Chemical Differentiation of Bilayer Surfaces in Functional Dialkylammonium Ion Vesicles: Observation of Surfactant Flip-Flop

摘要

The controlled generation of dissymmetrically surface-functionalized, synthetic bilayer vesicles of liposomes is an essential step in the successful modeling of biological membranes. Cationic p-nitrophenyl carbonate and p-nitrophenyl benzoate functionalized di-n-octadecylmethylammonium ion surfactants 6 and 7 were synthesized. Vesicles of 6 or covesicles of (1:9) 7 and 5, created at pH 3.9, gave rapid, partial p-nitrophenylate cleavage at Ph 7.9-8.0 (from 6) or rapid, partial benzoate cleavage by external thiolate ions at pH 7.9-8.0 (from 7), attributed to surface-specific exovesicular reactions of 6 or 7.5. These exovesicular cleavages at ph 8 and 25 C are apparently faster than reagent permeation across the bilayers to the endovesicular functional groups at pH 3.9. The dioctadecylammonium ion vesicles, in contradistinction to their dihexadecyl analogues, are able to maintain the indicated pH gradient long enough at 25 C to permit the surface-specific esterolyses. Relaxation of the pH gradient and endovesicular cleavages follow upon enhancement of the fluidity of the vesicle bilayers either with the application of heat or with additives such as 1-hexanol or dioctyldimethylammonium chloride. In the surface-differentiated 7/5 covesicles, flip-flop of intact 7 from endovesicular to exoversicular sites can be promoted and visualized by experiments that involve incubation of the vesicles at 38-40 C, pH 3.9, for 1-12 min. Reprints. (AW)