Studies of the Crystalline-Liquid Crystalline Phase Transition of Lipid Model Membranes. II. Analysis of Electron Spin Resonance Spectra of Steroid Labels Incorporated into Lipid Membranes

Esr spectra of androstane spin labels incorporated into dipalmitoyllecithin model membranes were measured as a function of temperature and label concentration in part I of this series. These spectra are analyzed in the present paper by computer simulation. The spectra measured at low label concentration (negligible radical interaction) can be simulated by a superposition of three Lorentzian lines of different line widths. The crystalline-liquid crystalline phase transition of the lipid material is indicated by a sharp decrease of the line width with increasing temperature. It was shown in part I that increasing label concentration produces broadened esr spectra. These spectra can be analyzed in terms of magnetic interactions between the radicals (dipole-dipole and spin-exchange interaction). Three methods were used to simulate the spectra. First we took into account the exchange interaction and second the dipole-dipole interaction; in a third approach these two effects were combined. It is demonstrated by the computer analysis that the line broadening observed with increasing label concentration is determined mainly by the exchange interaction which is characterized by the rate of exchange W_ex between the radicals. The lipid-phase transition produces a sharp decrease of W_ex with increasing temperature at about 35°. This temperature dependence is in complete contrast to the behavior of free radicals in solution and suggests a structural change of the steroid-lecithin system at the phase transition. Different functional dependencies of W_ex on the molar ratio c label: lipid are observed at temperatures below and above the phase transition. Above T_t the exchange frequency W_ex increases linearly in c, whereas below T_t the exchange frequency is a linear function of which is proportional to the average label separation d_1a. An explanation for these findings is given in an accompanying paper, part III of this series.