First principles study of coenzyme B12. crystal packing forces effect on axial bond lengths

In this work we analyze the structure of coenzyme B-12 (AdoCbl) by means of periodic density functional theory (DFT) in order to elucidate the influence of the corrin side chains and the crystalline environment on the properties of axial bonds. The Co-N-ax axial bond is very weak and its strength of less than 8 kcal/mol is four times smaller than Co-C which in solution is similar to 31 kcal/mol. The proper description of the Co-N-ax distance has been problematic in previous DFT calculations and the source of disagreement between experiment and theory remained unexplained. To resolve this discrepancy, periodic DFT calculations within the Car-Parrinello molecular dynamics (CPMD) framework were carried out on three different structural models of increased complexity. The simplest model (DBI-Ado(+)) contains the naked corrin ring with a total of 96 atoms. The second model is the full coenzyme B-12 (AdoCbl) with 209 atoms which has been taken from crystallographic analysis. To understand the extent to which the crystal packing forces influence the structural properties of AdoCbl the complete crystal consisting of four AdoCbl molecules plus 48 water molecules periodically repeated in space was analyzed (1008 atoms). The results show that the properties associated with the Co-C bond can be well reproduced using truncated models. This does not apply to the Co-N-ax axial bond and the presence of the local environment appears to be essential for the correct prediction of its bond length. The most interesting outcome of the present analysis is the finding that the actual length of the Co-N-ax bond (2.262 A) is largely influenced by crystal packing forces.