Phase transitions in phospholipid vesicles : Excited state prototropism of 1-naphthol as a novel probe concept

1-Naphthol (1-ROH), an excited state acid, undergoes partial dissociation in the excited state in liposome membrane, giving two fluorescence peaks as against a single peak in water. 1-Naphthol distributes between two different sites of different excited state reactivity in liposome, and the relative population in the two different sites changes with any perturbations in the membrane fluidity. This property is made use of in studying the thermotropic phase changes and cholesterol effect on the transition temperature of DMPC (dimyristoylphosphatidylcholine) and DPPC (dipalmitoylphosphatidylchloline) liposomes. The change in the permeability of the membrane due to thermotropic phase changes and the phase transition temperature of liposomes could be easily monitored by fluorescence spectroscopy from a conspicuous change in the emission of the neutral form of 1-naphthol. The applicability of 1-naphthol probe to mixed lipid sytems has also been examined.