New pH-sensitive vesicles. Release control of trapped materials from the inner aqueous phase of vesicles made from triple-chain amphiphiles bearing two carboxylate groups

The pH-induced release of trapped materials from the inner aqueous phase of vesicles made from the synthetic triple-chain amphiphile 1 bearing two car boxyl groups was investigated. While a particular pa-dependence of the rele ase of trapped materials from the inside of vesicles made from distearoylph osphatidylcholine (DSPC) was not observed, vesicles made from compound 1 sh owed unique pH-sensitive character toward the pH of the outer bulk phase as follows: it released less than 10% of 5(6)-carboxyfluorescein (CF) as a wa ter-soluble marker at pH 7.5 after 1 year. But the vesicular assemblies wer e broken below pH 3.5-4.0, resulting in the liberation of the trapped CF in to the bulk phase. The latter result is associated with a full-protonation of the carboxylate groups of compound I. In the case of vesicles made from a mixture of DSPC and compound 1, the addition of cholesterol into the vesi cle membranes was effective in providing clear pH-sensitive character for t his type of vesicle. The zeta potentials of a series of vesicles containing compound 1 at various pH synchronized well with the pK(a1) value of vesicl es made from compound 1 and the pH-dependence of the release of trapped CF from these vesicles.