AN ELECTRON-SPIN-RESONANCE STUDY OF INTERACTIONS BETWEEN GRAMICIDIN-A' AND PHOSPHATIDYLCHOLINE BILAYERS

The model of microscopic order and macroscopic disorder was used to si mulate electron spin resonance spectra of spin-labeled lipids, 5-PC, 1 0-PC, and 16-PC in multilamellar vesicles of dipalmitoylphosphatidylch oline (DPPC) containing gramicidin A' (GA) at temperatures above the g el-to-liquid crystal transition of DPPC. The simulations show that at a lower concentration of GA (i.e., molar ratios of DPPC/GA greater tha n 3), GA has only a slight effect on the acyl chain dynamics. The rota tional 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 co nsistent with the well-known concept of ''boundary lipids,'' that is, the peptide induces structural inhomogeneity in lipid bilayers. Howeve r, the spectra were significantly better fit with additional component s 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 e nd of the acyl chains, where the average local order parameter S(t)BAR 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 wit h increasing temperature. The hyperfine tensor component A(zz) increas es for 10-PC and 16-PC when GA is incorporated into DPPC bilayers, ind icating that water has deeply penetrated into the DPPC bilayers. Simul ations of published electron spin resonance spectra of 14-PC in dimyri stoylphosphatidylcholine/cytochrome oxidase complexes were also better fit by additional components that were more ordered, rather than immo bilized. The average local order parameter in this case is found to in crease from 0.11 for pure dimyristoylphosphatidylcholine to 0.61 for a lipid/protein ratio of 50. These spectra and their simulations are si milar 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.