A proton magnetic resonance relaxation time study of several vitamin D derivatives

Proton magnetic resonance relaxation time studies at 200 and 400 MHz have been measured for vitamins D2, D3, as well as the biologically active 1α,25-(OH)2D3. A comparison of these relaxation times at the two frequencies allows the evaluation of the dipolar correlation times for the different protons in the molecule. These are very short, and range from < 0.7 × 10−10 s to 1.2 × 10−10 s. Application of the Solomon equation to the relaxation times of the 19Z and 19E protons allows a calculation of the internuclear distance since these two geminal protons relax each other predominantly by the dipolar mechanism. Using the T1 values, the calculated distances are 1.98 ± 0.10 Å for 1α,25-(OH)2D3, 2.01 ± 0.10 Å for D3, and 1.95 ± 0.10 Å for D2. The calculated distance based on a neutron diffraction distance for the C—H bond is 1.91 Å. The measurement of the relaxation times also allows a definitive spectral assignment of the 21- and 28-methyl protons for vitamin D2.