Miscibility of octyl glucoside-phosphatidylcholine micellar solutions. Partition of the nicotinic acetylcholine receptor into the surfactant-rich phase.

Phospholipid phosphatidylcholines (PC) and the nonionic detergent octyl glucoside (OG) were found to form a miscibility gap in aqueous solution at a molar ratio [OC]T/[PC]T=2.6-2.4. This ratio is just below the critical ratio for the formation of vesicles. At [PC] greater than 1.3 g/l the transition from micelles to vesicles is not direct but via a surfactant-rich phase (SRP). The various stability regions are summarized in a phase diagram. Though the partition coefficient gamma=[Db]/[Dw] increases markedly with temperature, the critical ratio for the phase separation R(c) is biased to higher detergent concentrations. This behaviour can be explained by increased motion of the hydrocarbon moiety at higher temperatures. The different phases are investigated by light microscopy, nuclear magnetic resonance and thin-layer chromatography. The miscibility gap of PC/OG is compared to the lamellar phase of Triton X-114; the miscibility gap has implications for membrane protein reconstitution and can be used for membrane protein purification. The Torpedo californica nicotinic acetylcholine receptor (n-AcChR) was found to partition into the SRP.