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Spin‐Probe Investigations of Head Group Behavior in Aqueous Dispersions of a Nonionic Amphiphilic Compound
Author(s) -
Nakagawa Kouichi
Publication year - 2007
Publication title -
lipids
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.601
H-Index - 120
eISSN - 1558-9307
pISSN - 0024-4201
DOI - 10.1007/s11745-007-3048-2
Subject(s) - electron paramagnetic resonance , spin probe , spin label , rotational diffusion , chemistry , membrane , amphiphile , aqueous solution , diffusion , phosphatidylcholine , analytical chemistry (journal) , organic chemistry , nuclear magnetic resonance , phospholipid , copolymer , thermodynamics , molecule , polymer , biochemistry , physics
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 3β‐doxyl‐5α‐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 ⊥ ≈ 1/(6τ R )) for the probes were 3.4 × 10 7 and 7.1 × 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.