Anisotropic motion of the steroid ring system of cholesteryl esters. Calculation of carbon-13 NMR relaxation times and nuclear Overhauser enhancements and comparison with experiment

Abstract

A quantitative model for the molecular dynamics of the steroid fused ring system of cholesteryl esters is discussed. The model describes rotational diffusive motion of the steroid rings as that of axially symmetric prolate ellipsoids and is used to generate predictions for the dependence of 13C NMR line widths, spin-lattice relaxation times, and nuclear Overhauser enhancements of C3 and C6 of cholesteryl esters on the correlation times for rotation about the symmetry axis and about the nonunique axes of the ellipse by using the spectral density functions developed by Woessner. The predictions are used to calculate correlation times for motion of the steroid rings of isotropic liquid-phase cholesteryl linoleate and cholesteryl oleate from NMR spectra at magnetic field strengths of 2.35 and 6.34 T [Tesla] and at various temperatures. Such calculations characterize steroid ring motion of cholesteryl esters as highly anisotropic, with motion about the symmetry axis 40-130 times faster than about the nonunique axes. The fact that the line width of the C6 resonance is consistently narrower than that of C3 is attributed to the high rotational anisotropy of cholesteryl esters and to the inclination of the C6-H internuclear vector at an angle with respect to the molecular symmetry axis that is very close to the magic angle.