Temperature effects on excited state protolytic dissociation in aqueous solutions, micelles and liposomes: influence of phase transitions in lipid bilayers

Abstract

The kinetics of the excited state protolytic dissociation of 1- and 2-naphthol, hydroxy- and chloro-substituted naphthols (ArOH) in aqueous solutions, micelles and liposomes was studied as a function of temperature (over the range 283–353 K) by using steady state and time-resolved fluorescence measurements. The fluorescence data suggest kinetic non-equivalence of ArOH undergoing excited state protolytic dissociation in liposomes. Naphthol molecules in lipid membranes can be subdivided into two main fractions which have quite different reactivities, i.e. rate constants of proton transfer. In suspensions of dipalmitoylphosphatidylcholine (DPPC) liposomes the fluorescence spectra and kinetics were complicated by incomplete solubilization of ArOH in lipid membranes. The temperature dependences of the rate constants of 1- and 2-naphthol photodissociation in water, micelles and bilayer membranes of egg lecithin are described by the Arrhenius equation. The increase in activation energy and prefactor observed in surfactant assemblies as compared with aqueous solutions can be explained by an increase in the enthalpy and entropy of the protolytic equilibrium for weak acid dissociation in less polar media. An abrupt change in the dissociation rate constant was observed near the temperature of the main phase transition in DPPC liposomes.