Accurate Redetermination of the X-ray Structure and Electronic Bonding in Adenosylcobalamin

The electronic structure of adenosylcobalamin (B12 coenzyme, AdoCbl) has been calculated by a density functional
method, using the orthogonalized linear combination of the atomic orbital method (OLCAO). Since a fixed accurately
determined geometry was needed in such calculations, the crystal structure of adenosylcobalamin has been redone
and refined to R = 0.065, using synchrotron diffraction data. Comparison with the recently reported electronic
structures of cyano- (CNCbl) and methylcobalamin (MeCbl) shows that the net charges and bond orders vary only
on the axial donors. The values in the three cobalamins suggest that the Co−C bond in MeCbl has a strength
similar to that in AdoCbl, but it is significantly weaker that that in CNCbl. Present results are compared with those
previously reported for the analogous corrin derivatives; i.e., simplified cobalamins with the side chains a−f replaced
by H atoms. Despite a qualitative agreement, a discrepancy in the calculated HOMO−LUMO gap is found.