Spin-Probe Investigations of Head Group Behavior in Aqueous Dispersions of a Nonionic Amphiphilic Compound |
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Authors: | Kouichi Nakagawa |
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Affiliation: | (1) RI Research Center, Fukushima Medical University, 1 Hikarigaoka, Fukushima 960-1295, Japan |
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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/(6τR)) for the probes were 3.4 × 107 and 7.1 × 107 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. |
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Keywords: | EPR Spin probe Rotational diffusion coefficients Activation energy Amphiphilic compound |
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