Kemp elimination in membrane mimetic reaction media: Probing catalytic properties of catanionic vesicles formed from double-tailed amphiphiles

The rate-determining deprotonation of 5-nitrobenzisoxazole (Kemp elimination) by hydroxide is efficiently catalyzed by vesicles formed from dimethyldioctadecylammonium chloride (C18C18+). Gradual addition of sodium didecyl phosphate (C10C10-) leads to the formation of catanionic vesicles, which were characterized by cryo-electron microscopy, and their main phase transition temperatures (DSC) and xi-potentials. Increasing percentages of C10C10- in the vesicular bilayers decrease the catalysis of the Kemp elimination. A detailed kinetic analysis, supported by consideration of substrate binding site polarities and counterion binding percentages, suggest that the catalytic effects of C18C18+/C10C10- catanionic vesicles are primarily determined by the binding of catalytically active hydroxide ions to the vesicular surface area. The formation of neutral microdomains between 10 and 30 mol% of C10C10- in the bilayer, as revealed by DSC, is not apparent from the catalytic effects found for these vesicles. Interestingly, the catalytic effects observed for 50 mol% C10C10- in the catanionic vesicles indicate an asymmetric distribution Of C18C18+ and C10C10- over the bilayer leaflets. The overall kinetic results illustrate the highly complex mix of factors which determines catalytic effects on reactions occurring in biological cell membranes.