Studies of the Crystalline-Liquid Crystalline Phase Transition of Lipid Model Membranes. III. Structure of a Steroid-Lecithin System below and above the Lipid-Phase Transition

Theoretical models for the organization of mixed steroid-lecithin membranes are developed and checked against electron spin resonance measurements. In part II of this series we had determined values of the exchange interaction W_ex between androstane spin labels within dipalmitoyllecithin model membranes by computer analysis of esr spectra. Comparison of such data with theoretical considerations permits one to determine the molecular organization of mixed membranes. A good criterion for the validity of theoretical models is provided by comparison with the measured dependence of W_ex on the molar ratio steroid:lecithin, c, and the temperature. The most important result is the observation that the organization of androstane-lecithin membranes depends critically upon the crystalline-liquid crystalline phase transition of the lecithin. Different functional dependencies of W_ex on the steroid concentration (c) are observed at temperatures below and above the transition point (T_t). Above T_t the steroid and lecithin molecules form an ideal mixture. The steroid molecules, and most probably also the lipid molecules, undergo translational diffusion within the plane of the membrane. The diffusion coefficient is determined as D_diff = 10^-8 cm²/sec, corresponding to an average travel distance of about 10,000 Å in 1 sec. Below the phase transition the lipid matrix is less fluid. Substantial translational diffusion within the plane of the membrane is not possible. Also the tumbling motion of the molecules is less rapid (v ≈ 10⁸ Hz). Below T_t the organization of the mixed system can be described as a mosaic structure with very small steroid clusters embedded within the lipid matrix. The concentration of the clusters, their size and shape, and the number of steroid molecules per cluster can be determined from the experimental data. The density n of clusters is independent of the molar ratio steroid:lipid (c). For T = 19°, n = 3.36 × 10¹¹/cm². The clusters increase with increasingsteroid concentration. For c = 0.035 the cluster radius is r = 30 Å, the average distance between the clusters is 285 Å, and about 65 steroid molecules are assembled per cluster. The thermal transition between the two organization forms is reversible. Upon heating the system above the phase transition the clusters dissolve and a homogeneous mixture is formed. The phase transition produces an expansion of the lipid lattice. The value of this expansion and the temperature dependence of the lattice constant, a, can be determined from the temperature dependence of the esr spectra. The possible importance of our results for the structure and function of biological membranes is discussed.