- Enantioselective cleavage of esters by histidine-containing tripeptides in micellar solutions of various hexadecyltrialkylammonium bromide surfactants
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Cleavage of chiral p-nitrophenyl esters derived from the amino acid phenylalanine by histidine-containing tripeptides has been studied in micellar solutions of four quaternary ammonium surfactants.Enzyme-like enantioselectivities up to kL/kD = 131 (at 0 deg C) are observed.The enantioselectivity can be rationalized by assuming a hydrophobically driven stabilizing hydrogen bond between the L enantiomer of the ester and the tripeptide in the transition state of the reaction.This hydrogen bond is absent in the reaction with the D enantiomer of the ester.The transition state has an amphipolar character and is stabilized by the micellar environment.The hydrophilic-hydrophobic balance of the reactants, which affects the transition state, was optimized by varying the composition of the tripeptide and the length of the N-protecting groups in the tripeptide and the substrate.The activities and enantioselectivities depend on the structure of the quaternary ammonium surfactant headgroup.Increasing the size of this headgroup leads to an increase in rate of hydrolysis of the L ester and hence to an increase in enantioselectivity.This effect is attributed to a change in the degree of ion-pair formation with a carboxylate group that is present in the peptides.Compared to previous studies the results indicate that a chiral surfactant is not required for obtaining high enantioselectivities.
- Cleij, Marco C.,Drenth, Wiendelt,Nolte, Roeland J. M.
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- Effect of Head-Group Size on Micellization and Phase Behavior in Quaternary Ammonium Surfactant Systems
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The micellization and phase equilibria in aqueous solutions of cationic quaternary ammonium bromide surfactants of the type alkyltrimethyl-, -ethyl-, -propy-, -butyl-, and -pentylammonium bromide have been investigated as a function of alkyl chain length, electrolyte concentration, and temperature.A large isotropic (micellar) phase is observed in triethyl- and tripropylammonium surfactants, which demixes into two conjugate phases on warming in the case of the alkyltributylammonium bromides.Liquid-liquid phase separation is also observed for tetradecyltripentylammonium bromide.Critical micelle concentrations are found to decrease with increasing hydrophobicity of the surfactant ion, but the free energy of micellization of tetradecyltributylammonium bromide is the same as that of tetradecyltrimethylammonium bromide over a range of temperatures.Some evidence is found for the formation of premicellar aggregates in tributylammonium surfactants at elevated temperatures, consistent with significant polydispersity and changing monomer concentrations observed previously.The results are discussed in terms of the hydrophobic and electrostatic contributions of the free energy of micellization, and some interferences are drawn regarding the nature of the interactions which give rise to phase separation.
- Buckingham, Scott A.,Garvey, Christopher J.,Warr, Gregory G.
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p. 10236 - 10244
(2007/10/02)
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- Micellar Enhancements of Rates of SN2 Reactions of Halide Ions. The Effect of Headgroup Size
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Observed first-order rate constants for reaction of methyl naphthalene-2-sulfonate with Cl- or Br- in micelles of cetyltrialkylammonium halide (C16H33NR3X, R = Me,Et,n-Pr,n-Bu, X=Cl,Br) increase monotonically with increasing surfactant and halide ion concentrations and tend to limiting values.The variation of rate constants with concentrations of surfactant and halide ions can be fitted to an equation that accounts for the distribution of both reactants between water and micelles.Fractional micellar ionization, α, increases, i.e., the affinity of the micelles for the halide ion decreases, with increasing bulk of the N-alkyl groups, but the second-order rate constants at the micellar surface and the overall rate constants increase.The rate effects of the micelles of the bromide ion surfactants can be understood in terms of NMR evidence on the micellar binding of the reactants and their location at the micellar surface.The reactivity increase with increasing headgroup bulk is related to the disruption of the hydration of Br- and to the electrostatic interaction of the naphthalene ?-system with cationic nitrogen.
- Bacaloglu, Radu,Bunton, Clifford A.,Ortega, Francisco
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p. 1497 - 1502
(2007/10/02)
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- Two-Phase Reaction of 1-Bromooctane with Sodium Acetate and Potassium Acetate Catalyzed by Diquaternary Ammonium Salts
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New unsaturated diquaternary ammonium salts (diquats) were prepared in two ways from ditertiary amines 3 and 1-bromoalkanes or from 1,4-dibromobut-2t-ene and tertiary monoamines.A comparison of their catalytic ability with that of saturated diquats and with that of corresponding monoquats was made.The reaction of n-C8H17Br with NaOAc or KOAc in the two phase system liquid-liquid without an additional solvent is catalyzed more effectively by the unsaturated diquats than by the saturated diquats and the monoquats.Most of the used quats catalyze the ester formation from NaOAc more effectively in the system liquid-liquid, however, the ester formation from KOAc more effectively in the system solid-liquid; KOAc is generally superior to NaOAc in both systems.Consequently, in the phase-transfer catalyzed ester formation from alkali acetate and alkyl halides not only the structure of the quat but also the inorganic cation of the acetate have influence on the rate of conversion.
- Schiefer, H.,Beger, J.,Lorenz, U.
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p. 383 - 398
(2007/10/02)
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- PHOSPHORORGANISCHE VERBINDUNGEN 95. Beschleunigte Hydrolyse von Organophosphorverbindungen durch Phasentransferkatalyse und H2O2 als Supernucleophil
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The alkaline hydrolyses of the physiologically active phosphor compounds: butyl-phenylfluorophosphonate 1 and di-isopropyl-fluorophosphate were studied under phase-transfer (PT) conditions.The influence of pH, the structure of the phase-transfer catalyst, the nature of the organic phase and the use of the supernucleophile HO2- on the hydrolysis was investigated.The hydrolysis of 1 is accelerated by a factor of ca. 550 (toluene) resp. ca. 520 (tetrachloroethylene) compared with the uncatalysed reaction.In concrete terms, the compound 1 can be completely hydrolysed in three (tuloene) resp. in one minute (tetrachloroethylene) under optimal conditions.
- Horner, Leopold,Kappa, H. Wilfried
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p. 339 - 348
(2007/10/02)
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