Singlet Oxygen Flux, Associated Lipid Photooxidation, and Membrane Expansion Dynamics Visualized on Giant Unilamellar Vesicles

Abstract

The physical properties of lipid membranes depend on their lipid composition. Photosensitized singlet oxygen (1O2) provides a handle to spatiotemporally control the generation of lipid hydroperoxides via the ene reaction, enabling fundamental studies on membrane dynamics in response to chemical composition changes. Critical to relating the physical properties of the lipid membrane to hydroperoxide formation is the availability of a sensitive reporter to quantify the arrival of 1O2. Here, we show that a fluorogenic α-tocopherol analogue, H4BPMHC, undergoes a >360-fold emission intensity enhancement in liposomes following a reaction with 1O2. Rapid quenching of 1O2 by the probe (kq = 4.9 × 108 M-1 s-1) ensures zero-order kinetics of probe consumption. The remarkable intensity enhancement of H4BPMHC upon 1O2 trapping, its linear temporal behavior, and its protective role in outcompeting membrane damage provide a sensitive and reliable method to quantify the 1O2 flux on lipid membranes. Armed with this probe, fluorescence microscopy studies were devised to enable (i) monitoring the flux of photosensitized 1O2 into giant unilamellar vesicles (GUVs), (ii) establishing the onset of the ene reaction with the double bonds of monounsaturated lipids, and (iii) visualizing the ensuing collective membrane expansion dynamics associated with molecular changes in the lipid structure upon hydroperoxide formation. A correlation was observed between the time for antioxidant H4BPMHC consumption by 1O2 and the onset of membrane fluctuations and surface expansion. Together, our imaging studies with H4BPMHC in GUVs provide a methodology to explore the intimate relationship between photosensitizer activity, chemical insult, membrane morphology, and its collective dynamics.