EPR Investigations of Spin‐Probe Dynamics in Aqueous Dispersions of a Nonionic Amphiphilic Compound

Abstract

Dynamics of various spin probes in aqueous dispersions of nonionic amphiphilic compound, [poly(oxyethylene) hydrogenated castor oil, HCO], were investigated by EPR (electron paramagnetic resonance) and saturation recovery (SR) spectroscopies. Partitioning, rotational correlation time (s R ), rotational diffusion coefficient, and electron spin-lattice relaxation time (T 1e ) in dispersions of the HCO membrane were obtained. The partitioning of water soluble spin probes, DTBN and TEMPO, in the aqueous and vesicle phases was determined by an EPR linewidth simulation as a function of temperature. The results suggest that DTBN and TEMPO have a similar partitioning in the vesicle phase throughout the temperatures studied. The longer s R and shorter T 1e (*0.33 ls) values of DTBN in the vesicle phase were obtained, and could be attributed to the probe environment in the membrane. The simulation results for fast tumbling probes were quite different from those of conventional intensity analysis (spectral parameter, f). Thus, the simulation and T 1e analyses have provided a quantitative understanding of the probe dynamics in both phases. Aliphatic spin probes, doxylstearic acids (DSAs) and 3b-doxyl-5a-cholestane (CHL), were used for monitor of various membrane motions. The EPR spectra were quantitatively analyzed by a slow tumbling simulation. The rotational diffusion coefficients and order parameter were obtained by the simulation. In addition, the direct observations of the behavior of the probes were measured by SR method. The results were consistent with T 1e obtained for spin probes. Thus, the quantitative results regarding EPR, SR method, various simulation analyses have provided detailed information regarding physicochemical properties of the various moieties of the probe region in the amphiphilic compound.

Spin-Lattice Relaxation Rates of Lipid Spin Labels as a Measure of Their Rotational Diffusion Rates in Lipid Bilayer Membranes

“…Similar dependences were previously investigated but only for small spin labels in simple isotropic glass forming solvents for a broad temperature range between 100 K and 300 K [ 23 ]. Interesting data were also presented by Nakagawa [ 24 ], who investigated spin-lattice relaxation rates and rotational diffusions of small and lipid type spin labels in dispersions of hydrogenated castor oil. This triglyceride derivative forms in water dispersion bilayer membranes.…”