Octyl-β-D-Glucopyranoside Shows Composition Dependent Disordering Effects in Ternary Lipid Bilayers

Solubilization of lipid bilayers by the nonionic detergent OG has been well characterized in one and two component lipid systems, but has not been extensively studied in ternary membranes containing mixed fluid phases. In order to understand the effects of membrane order on solubilization by OG, LUVs of pure POPC were compared to those composed of POPC plus varying fractions of sphingomyelin (SM) and cholesterol. Detergent partitioning from the aqueous phase into the bilayer and membrane solubilization were monitored by ITC. The fluorescent membrane probe, DPH, was observed by time-resolved fluorescence intensity and polarization decay in the presence of increasing detergent concentrations to determine the effects on membrane order and dynamics. The results of partitioning experiments showed approximately a 50% decrease in the partition constant between pure POPC and the most ordered samples. ITC solubilization experiments showed clear boundaries for the micelle-bilayer mixed aggregate coexistence region in samples of low initial order, but the distinct thermodynamic signature associated with this coexistence region was not present in the cholesterol and SM rich samples. The average excited state lifetime of DPH, an indicator of water penetration, in vesicles rich in POPC showed a rapid increase at the onset of solubilization, whereas vesicles rich in SM and cholesterol showed a corresponding rapid decrease. Dynamic fluorescence depolarization data analyzed in terms of a Brownian rotational diffusion model revealed an increase in the occupancy of the bilayer midplane by DPH prior to solubilization in less ordered membranes coupled with a constant rotational correlation time. Membranes with higher initial order showed little increase in midplane occupancy and increasing rotational correlation time in the same detergent concentration range, suggesting that two modes of membrane disruption are possible and depend on the lipid makeup of the bilayer.