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

Abstract

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.

Investigating Magnetically Aligned Phospholipid Bilayers with EPR Spectroscopy at 94 GHz

“…We attribute the increase to an enlargement in the amplitude or rate of the random walk motion (particularly R ⊥ ) and a decrease in the overall alignment of the oriented bilayer discs caused by the increase in temperature. Previous solid-state NMR studies of lanthanide-doped bicelles indicate that phase instability can occur at temperatures greater than 50• C, which would lead to a decrease in bicelle alignment (36)(37)(38). This is also justified by the least-squares simulations shown in Fig.…”

Application of the out-of-phase absorption mode to separating overlapping EPR signals with different T1 values

“…The more motionally restricted component (b) is usually assigned to the boundary lipids contacting the GA aggregates [22][23][24][25], in analogy with similar studies on integral transmembrane proteins [26]. The weakly immobilized component (f) corresponds to the fluid (free) lipids, although the EPR spectra of the latter indicate considerably lower rotational mobility as compared with membranes of fluid lipids alone.…”

“…In previous studies [22][23][24][25], mostly on membranes with rather high gramicidin A contents (GA/lipid P 0.1 mol/mol), it was found that the conventional low-power EPR absorption spectra of phospholipids spin-labelled close to the terminal methyl group of the sn-2 chain contain two components. The more motionally restricted component (b) is usually assigned to the boundary lipids contacting the GA aggregates [22][23][24][25], in analogy with similar studies on integral transmembrane proteins [26].…”

Organization of lipids in fiber-cell plasma membranes of the eye lens

“…Esser and Lanyi [65], investigating properties of the lipid phase in cell envelope vesicles from Halobacterium cutirubrum , were the first to note, that membrane integral proteins can decrease hydrophobicity of the membrane. Freed’s group showed that gramicidin A significantly decreased hydrophobicity of PL membranes [66, 67]. Based on these works Ge and Freed [67] suggested that it might be a general rule that integral membrane proteins and membrane soluble peptides decrease the hydrophobicity of membrane interior.…”

“…Freed’s group showed that gramicidin A significantly decreased hydrophobicity of PL membranes [66, 67]. Based on these works Ge and Freed [67] suggested that it might be a general rule that integral membrane proteins and membrane soluble peptides decrease the hydrophobicity of membrane interior. Later works showed, however, that transmembrane peptides affect membrane hydrophobicity differently, depending on their structure and amino acid composition.…”