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 (T_1e) became shorter. The shorter T_1e indicates more flexibility around the probe moiety. Also, T_1e for the DSAs became shorter when the temperature increased. The values of T_1e obtained were consistent with perpendicular diffusion coefficients. In addition, no significant difference in T_1e 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 T_1e's obtained.