A (13)C solid-state NMR analysis of vitamin D compounds.

(13)C cross-polarization/magic-angle spinning (CP/MAS) solid-state NMR spectroscopy has been employed to analyze four vitamin D compounds, namely vitamin D3 (D3), vitamin D2 (D2), and the precursors ergosterol (Erg) and 7-dehydrocholesterol (7DHC). The (13)C NMR spectrum of D3 displays a doublet pattern for each of the carbon atoms, while that of Erg contains both singlet and doublet patterns. In the cases of 7DHC and D2, the (13)C spectra display various multiplet patterns, viz. singlets, doublets, triplets, and quartets. To overcome the signal overlap between the (13)C resonances of protonated and unprotonated carbons, we have subjected these vitamin D compounds to 1D (1)H-filtered (13)C CP/MAS and {(1)H}/(13)C heteronuclear correlation (Hetcor) NMR experiments. As a result, assisted by solution NMR data, all of the (13)C resonances have been successfully assigned to the respective carbon atoms of these vitamin D compounds. The (13)C multiplets are interpreted due to the presence of s-cis and s-trans configurations in the alpha- and beta-molecular conformers, consistent with computer molecular modeling determined by molecular dynamics and energy minimization calculations. To further characterize the ring conformations in D3, we have successfully extracted chemical shift tensor elements for the (13)C doublets. It is demonstrated that (13)C solid-state NMR spectroscopy provides a robust and high sensitive means of characterizing molecular conformations in vitamin D compounds.