THEORY AND APPLICATION OF FULLY ANISOTROPIC OVERALL MOLECULAR TUMBLING WITH RESTRICTED GROUP INTERNAL-ROTATION - C-13 NMR RELAXATION STUDIES OF SODIUM TAUROCHOLATE AND GINSENOSIDE-RE

A theory is presented for fully anisotropic overall molecular tumbling with restricted group internal rotation, This formalism is used with C-13 NMR relaxation data and results from molecular mechanics to compu te the rotational diffusion rates and methyl group internal rotation r ates for sodium taurocholate and ginsenoside-Re in organic solvents, A lthough existing theories for steroid derivatives describe methyl inte rnal rotation in terms of free isotropic motion, methyl rotation in gi nsenoside-Re is found to be both sluggish and spatially restricted, Th e energetic barrier to methyl internal rotation increases significantl y with the bulkiness of flexible side-chains in a series of steroid na tural products.