Vesicular catalysis of the decarboxylation of 6-nitrobenzisoxazole-3-carboxylate. The effects of sugars, long-tailed sugars, cholesterol and alcohol additives

The effects of the addition of sugars, long-tailed n-alkyl pyranosides, n-alkyl glycerol ethers and nalcohols on the properties of di-n-hexadecyldimethylammonium bromide (DHAB) vesicles have been studied. Properties that were examined include the stability, morphology, phase of the tails, and catalytic rate acceleration of the unimolecular decarboxylation of the 6-nitrobenzisoxazole-3-carboxylate anion (6-NBIC). The kinetic data were analyzed on the basis of the pseudophase model and show a rate acceleration of a factor of about 1000 relative to the reaction in water. Upon addition of most additives an inhibiting effect on the decarboxylation reaction of 6-NBIC is observed relative to the reaction in vesicles without any additive. The largest inhibition was observed in the case of cholesterol. Contrastingly, n-dodecyl-β-maltoside (in the spherical vesicle region) and trehalose accelerate the reaction. The activation parameters show that the most significant contribution to the Gibbs energy of activation is the enthalpic factor, with a partly compensating entropic contribution.