Effects of surfactant charge and structure on excited-state protolytic dissociation of 1-naphthol in vesicles

Abstract

The kinetics of protolytic photodissociation of 1-naphthol in the bilayer membrane of cationic vesicles of didodecyldimethylammonium bromide (DDAB) and dioctadecyldimethylammonium bromide (DOAB) have been studied in comparison with the same reaction in vesicles of a zwitterionic lipid dipalmitoylphosphatidylcholine (DPPC). Evidence for the existence of two fractions (two types of site) of 1-naphthol molecules in vesicles of cationic surfactants which differ strongly in their rate constants for excited-state proton transfer was found, similar to the case for zwitterionic vesicles. The rate constants of the excited-state proton transfer for both fractions are much higher in bilayer membranes of cationic surfactants than for zwitterionic lipids (DPPC and egg lecithin). The activation enthalpy of excited-state proton transfer (ESPT) for both fractions of ArOH in the membrane of DDAB is ca. 40 kJ mol–1, which is much higher than in homogeneous solutions and zwitterionic surfactants. Fluorescence kinetic data for DOAB vesicles allow no reliable conclusions to be drawn as to the temperature dependence of excited-state protolytic dissociation rate constants in these vesicles because the reaction rate is too fast. No significant decrease in the excited-state proton-transfer rate constants at the membrane phase-transition temperature of vesicles of cationic surfactants is observed, in contrast to the zwitterionic lipids. All these features characterize distinctions between the properties of the membranes of the vesicles of cationic and zwitterionic surfactants in proton-transfer reactions.