EFFECTS OF SURFACTANT CHARGE AND STRUCTURE ON EXCITED-STATE PROTOLYTIC DISSOCIATION OF 1-NAPHTHOL IN VESICLES

The kinetics of protolytic photodissociation of 1-naphthol in the bila yer membrane of cationic vesicles of didodecyldimethylammonium bromide (DDAB) and dioctadecyldimethylammonium bromide (DOAB) have been studi ed in comparison with the same reaction in vesicles of a zwitterionic lipid dipalmitoylphosphatidylcholine (DPPC). Evidence for the existenc e of two fractions (two types of site) of 1-naphthol molecules in vesi cles of cationic surfactants which differ strongly in their rate const ants for excited-state proton transfer was found, similar to the case for zwitterionic vesicles. The rate constants of the excited-state pro ton transfer for both fractions are much higher in bilayer membranes o f cationic surfactants than for zwitterionic lipids (DPPC and egg leci thin). 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 zwitterioni c surfactants. Fluorescence kinetic data for DOAB vesicles allow no re liable conclusions to be drawn as to the temperature dependence of exc ited-state protolytic dissociation rate constants in these vesicles be cause the reaction rate is too fast. No significant decrease in the ex cited-state proton-transfer rate constants at the membrane phase-trans ition temperature of vesicles of cationic surfactants is observed, in contrast to the zwitterionic lipids. All these features characterize d istinctions between the properties of the membranes of the vesicles of cationic and zwitterionic surfactants in proton-transfer reactions.