme, and physical reaction parameters on the enzymic hydrolysis. Using a combination of the these three variables, it is demonstrated that this is a versatile approach for the optimisation of an enzymic hydrolysis for a given substrate. Additionally, we also report on an efficient, multigram synthesis of oxiranedimethanol diesters from cheap achiral precursors, their successful enzymic hydrolyses to alcohols in good chemical yield and e.e., and their conversion to synthetically useful aldehydes in high yield. Both enantiomeric series can readily be accessed from a single enzymic hydrolysis result.     

Kinetics of metal ion complex formation in micellar media. A comparison between normal and polymerized micelles

Micellar media can be used to investigate the rate of complex formation between hydrophobic extractants and metal ions. Acceleration or retardation effects may be obtained, depending on factors such as the nature of the surfactant, the hydrophilic/lipophilic character and ionization state of the extractant, etc. The present work was aimed at studying the influence of some specific parameters in producing retardation effects, which have potential application for performing separation of metal ions on a kinetic basis. The stopped-flow technique was used to measure the rate of complex formation between Cu²⁺ and complexing agents with varying alkyl chain length. Micelles made of the nonionic surfactants C₁₂EO₆, Triton X-100 and Brij 35 were first considered, which did not show any direct correlation between the rate of complex formation and the thickness of the hydrophilic layer constituted by the polar heads of the surfactant molecules.In a second approach, we have used polymerized micelles obtained from undec-10-enyltrimethyl ammonium bromide, which we assumed to be more rigid than normal micelles. Although the absence of CMC was confirmed, the rates measured in these polymerized micelles were larger than those obtained in micellar solutions of CTAB and DTAB. The results are discussed in relation with the nature of the hydrophobic domains in the different situations.Institut Nancéien de Chimie Moléculaire (I.N.C.M.), FU CNRS n=8     

Zinc chloride homogeneous catalysis in the tritylation of hydroxyl- and amide-bearing molecules

A tritylation protocol based on the transfer of the triphenylmethylcarbenium ion from trityl acetate to substrates having hydroxyls, in the presence of catalytic amounts of ZnCl₂, is described. The advantages of this method are broad scope, mild conditions, and easy handling. The comparison with the procedure based on the use of equimolar mixture of TrCl and ZnCl₂ in the presence of TEA shows that comparable results are obtained. However, only this method allows reactions of secondary or benzylic alcohols such as oxidation or formation of symmetric ethers to be suppressed. Both procedures are successfully extended to simple and substituted amides. Irrespective of its low solubility in acetonitrile, even asparagine can be directly tritylated on its amide group.     

Synthesis of stearic acid triethanolamine ester over solid acid catalysts

<正>The synthesis of stearic acid triethanolamine ester over solid acid catalysts was investigated.The results showed that the catalytic activity and selectivity of zirconium sulfate supported on SBA-15(6)(pore diameter 6 nm) is better than that of commonly used hypophosphorous acid,zirconium sulfate supported on MCM-41 and zirconium sulfate supported on SBA-15(9)(pore diameter 9 nm).     

Forward and backward extractions of cytochrome c using reverse micellar system of sucrose fatty acid ester

The effects of solvent, pH, and ionic strength on the reverse micellar extraction of cytochrome c have been examined, when sucrose fatty acid esters were employed as surfactants of reverse micelles. The transparent and stable reverse micellar organic phase was formed, when the mixture of isooctane and n-butanol (7:3 v/v) was used as the bulk organic phase. The high forward extraction ratio was obtained under mild alkaline and low ionic strength conditions, while the high backward extraction ratio was obtained for acidic pH values or at high ionic strength. The activity of cytochrome c recovered from the reverse micellar phase was sufficiently retained.     

Dynamics of HCl collisions with hydroxyl- and methyl-terminated self-assembled monolayers

Molecular beam scattering techniques are used to explore the energy exchange and thermal accommodation efficiencies of HCl in collisions with long-chain OH- and CH(3)-terminated self-assembled monolayers (SAMs) on gold. Upon colliding with the nonpolar methyl-terminated SAM, HCl (E(i) = 85 kJ/mol) is found to transfer the majority, 83%, of its translational energy to the surface. The extensive energy loss for HCl helps to bring the molecules into thermal equilibrium with the monolayer. Specifically, 72% of the HCl approaches thermal equilibrium prior to desorption. For the molecules that do not thermally accommodate, but scatter after an impulsive collision with the surface, the final translational energy is observed to be directly proportional to the surface temperature as the thermal surface energy and gas translational energy exchange during the collision. For the OH-terminated SAM, the impulsively scattered HCl escapes from the surface with slightly more average energy. The rigid nature of the OH-terminated SAM is due to the extended intra-monolayer hydrogen-bonding network that restricts some of the low-energy modes of the surface. However, despite the rigid nature of this system, the extent of thermal accommodation for HCl on these two surfaces is remarkably similar. It appears that the potential energy well between the impinging HCl and the polar surface groups is sufficient enough to trap HCl molecules that would otherwise scatter impulsively from this rigid SAM.     

