Effect of oxidative stress on membrane structure: small-angle X-ray diffraction analysis.

Free radical damage to cellular membranes appears to underlie alterations in function in aging and various pathological processes, including cardiovascular disease. The objective of this study was to directly characterize changes in the molecular structure of membrane lipid bilayers resulting from oxidative stress. Membrane samples reconstituted from either synthetic or cardiac phospholipids enriched with polyunsaturated fatty acids were examined at high resolution using small-angle x-ray diffraction methods. In this study, Fe2+/ascorbate-induced lipid peroxidation produced significant and dose-dependent alterations in the basic physical structure of the phospholipid bilayer. Electron density profiles (A vs. electrons/A3) calculated from the x-ray diffraction data showed a marked reduction in the hydrocarbon core width of dilinoleoyl phosphatidylcholine (DLPC) bilayers from 36 A to 32 A, and a decrease in overall membrane width, including surface hydration, from 48.7 A to 44.6 A. In addition, a broad decrease in molecular volume was observed +/-3-10 A from the center of the membrane bilayer, along with interdigitation of the terminal methyl segments. Pronounced changes in the lipid bilayer structure following oxidative stress were also observed in membranes reconstituted from cardiac lipids, including a 4 A reduction in hydrocarbon core width from 40 A to 36 A and interdigitation of the terminal methyl segments. These data provide direct evidence for changes in membrane hydrocarbon core width and molecular volume resulting from phospholipid peroxidation, which may contribute to perturbations in membrane structure/function relationships associated with aging and cardiovascular disease.