ESR and DSC comparative study of phase separation in γ-irradiated lipid membranes

ESR and DSC techniques have been applied to the study of aqueous dispersions of dipalmitoyl phosphatidylcholine multilayers both unirradiated and γ-irradiated, with TEMPO spin-label added to them. The examen of ESR spectra gave information on the molar fraction (f) of TEMPO bound to the lipid phase. While for the unirradiated samples one observes a sharp variation offat the main transition temperature, in the case of irradiated samples the speed of variation is greatly reduced, and other significant details do appear in thefvs.Tcurves. Moreover, always around the transition zone, the irradiation products, when in low concentration, show quite complete miscibility in the lipid bilayers. When their concentration is sufficiently higher, due to a greater irradiation dose, the products of irradiation lead to the formation of a domain-like organization of precipitates, whose composition and fluidity depend on the actual absorbed dose as well as on the temperature.