Does hydrogen bonding contribute to lipoperoxidation-dependent membrane fluidity variation? An EPR-spin labeling study

This study is aimed at making clear the relationship between oxidative stress of the phospholipid bilayer and membrane fluidity. Di-(hydroperoxylinoleoyl)-phosphatidylcholine (diHpLPC) was used as a highly hydroperoxidized and unsaturated phospholipid species in order to investigate the issue. Hydrophylic Interaction Liquid Chromatography-ElectroSpray Ionization-Mass Spectrometry (HILIC-ESI-MS) and NMR spectroscopy were employed to define the structure of the peroxidized phospholipid as 1-(9-hydroperoxy-10c,12t)octadecadienoyl-2-(9t,11c-13-hydroperoxy)octadecadienoyl-sn-glycero-3-phosphorylcholine. This phospholipid's ability to form vesicular structures was confirmed by Sepharose 4B gel filtration and Dynamic Light Scattering (DLS) of its aqueous suspensions. Fatty acid misalignment and fluidity gradient were studied in the bilayer of both supported planar bilayers (SPB) and multilamellar vesicles (MLV) made of different DLPC/diHpLPC mixtures by means of spin labelling-EPR spectroscopy of either n-DSPC or 3-doxylcholestane spin labels embedded in the membranes. It was found that diHpLPC increases both fatty acid misalignment and rigidification with increasing molar ratio in spite of increasing unsaturation of the fatty acid core. Basing on our observations, the observed ability of pure diHpLPC to form rigid and disordered SPB and MLV bilayers is proposed to be dependent on the cross bridging of oxidized linoleoyl chains by mutual hydrogen bonding of hydroperoxyl groups. However, the contribution to the observed overall rigidification of the model membranes by trans double bonds in the peroxidized chains should not be neglected, as a second membrane fluidity effector also arising from lipid peroxidation.