Characterization of mixed micelles of phospholipids of various classes and a synthetic, homogeneous analogue of the nonionic detergent Triton X-100 containing nine oxyethylene groups.

The synthesis and high-pressure liquid chromatographic purification of the homogenous nonionic surfactant p-(1,1,3,3-tetramethylbutyl)phenoxynonaoxyethylene glycol (OPE-9) in quantities suitable for membrane solubilization studies is reported. Micelles of OPE-9 and mixed micelles of OPE-9 with dimyristoyl and dipalmitoyl phosphatidylcholine as well as phosphatidylserine, phosphatidylethanolamine, lysophosphatidylcholine, sphingomyelin, and palmitic acid were characterized by column chromatography on 6% agarose. It was found that at 28 degrees C OPE-9 micelles have a Stokes' radius of 32 A, giving a molecular weight for a spherical micells of about half that of micelles of the polydisperse nonionic surfactant Triton X-100 under the same conditions. The micelle size is temperature dependent: at 40 degrees C the OPE-9 micelles have a Stokes' radius of 44 A, giving a molecular weight for a spherical micelle of about twice that of the OPE-9 micelles at 28 degrees C. The size of the mixed micelles varies linearly (as measured by Kav) with the mole fraction of phospholipid. The mixed micelle size was found to be relatively independent of the absolute concentration of surfactant over a four-fold range if the mole fraction of phospholipid is kept constant. The usefulness of the OPE-9/phospholipid mixed micelle system for lipolytic enzyme substrates and membrane-related studies is considered.