The Kemp elimination in membrane mimetic reaction media. Probing catalytic properties of cationic vesicles formed from a double-tailed amphiphile and linear long-tailed alcohols or alkyl pyranosides

摘要

Vesicles formed from synthetic, double-tailed amphiphiles are often used as mimics for biological membranes. However, biological membranes are a complex mixture of various compounds. In the present paper we describe a first attempt to study the importance of additives on vesicular catalysis. The rate-determining deprotonation of 5-nitrobenzisoxazole (Kemp elimination) by hydroxide ion is efficiently catalysed by vesicles formed from dimethyldi-n-octadecylammonium chloride (C18C18+) as a result of (partial) dehydration of the reactants (especially the hydroxide ion) at the vesicular binding sites. Gradual addition of linear alcohols, such as n-decanol (C10OH), n-octadecanol (C18OH) and batyl alcohol (C(18)GlyOH) leads to a decrease in the observed catalysis. By contrast, gradual addition of oleyl alcohol, n-dodecyl-beta-glucoside (C(12)Glu) and n-dodecyl-beta-maltoside (C(12)Mal) leads to an increase in the observed catalysis. A detailed kinetic analysis, taking into account substrate binding site polarities, counterion binding percentages and binding affinity of the kinetic probe, suggests that the catalytic changes depend strongly on subtle changes in the structure of the additive. Whereas the C(12)Glu-induced effect can be explained by an increase in the vesicular rate constant, the effect of C(12)Mal can only be explained by an increase in the binding constant of the kinetic probe. However, for these pyranoside-containing vesicles others factors, such as a more extensive dehydration of the hydroxide ion, and micelle formation have to be considered. For the linear alcohols, besides a decrease in the counterion binding, changes in the vesicular rate constant and the binding constant should be taken into account. These two parameters change to a different extent for the different alcohols. The kinetic analysis is supported by differential scanning calorimetry (DSC), E-T(30) absorbance data and Nile Red, Laurdan, ANS and pyrene fluorescence measurements. The overall kinetic results are illustrative for the highly complex mix of factors which determines catalytic effects on reactions occurring in biological cell membranes.