Molecular dynamics study of oxidized lipid bilayers in NaCl solution.

Polyunsaturated lipids are major targets of free radicals forming oxidized lipids through the lipid peroxidation process. Thus, oxidized lipids play a significant role in cell membrane damage. Using atomistic molecular dynamics (MD) simulations to investigate the dynamics of oxidized lipid bilayers, we examined the effects of NaCl on them. Lipid bilayer systems of 1-palmitoyl-2-linoleoyl-sn-glycero-3-phosphatidylcholine (PLPC) and 4 oxidation products, namely, 9-tc-hydroperoxide linoleic acid, 13-tc-hydroperoxide linoleic acid, 9-oxononanoic acid, and 12-oxo-9-didecadienoic acid in 0, 0.06, and 1 M NaCl solution were studied. These 51 systems, combined over 15 μs of total simulation time, show Cl(-) anions remaining in the water phase and Na(+) cations permeating into the headgroup region of the bilayer leading to membrane packing. The effects of NaCl on thickness and area per molecule were found to be independent of the concentration of oxidized lipids. NaCl disturbed the bilayers with aldehyde lipids more than those with peroxide lipids. The key finding is that oxidized lipids bend their polar tails toward the water interface. This behavior was monitored by following the time evolution of hydrogen bonds between the oxidized functional groups of different lipids, and the concomitant increase of hydrogen bonds between oxidized functional groups and water molecules. Our results also show that the number of hydrogen bonds should be considered as an equilibration parameter: Very long simulations are needed to equilibrate systems with high NaCl concentrations.