Temperature dependence of the vesicle-micelle transition of egg phosphatidylcholine and octyl glucoside

The temperature dependence of octyl glucoside micellization was determined and compared to the phase behavior of the octyl glucoside--egg phosphatidylcholine (PC) mixed system in excess water to help elucidate the process of vesicle formation from mixed surfactant-phospholipid micelles. The critical micelle concentration of octyl glucoside (OG) was determined from the sharp increase of ANS fluorescence at micellization in an NaCl buffer at temperatures ranging from 5 to 40 degrees C. The cmc decreased with increasing temperature from 31 mM at 5 degrees C to 16 mM at 40 degrees C. A similar but less steep temperature dependence is observed for the solubilization of egg PC vesicles by OG as monitored by the surfactant-dependent changes in (1) solution turbidity and (2) the resonance energy transfer between NBD-PE and Rho-PE incorporated in the vesicles. These assays identify two breakpoints, most likely the boundaries of the cylindrical micelle and spheroidal micelle coexistence region. The [OG]aq values at these two breakpoints have similar temperature dependencies. However, the cylindrical mixed micelles at the boundary have nearly identical OG:PC ratios over the temperature range studied, whereas the spheroidal mixed micelles have relatively more OG at the higher temperatures (OG:PC ratio increases from 2.92 to 3.72 between 5 and 35 degrees C). Estimation of the acyl volume to surface area ratio for the compositions observed suggests that this parameter remains constant over temperature. The spheroidal mixed micelles, but not the cylindrical PC-OG micelles, exhibit ideal mixing between the two components at all temperatures (5-35 degrees C). This temperature sensitivity may be utilized to improve the efficacy of membrane protein reconstitution.(ABSTRACT TRUNCATED AT 250 WORDS)