PROPERTIES OF MIXED MONOLAYERS OF LECITHIN AND SPIN PROBE .2. ANALYSIS OF THE MOLECULAR-ORGANIZATION

This paper seeks to investigate the molecular organization of fatty acid spin label (4-palmitamido2,2,6,6-tetramethylpiperidine-1-oxyl) and dipalmitoylphosphatidylcholine molecules in monolayer vesicles. The molar ratio of the spin probe was varied from 1 to 22% and the corresponding electron spin resonance spectra were measured at 25 and 48°C. These spectra were analyzed in terms of the dipole-dipole and exchange interactions occurring between the radicals. From a computer simulation, a quantitative analysis of these interactions could be made. Comparison of these data with theoretical models indicate that above the lipid phase transition, i.e., at 48°C, the membrane is homogeneous and the label molecules diffuse randomly in the lecithin bulk phase (D = 3.8 x 10-8 cm2/sec). At 25°C, the membrane model can be viewed as a mosaic structure composed of spin probe clusters (7.9 x 1011 clusters/cm2). The size of the clusters as well as the minimal mean distance between two cluster edges could be calculated as a function of the spin label molar ratio. These results are used to interpret the observations made in the first paper of this series [Chatelain et al., J. Colloid Interface Sci. 72, 287 (1979)] concerning the surface viscosity of the mixed monolayer and the kinetics of the enzymatic activity of phospholipase A2. It can be concluded that above a critical spin label concentration (10-15%) the properties of the entire lecithin phase are influenced by the presence of the spin probe clusters. © 1979.