QCM-D and molecular dynamics: Combined study of oxidized phosphatidylcholines in model membranes

Oxidized phosphatidylcholines (oxPC) in cell membranes are biomarkers for oxidative stress-related cardiovascular diseases that are among the top causes of global mortality. Under oxidative stress, phospholipid acyl tails interact with reactive oxygen species to yield products with truncated tails containing polar functional groups. Previous research shows that oxPC differ in conformation to their non-oxidized counterparts, and that lipid bilayers containing oxPC have altered structure and functionality. The effects of a few oxPC have been characterized, but many species remain. Here, we combined biophysical and computational methods to study the effects of KDdiA-PC and KOdiA-PC, in model membranes. These two oxPCs are linked to atherosclerosis and inflammatory immune responses. We show using quartz crystal microbalance with dissipation monitoring (QCM-D) that supported lipid bilayers (SLBs) with significant concentrations of KOdiA-PC or KDdiA-PC contain less mass than SLBs formed without oxPC. Additionally, increasing the concentration of these two oxPCs in vesicles shifts SLB formation from a two-step accumulation-rupture process to a one-step rupture-on-contact process, which agrees with previous studies probing other oxPC species. Using molecular dynamics simulations, we also show trends of increasing area-per-molecule and decreasing membrane thickness in simulations with increasing concentrations of KOdiA-PC and KDdiA-PC. We also show that protonation of the carboxylic acids on KOdiA-PC and KDdiA-PC significantly alter their tailgroup conformations in bilayer membranes. We attribute these area-per-molecule and membrane thickness results to conformational changes in the polar oxPC tails, which in turn affect the dynamic between vesicle cohesion and adsorption to hydrophilic supports that occurs during SLB formation. Characterizing lipid bilayers containing oxPC provides a clearer understanding of what is occurring on the molecular level in oxidative stress-related disease and subsequent immune response.