Catalysis vesicular

Most of the characteristics invoked to explain rate accelerations and rate retardations by micelles are valid for vesicles as well. For example, the alkaline hydrolysis of A-methyl-A-nitroso-p-toluenesulfonamide is accelerated by cationic vesicles (dioctade-cyldimethylammonium chloride). This rate acceleration is the result of a higher local OH concentration which more than compensates for the decreased polarity of the vesicular pseudophase (compared to both water and micelles) resulting in a lower local second-order rate constant. Similar to effects found for micelles, the partial dehydration of OH and the lower local polarity are considered to contribute significantly to the catalysis of the Kemp elimination " by DODAB vesicles. Even the different... 

The effect of additives betrays the intricacy of the balance of rate effects even more. The addition of cholesterol to catalytic bilayers has been found to be beneficial for the Kemp eleminiation but to inhibit the decarboxylation of 6-NBIC. In general, the effects of additives on the decarboxylation of 6-NBIC appear to subtly depend on the structure of the hydrophobic tail and hydrophilic headgroup of additives. Similarly subtle effects were found for the Kemp elimination and nucleophilic attack by Br and water on aromatic alkylsulfonates depending on the choice of additive, hydrogen bonding effects, reactivity of partially dehydrated OH , and local water concentrations all played a role and vesicular catalysis could be increased or decreased. 

It is tempting to speculate about the reasons for the observation that surfactant aggregates often do not appear to be as effective as hoped. In the author s opinion, the reasons for this could well be (1) the choice of reactions and (2) the way in which reaction rates are compared. Starting with the first point, it appears as if micellar and vesicular catalysis is often studied for reactions for which water is intrinsically a good solvent, that is, a better solvent than less polar organic solvents. By using the less polar pseudophase formed by surfactant aggregates as a base for catalysis, part of... 

Having successfully accelerated the reversible isomerization between the aldimine and ketimine Schiff bases, Murakami et al. then studied how to obtain turnovers in the full transamination reaction between one amino acid and one keto acid [25]. They found that the bilayer vesicle system constituted with 33, 36, and Cu(n) ions showed some turnovers for the transamination between L-phenylalanine and pyruvic add. However, such turnover behavior was not observed in a vesicular system composed of 32, 36, and Cu(n) ions, and an aqueous system involving N-methylpyridoxal and Cu(n) ions without amphiphiles. Therefore, both the hydrophobic effect and the imidazole catalysis effect were proposed as important for the turnover behavior. 

Akinetic study was also performed in a variety of vesicular solutions (DDAB, DODAB, DODAC [NaOH] = 2.25mM, 25 °C). Interestingly, the vesicles possess stronger catalytic reaction environments than the micelles. The rate-determining proton transfer from carbon to the hydroxide ion was accelerated up to 850 fold in di- -dodecyldimethylam-monium bromide (DDAB) vesicles. This is evidence that the reaction sides are less aqueous than those in micelles, as anticipated. Application of the pseudophase model afforded the bimolecular rate constants in the vesicles (kves). For the different vesicles, ves is significantly higher (ca. 12 times for DODAB) than the second-order rate constant in water. This shows that the catalysis is due to both a medium effect and a concentration effect. It was assumed that there was a fast equilibrium for substrate binding to the inner and outer leaflets of the bilayer, in accord with the fact that no two-phase kinetics were found. 

M. G. M. Jongejan, J. E. Klijn, J. B. F. N. Engberts, Vesicular catalysis of the decarboxylation of 6-nitrobenzisoxazole-3-carboxylate. The effects of sugars, long-tailed sugars, cholesterol and alcohol additives, J. Phys. Org. Chem., 2006, 19, 249-256. 

Key words Vesicles - reaction sites in vesicles - kinetic analysis in vesicles - vesicular catalysis - reaction rate -control with vesicles... 

Vesicular catalysis, reagent binding and kinetic analysis... 

Alkaline hydrolysis of ethyl caprylate (itself insoluble in water) yields sodium caprylate, initially at a very slow rate bnt as soon as sufficient caprylate was formed for aggregation into micelles to take place, the authors observed an exponential increase in reaction rate owing to micellar catalysis. These self-assembling surfactant strucmres may consequently provide a model system for studies of pre-biotic chemistry. The possible relevance of this process to prebiotic chemistry was emphasized by their observation that the micelles can be converted into more robust vesicles by a pH change induced by dissolved CO2, and latter on, Luisi extended this approach to vesicular systems (see Section 3.3). Kinetic models for this kind of autocatalytic dynamic systems were also developed in the literature." ... 

It was natural that extensive attempts were made to further increase reaction rates in aqueous media by applying Lewis acid catalysis and, more specifically, (transition) metal ion and rare earth metal ion catalysis. Also combinations of Lewis acid catalysis with micellar or vesicular catalysis were sometimes found to be highly effective. And, of course, the possibility of stereospecific catalytic processes in water was examined in detail. ... 

Furthermore, using the unique structures of amphiphilic pillararenes and particular properties of their assemblies, more investigation should be carried out to explore their potential applications in biomedicine, liquid crystals and catalysis. It is expected that further research based on pillararenes and their micellar, vesicular and tubular assemblies will boost this emerging area to a new level. ... 

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