TRANSITION STRUCTURE FOR THE HYDRIDE TRANSFER-REACTION FROM FORMATE ANION TO CYCLOPROPENYL CATION - A SIMPLE THEORETICAL-MODEL FOR THE REACTION CATALYZED BY FORMATE DEHYDROGENASE

The transition structure (TS) for the reaction of formate anion with the hydride acceptor cyclopropenyl cation (CP+) has been calculated with analytical gradients at an ab initio MO SCF level. The saddle point characterizing the reaction in vacuum was calculated with two basis sets: an extended (9s, 5p) and the 4-31G. The geometry, electronic structure and transition vector components are qualitatively basis-set independent. Information extracted from the theoretical TS is contrasted with experimental kinetic isotopic substitution measurements for the reaction leading from formate to CO2 in the reaction catalyzed by yeast formate dehydrogenase. With the simple model chosen here where the reactants are moulded into the TS structure, the isotope effects can be rendered in fairly good qualitative agreement. Furthermore, by comparison of the TS with the hydride transfer model in liver alcohol dehydrogenase and with the transition structure for carbon dioxide interconversion in carbonic anhydrase, the structures of the moieties in the TS appear to be fairly transferable.