Conformational flexibility in coenzyme B-12: A nuclear magnetic resonance and molecular mechanics study

The conformation of 5'-deoxyadenosylcobalamin (AdoCbl, coenzyme B-12) with respect to rotation of the Ado ligand about the Co-C bond has been studied by rotating-frame Overhauser enhancement NMR spectroscopy. Previous molecular-mechanics calculations suggest that in addition to the conformation seen in the solid state, in which the Ado ligand lies over the ''southern'' quadrant of the corrin ring, relatively low-energy conformations exist in which the Ado lies over the ''eastern'', ''northern'' and ''western'' quadrants as well. At 27 degrees C, 12 NOE cross peaks are observed between Ado protons and protons elsewhere on the molecule, all of which are consistent with the calculated ''southern'' or ''eastern'' conformations, or both. However, at 60 degrees C, several of the Ado protons undergo significant chemical shift changes and of the 27 observed NOE cross peaks, eight are compatible only with the ''northern'' or ''western'' conformations or both. The effect of the apparent population of these additional conformations at elevated temperatures on the activation parameters for thermal carbon-cobalt bond homolysis is discussed in the light of molecular-mechanics calculations on the influence of Co-C bond geometry and acetamide side-chain rotational motions on the energetics of this reaction.