Characterization of Molecular Assemblies Formed in Aqueous C10E7/DPPC Mixture by Spin Label and Fluorescence Probe Techniques and Mechanism of Micelle-to-Vesicle Transformation

Mixtures of polyoxyethylene (POE)-type nonionic surfactants and diacylphosphatidylcholines (PC) form diverse molecular assemblies in dilute aqueous solutions depending on temperature and composition, namely, vesicles, mixed micelles, large aggregates with highly polydisperse size distribution, and oil droplets separated from the aqueous phase. In order to get more detailed information on these molecular aggregates, we utilized the spin label and fluorescence probe techniques. The electron spin resonance (ESR) spectra of the spin probe, 5-nitroxide stearic acid, demonstrate that the lipid bilayer structure is partly retained within the mixed micelles of relatively low surfactant content. It is also suggested by the ESR spectra that the large aggregates are composed of lamellar fragments resulting from the destruction of vesicles. The results obtained by the use of fluorescence probes, i.e., the fluorescence depolarization of 2- and 12-(9-anthroyloxy) stearic acids and the pyrene III/I ratio, are consistent with the conclusions derived by the ESR experiments. Based on the structural model postulated for C10E7/DPPC mixed micelles, a probable mechanism is proposed for temperature-induced reversible transformation between mixed micelles and vesicles, which is a characteristic feature observed in dilute aqueous POE/PC mixtures.