Diffusion Coefficient and Relaxation Time of Aliphatic Spin Probes in a Unique Triglyceride Membrane.

The molecular dynamics of aliphatic spin probes in a unique triglyceride membrane were investigated by continuous wave electron paramagnetic resonance (CW EPR) and saturation recovery spectroscopic methods. Rotational diffusion coefficients (R⊥ and R∥) obtained by a slow-motional EPR spectral simulation for 7-doxylstearic acid (7-DSA), 12-doxylstearic acid (12-DSA), and 16-doxylstearic acid (16-DSA) in the membrane at various temperatures were obtained. The activation energies calculated using R⊥ values for 7-, 12-, and 16-DSA in the membrane were 19 ± 0.9, 24 ± 1.2, and 37 ± 1.8 kJ/mol, respectively. The higher activation energy implies that the perpendicular motion of 16-DSA is more sensitive to temperature. As the nitroxide group of the spin probe was moved further down the stearic chain, electron spin-lattice relaxation times (T1e) became shorter. The shorter T1e indicates more flexibility around the probe moiety. Also, T1e for the DSAs became shorter when the temperature increased. The values of T1e obtained were consistent with perpendicular diffusion coefficients. In addition, no significant difference in T1e between the H2O and the D2O solutions was observed for the different DSAs. Therefore, it is concluded that steric effects and the local rotational mobility of the nitroxide moiety influence the T1e's obtained.