Computational exploration of rearrangements related to the vitamin B12-dependent ethanolamine ammonia lyase catalyzed transformation

DFT (B3LYP/6-31G*) and ab initio molecular orbital theory (QCISD/cc-pVDZ) are used to investigate several possible mechanisms involving free radical intermediates as well as their protonated forms for processes related to the coenzyme B-12-dependent rearrangement catalyzed by ethanolamine ammonia lyase. Two major types of rearrangements are discussed in detail, intramolecular migration and dissociation of the amine/ammonia groups, for both of which several scenarios are considered. According to the calculations, the complete dissociation of the migrating group and its subsequent association constitute an unlikely route for both the protonated and the unprotonated reactant because of the high-energy barriers (more than 23 kcal/mol) involved in these steps. Direct migration of the protonated amine group is far more favorable (10.4 kcal/mol) and therefore presents the most likely candidate for the actual enzymatic reaction. The calculations further imply that the direct loss of an ammonium cation (10.6 kcal/mol) represents a feasible pathway as well. Comparing the rearrangements for the aminoethanol radical and its protonated counterpart, in line with previous findings reported by Golding, Radom, and co-workers, we find that the migration of a protonated group is in general associated with lower energy barriers, suggesting that the actual enzyme substrate quite likely corresponds to (partially) protonated aminoethanol. As the extent of the substrate protonation/cleprotonation by the active site of the enzyme may vary, the actual energy barriers are expected to range between the values calculated for the two extreme cases of a substrate, that is, the aminoethanol radical 2 and its fully protonated form 6.