A multinuclear solid-state NMR analysis of vitamin B-12 in its different polymorphic forms

Multinuclear solid-state nuclear magnetic resonance methods (Co-59, C-13, N -15, and P-31 NMR) were applied at natural abundance to the structural and dynamic analysis of cyanocobalamin ( vitamin B-12) polymorphs. These studie s involved recrystallizing a series of samples under different conditions a nd from various solvents, and subsequently recording their powder NMR spect ra at different temperature. Two pofq morphs could be identified in these s tudies, in correspondence with the two structures described by Hodgkin and co-workers in their seminal vitamin B-12 crystallographic analyses, Most in formative about the molecular differences characterizing these two forms we re the C-13 NMR data, which showed sharp and well-resolved resonances indic ative of high sample crystallinity. Diagnostic differences between these tw o forms could be observed in the chemical shifts of particular resonances, many of which could be assigned to their molecular sites on the basis of so lution-state Literature and of solid-state spectral editing procedures. The se shifts indicated a conformational variability that involved a few specif ic sites in the corrinoid ring and which was not entirely evident from diff erences among the X-ray structures previously reported for the forms. Furth er evidence about the conformational flexibility of these sites in the soli d is furnished by C-13 spectral shifts observed upon changing temperatures. The relevance of these observations within the context of the extensive st ructure/ activity relation studies that have been carried out on this class of systems is briefly addressed.