Synthesis,structures and electrochemical properties of hydroxyl- and pyridyl-functionalized diiron azadithiolate complexes

The hydroxyl- and pyridyl-functionalized diiron azadithiolate complexes [{(μ-SCH₂)₂N(CH₂CH₂OH)}Fe₂(CO)₆] (1) and [{(μ-SCH₂)₂N(CH₂CH₂OOCPy)}Fe₂(CO)₆] (Py = pyridyl) (2) were prepared as biomimetic models of the active site of Fe-only hydrogenases. Both complexes were characterized by MS, IR, ¹H NMR spectra and elemental analysis. The molecular structures of 1 and 2 were determined by single crystal X-ray analysis. A network is constructed by intermolecular H-bonds in the crystals of 1. An S?O intermolecular contact was found in the crystals of 2, which is scarcely found for organometallic complexes. Cyclic voltammograms of 1 and 2 were studied to evaluate their redox properties.     

Bulky diarylammonium arenesulfonates as mild and extremely active dehydrative ester condensation catalysts

More environmentally benign alternatives to current chemical processes, especially large-scale, fundamental reactions like ester condensations, are highly desirable for many reactions. Bulky diarylammonium pentafluorobenzenesulfonates and tosylates serve as extremely active dehydration catalysts for the ester condensation reaction of carboxylic acids with equimolar amounts of sterically demanding alcohols and acid-sensitive alcohols. Typically, the esterification reaction is performed in heptane by heating at 80 °C in the presence of 1 mol% of the catalyst without removing water. Esterification with primary alcohols proceeds without solvents even at room temperature. Furthermore, 4-(N-mesitylamino)polystyrene resin-bound pentafluorobenzenesulfonate can be recycled more than 10 times without a loss of activity.     

Extinction coefficients of hydroxyl- and deuteroxyl groups on silica and alumina

Summary Extinction coefficient of the hydroxyl groups on silica, and of deuteroxyl groups on silica and alumina were investigated, using FTIR spectroscopy. A desorption method was applied, i.e. the spectrum of the oxide was taken after a heating period, and an other one obtained after heating to a next higher temperature was subtracted. The amount of water desorbed during this temperature interval was determined from gas phase IR spectra.

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Extinktionskoeffizienten von Hydroxyl- und Deuteroxylgruppen an Siliciumdioxid und Aluminiumoxid

     

Solubility equilibria of solid solutions of hydroxyl- and chlor-apatite of arsenic

The solubility equilibria of arsenic hydroxylapatite, Ca₁₀(AsO₄)₆(OH)₂, chlorapatite, Ca₁₀(AsO₄)₆(Cl)₂, and their nine solid solutions were investigated at 37°C in the pH range, 4.6 to 7.8, in buffered systems through microanalytical determination of calcium, arsenic and chloride in their saturated solution. Stoichiometric dissolution of the samples was proved by the constancy of the ionic products of the apatite phase in the saturated solution.     

A comparison of ionic liquids to molecular organic solvents as additives for chiral separations in micellar electrokinetic chromatography

In this study, we report the effects of adding ionic liquids (ILs), as compared to adding conventional molecular organic solvents (MOSs), to aqueous buffer solutions containing molecular micelles in the separation of chiral analyte mixtures in micellar EKC (MEKC). The molecular micelle used in this study was polysodium oleyl-L-leucylvalinate (poly-L-SOLV). The ILs were 1-alkyl-3-methylimidazolium tetrafluoroborate, where the alkyl group was ethyl, butyl, hexyl, or octyl. These ILs were chosen due to their hydrophobicity, good solvating, and electrolyte properties. Thus, it was expected that these ILs would have favorable interactions with chiral analytes and not adversely affect the background current. Common CE buffers, mixed with a molecular micelle, and an IL or a MOS, were used for these chiral separations. The buffers containing an IL in the concentration range of 0.02-0.1 v/v were found to support a reasonable current when an electric field strength of 500 V/cm was applied across the capillary. However, a current break down was observed for the buffers containing more than 60% v/v MOS on application of the above-mentioned electric field. The chiral resolution and selectivity of the analytes were dependent on the concentration and type of IL or MOS used.