Model studies of membrane disruption by photogenerated oxidative assault

We have investigated the response of solid-supported phospholipid bilayers to short doses of photogenerated oxidative stress to characterize physical membrane changes during early phases of membrane oxidation. The low-dose oxidative stress is generated by uniformly exposing the bilayer samples using short-wavelength UV radiation (184-257 nm) for short periods (approximately 3 min) and resulting membrane morphological transformations characterized using a combination of wide-field epifluorescence microscopy and imaging ellipsometry measurements. Our results establish that the early phase of membrane oxidation is characterized by the nucleation and growth of discrete microscopic voids within the bilayer. The locations of the voids are randomly distributed throughout the sample surface, despite the uniform illumination. Over longer time scales, the voids continue to grow after the termination of the UV radiation. We also find that the voids heal as sample temperature is raised and that the supported bilayers consisting of fully saturated lipids are less susceptible to the mild oxidation conditions used, regardless of phase state. Analyzing these results in terms of (1) reactive-oxygen species mediated oxidative attack, (2) in situ generation of membrane oxidation products, and (3) their reequilibration between the membrane and the bulk aqueous phase explains the membrane morphological changes observed and provides insights into membrane perturbations following oxidative assault. Specifically, molecular properties of oxidation products (e.g., intrinsic curvature) account for formation and stabilization of voids within contiguous bilayers, and the long-term structural evolution is consistent with slow kinetics of the desorption of these oxidation products from the bilayer into bulk solution. A corollary benefit from our study is that the thermal properties of voids appear to offer a useful means to measure the thermal expansivity of supported membranes.