Mt. Ge et Jh. Freed, AN ELECTRON-SPIN-RESONANCE STUDY OF INTERACTIONS BETWEEN GRAMICIDIN-A' AND PHOSPHATIDYLCHOLINE BILAYERS, Biophysical journal, 65(5), 1993, pp. 2106-2123
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.