Solubilization of phosphatidylcholine bilayers by octyl glucoside

The solubilization of large, unilamellar egg phosphatidylcholine vesicles by the nonionic detergent octyl glucoside (OG) was investigated by nuclear magnetic resonance (NMR), fluorescence anisotropy, turbidity, electron microscopy, and centrifugation followed by compositional analysis. The solubilization process is well described by the three-stage model previously proposed for other detergents. In stage I, the OG partitions between the bilayer and aqueous phases with a molar partition coefficient of 59 +/- 6. The presence of OG in the bilayers produces a small "fluidizing" effect, as indicated by changes in the NMR and fluorescence anisotropy parameters. A rearrangement that forms large mixed bilayers occurs in the latter part of stage I. Stage II, the conversion of detergent-saturated bilayers into mixed micelles, begins at a ratio of total OG concentration minus the critical micelle concentration to total phosphatidylcholine concentration of approximately 1.5 and continues until this ratio reaches about 3.0. The correction for the critical micelle concentration of the OG is necessary for comparison of experimental results obtained at different lipid concentrations. The mixed bilayer-mixed micelle interconversion is quantified by the centrifugation experiments and by 31P NMR. The agreement between the two methods is excellent. Advantages of the NMR method are discussed. In stage III, which was not studied in detail here, all of the phosphatidylcholine is present as mixed micelles. Evidence is presented that the various structures present in the dispersions are in equilibrium with one another during these experiments.