Model membrane studies of spin-label probes. Part 1. Mixed monolayers of 12-nitroxide stearic acid and myristic acid

Abstract

Pure and mixed monomolecular films of a cell membrane spin label probe, 12-nitroxide stearic acid have been studied where myristic acid was selected as the host lipid. The behavior of 12-nitroxide stearic acid at the air water interface is understood in terms of two molecular configurations: erect (with only the carboxyl group in the interface) and bent (with both the carboxyl group and the oxazolidine ring in the interface). In mixed films both of these conformations play a role at high surface pressures. At low probe concentrations, 12-nitroxide stearic acid is primarily in an erect conformation, while at high probe concentrations the reverse is true. This particular host lipid appears capable of erecting the probe molecule with only small concentrations of myristic acid. In a condensed host lipid, the probe is partially immiscible, and segregates to form a heterogeneous film from which it is readily collapsed. The probe is seen to perturb the molecular packing in this mixed system and the perturbation to be dependent on both the molecular shape and nature of the probe.