Vitamin D in solution: conformations of vitamin D3, 1alpha,25-dihydroxyvitamin D3, and dihydrotachysterol3.

Solution conformations of the A and seco B rings of vitamin D(3), 1(alpha), 25-dihydroxyvitamin D(3), 1(alpha)-hydroxyvitamin D(3), and dihydrotachysterol(3) have been established by high resolution, 300-megahertz proton magnetic resonance spectroscopy. The A ring of these steroids is dynamically equilibrated between two chair conformers. For vitamin D(3), 1(alpha)-hydroxyvitamin D(3), and 1(alpha),25-dihydroxyvitamin D(3) the relative proportions of the two conformers are 1 : 1, whereas dihydrotachysterol3 exists principally as only one conformer. Thus, the substituent groups on the A ring may be either equatorially or axially oriented, and suggests a refinement of the existing topological model for vitamin D hormonal activity.