5-PC as a Lipid Probe Molecule and as a Second Phospholipid in Binary Phospholipid Mixtures: Saturation Recovery EPR Studies.

Mixtures of two phospholipids (PLs) with different main phase transition temperatures were investigated. Host PLs (HPLs) were represented by DMPC, DPPC, DSPC, and DMPE. The admixed PL was the spin-labeled phosphatidylcholine 5-PC(1-palmitoyl-2-(5-doxylstearoyl)phosphatidylcholine), with a unique opportunity to monitor the properties and the local environments of all admixed PL molecules using saturation recovery EPR methods. Below the HPL phase transition temperatures, 5-PC mixes with HPL to form two distinct pools with different rotational diffusion rates. The fluidity of the local environment in these two pools is very different, being more fluid for molecules with greater rotational diffusion rates. Above the HPL phase transition temperature, 5-PC mixes with HPL uniformly. This is independent of the HPL, observed for 5-PC concentrations from 0.25 mol% up to 20 mol% and for the wide temperature range. Assuminga very low concentration of 5-PC is an ideal probe molecule, we can conclude that small fluid phase domains made of HPL molecules are formed below the phase transition temperature of the HPL bilayers. In binary mixtures of HPLs with 5-PC, below the phase transition of HPL bilayers, fluid phase domains are created within the bulk gel phase of HPL lipids by the admixed second PL, namely 5-PC.