title: Spin-probe investigations of head group behavior in aqueous dispersions of a nonionic amphiphilic compound.
abstract: Head group behavior of nonionic amphiphilic compound, (poly(oxyethylene) hydrogenated castor oil, HCO), in aqueous dispersions were investigated by EPR (electron paramagnetic resonance) in conjunction with a modern slow-tumbling simulation. The aliphatic spin probes, 5-doxylstearic acid (5-DSA) and 3beta-doxyl-5alpha-cholestane (CHL), were used to obtain fluidity of the surface region of the membrane. The order parameter (S (0)) using the simulation for 5-DSA and CHL in the region were approximately 0.4 and 0.2, respectively. The ordering results suggest that the head group region of the membrane is somewhat fluid. The rotational diffusion coefficients (R ( perpendicular) approximately 1/(6tau(R))) for the probes were 3.4 x 10(7) and 7.1 x 10(7) s(-1), respectively. Activation energies, calculated using the temperature dependence of diffusion coefficients, were 18 and 17 kJ/mol for the probes. The EPR results imply that the CHL probe in the HCO membrane has quite different behavior in comparison with that of PC (phosphatidylcholine) from egg. Thus, the present EPR analyses have provided quantitative insight into the surface region of the amphiphilic membrane.
authors.affiliation: RI Research Center, Fukushima Medical University, 1 Hikarigaoka, Fukushima, 960-1295, Japan. nakagawa@fmu.ac.jp
authors.forename: Kouichi
authors.initials: K
authors.lastname: Nakagawa
mesh_terms: D002368:Castor Oil / Q000031:analogs & derivatives* / Q000737:chemistry; D003497:Cyclic N-Oxides / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy; D008560:Membrane Fluidity; D013113:Spin Labels; D013499:Surface Properties; D013501:Surface-Active Agents / Q000737:chemistry*; D013696:Temperature; D014867:Water / Q000737:chemistry
chemical_list: D003497:Cyclic N-Oxides; D013113:Spin Labels; D013501:Surface-Active Agents; D014867:Water; C005072:5-doxylstearic acid; C023040:polyethoxylated castor oil; D002368:Castor Oil