AN AB-INITIO COMPUTATIONAL MOLECULAR-ORBITAL STUDY OF RADICAL, PROTONATED RADICAL (RADICAL-CATION), AND CARBOCATION SPECIES THAT HAVE BEEN PROPOSED IN MECHANISMS FOR THE TRANSFER PROCESS IN THE ENZYME COENZYMEB-12-CATALYZED DEHYDRATION OF 1,2-DIHYDROXYETHANE

The mechanism of action of the enzyme diol dehydrase has variously bee n suggested to involve a radical, a protonated radical (radical cation ), or a carbocation in the apparent transfer of HO from one carbon ato m to the other to give acetaldehyde and water. This adenosylcobalamin- requiring B-12 enzyme catalyzes dehydration of 1,2-dihydroxyethane. It s active site is buried within a hydrophobic cavity. Each of the three possibilities are examined by ab initio molecular orbital calculation s. Total molecular energies have been calculated with full geometry op timization at the MP2(FC)/6-31G level and, in addition, single point energies using the MP2(FC)/6-311++G* basis set. Vibrational frequenci es were also calculated. Transfer of an HO group within a HOCH-CH2OH i nverted left perpendicular (.) radical (postulated to be formed by the initial reaction of the diol with the deoxyadenosyl radical via a bri dge structure) was ruled out because the activation energy is much too high in relation to the observed rate constant for the enzyme reactio n. A carbocation mechanism also presents problem, quite apart from the necessity of postulating some acceptor for the electron from the HOCH -CH2OH inverted left perpendicular (.) radical. In principle acetafdeh yde could be formed via the 2,2-dihydroxyeth-1-yl cation which was fou nd to undergo a spontaneous 1,2-hydride ion shift, giving protonated a cetic acid. But, although the 1,2-dihydroxyethyl cation (otherwise pro tonated glycolaldehyde) is well established as a stable species, the i ntermediary bridge structure could not be found. The radical cation HO CH-CH2OH2 inverted left perpendicular (.+), formed by proton transfer from an active-site group, was found to be inherently unstable, transf orming without activation into a stable hydrogen-bonded hydrate of the anti-vinyl alcohol radical cation, H2O ... CH2 inverted left perpendi cular (.+). Deprotonation and H-atom transfer (from AdCH(3)) were then found to give stable hydrogen-bonded hydrates of the formylmethyl rad ical, protonated acetaldehyde, and acetaldehydeitself. The ultimate fo rmation of acetaIdehyde and water can be attributed either to the diss ociation of acetaldehyde hydrate or the prior dissociation of theformy lmethyl radical hydrate followed by the H-atom transfer step. Because H2O is already formed as a discrete entity in the initial protonation step and no transfer of a bonded HO, H2O,-or H2O+ group from one carbo n atom to the other actually occurs, this series of reactions may be t ermed a ''predissociation'' mechanism. The overall proton transfer and the formation of water are complicated interconnected processes. A co nsideration of whether or not the cobalt participates in one of the la ter reaction steps is undenvay.