Motional Anisotropy-Induced Unequal Deuterium NMR Spin-Lattice Relaxation of 3α-Deuteriocholesterol and 3β-Deuterioepicholesterol in Solution as a Measure of Sterol Motion about the Molecular Axis

Article abstract of DOI:10.1002/mrc.1260230406

Deuterium spin-lattice relaxation (T₁) data for 3α-deuteriocholesterol and 3β-deuterioepicholesterol in solution were obtained at 12.21 MHz as a function of solvent viscosity (η).The correlation times (τ_c) for both 3-deuteriosterols varied in a linear fashion over the range of viscosities studied.The data showed that although both 3α-deuteriocholesterol and 3β-deuterioepicholesterol undergo rapid reorientations (τ_c ca. 10^-11 s) in solution, the relaxation of the deuterium in α-deuteriocholesterol was approximately 2.5 times faster than in 3β-deuterioepicholesterol.These unequal rates of relaxation are due to the difference in the angles between the C-D bond and the molecular axis for the two sterol isomers.An equation containing an angular-dependent term was derived to analyze the T₁ data further.The effective correlation times (τ_eff), thus calculated, converged for identical viscosities.In the case of 3α-deuteriocholesterol, it was determined that rotation about the molecular axis comprised up to 60percent of the motions relaxing the deuteron.The results show that for molecules having an inherent anisotropy and undergoing isotropic motions, it is possible to separate the portion of motion due to rotation about the molecular axis from other motions, it is possible to separate the portion of motion due to rotation about the molecular axis from other motions which contribute to the relaxation processes at a specific center.