An electron spin resonance study of interactions between gramicidin A' and phosphatidylcholine bilayers

The model of microscopic order and macroscopic disorder was used to stimulate electron spin resonance spectra of spin-labeled lipids, 5-PC, 10-PC, and 16-PC in multilamellar vesicles of dipalmitoylphosphatidylcholine (DPPC) containing gramicidin A' (GA) at temperatures above the gel-to-liquid crystal transition of DPPC. The simulations show that at a lower concentration of GA (i.e., molar ratios of DPPC/GA greater than 3), GA has only a slight effect on the acyl chain dynamics. The rotational diffusion rate around the axis parallel to the long hydrocarbon chain remains unchanged or increases slightly, while the rate around the perpendicular axes decreases slightly. These spectra from DPPC/GA mixtures could only be fit successfully with two or more components consistent with the well-known concept of "boundary lipids," that is, the peptide induces structural inhomogeneity in lipid bilayers. However, the spectra were significantly better fit with additional components that exhibit increased local ordering, implying decreased amplitude of rotational motion, rather than immobilized components with sharply a reduced rotational rate. The largest relative effects occur at the end of the acyl chains, where the average local order parameter St of 16-PC increases from 0.06 for pure lipid to 0.66 for 1:1 DPPC/GA. The inhomogeneity in ordering in DPPC bilayers due to GA decreases with increasing temperature. The hyperfine tensor component Azz increases for 10-PC and 16-PC when GA is incorporated into DPPC bilayers, indicating that water has deeply penetrated into the DPPC bilayers. Simulations of published electron spin resonance spectra of 14-PC in dimyristoylphosphatidylcholine/cytochrome oxidase complexes were also better fit by additional components that were more ordered, rather than immobilized. The average local order parameter in this case is found to increase from 0.11 for pure dimyristoylphosphatidylcholine to 0.61 for a lipid/protein ratio of 50. These spectra and their simulations are similar to the results obtained with 16-PC in the DPPC/GA mixtures. The relevance to studies of lipid-protein interactions for other proteins is briefly discussed.