Studies of the Crystalline-Liquid Crystalline Phase Transition of Lipid Model Membranes. I. Use of Spin Labels and Optical Probes as Indicators of the Phase Transition

The crystalline-liquid crystalline phase transition of dipalmitoyllecithin model membranes with a transition temperature (T_t) of 41° was studied by optical methods and by the spin-labeling technique. Structural sensitive fluorescent and absorption probes were used to indicate changes in the arrangement of the lipid polar head groups of the membranes. The number of binding sites for these probes on the membrane surface increases by a factor of about 3 when the system is heated above the transition temperature. This effect is due to a lateral expansion of the membrane and to a loosening of the polar group structure at the transition temperature. The spin-labeling technique was used to study changes within the hydrocarbon phase of the lipid model membranes. An androstane derivative with a paramagnetic N-0 group was used as a steroid spin label. Several steroid-lecithin mixtures with a steroid content ranging from c = 0.01 to c = 0.27 (c = molar ratio steroid: lecithin) were investigated at temperatures between 18 and 55°. For low label concentrations (c < 0.01) clear triplet spectra are observed at all temperatures. The line width of these spectra decreases sharply when the system is heated above the phase transition, indicating a sharp increase in the tumbling motion of the label molecules. With increasing steroid concentration the shape of the esr spectrum is modified due to increasing magnetic interactions (dipolar and exchange interaction) between the label molecules. For T < T_t increasing label concentration produces a gradual broadening of the spectra until for c > 0.035 the spectra coalesce into broad one-line spectra. When the system is heated above T_t these broad spectra resolve into sharp triplet spectra. The changes in the esr spectra caused by the magnetic interaction between the steroid labels were used to study (1) the molecular organization of the steroid-lecithin system and (2) the influence of the lipid-phase transition on the structure of the mixed membranes. As a parameter which reflects the membrane structure we have evaluated the exchange frequency W_ex denoting the rate of spin exchange between the steroid molecules. With increasing temperature W_ex decreases sharply at the phase transition. This characteristic temperature dependence is in complete contrast to the behavior of paramagnetic radicals in organic solvents and suggests a sudden change in the internal structure of the mixed steroid-lecithin system at the lipid-phase transition. The details of these structural changes are discussed in two subsequent papers.