DEHYDROGENATION OF FORMATE BY 10-METHYLACRIDINIUM ION

Article abstract of DOI:10.1016/S0040-4020(01)87505-0

Formate is dehydrogenated to CO2 by 10-methylacridinium ion, mimicking formate dehydrogenase.The hydrogen and carbon kinetic isotope effects are 2.74 and 1.027 in a mixed solvent consisting of dimethylformamide and water in a 4 : 1 ratio, at 50 deg C.These values are similar to those observed in the enzymatic reaction (2.27 and 1.042, respectively) suggesting that the mechanisms of enzymatic and nonenzymatic reactions are the same, and transition state structures not too different.Marcus theory of atom and group transfer is used to locate the transition state for the nonenzymatic reaction 0.4 of the distance along the minimum energy path from precursor configuration to successor configuration.It is concluded, following Cleland and coworkers, that the protein of the enzyme dehydrates the formate and deforms the cofactor NAD⁺ so as to make the reaction more spontaneous.This produces an enzymatic transition state in which the covalency changes around hydrogen are less advanced than in the non-enzymatic transition state, but the environment of the carboxylate is much more suitable to the product, CO2.Yeast formate dehydrogenase brings about the oxidation of formate to CO2, in the process, transferring a hydride ion to the enzyme cofactor, Nicotinamide Adenine Dinucleotide (NAD⁺), reducing the latter to the corresponding 1,4-dihydropyridine, NADH (eq. 1) HCO2^- + NAD⁺ ----> O2 + NADH Cleland and coworkers have made extensive studies of isotope effects on these reactions.They have shown that the reaction is essentially irreversible, and that the step involving the rearrangement of covalent bonds is fully rate-limiting.They have found that the NAD⁺ could be replaced with other pyridinium ions, and measured the changes in isotope effects which attended these replacements.They have found that N3^- is a much more effective competitive inhibitor than NO3^-.From these studies they have reached the following conclusions: 1.) Prior to the hydride transfer, the pyridinium ring is strongly distorted in the direction of dihydropyridine geometry, considerably increasing its hydride affinity (reduction potential). 2.) The transition state in the enzyme catalyzed reaction resembles the products much more closely than the reactants. 3.) Relatively small increases in the reduction potential of the cofactor significantly shift the transition state structure toward that of the reactants.Attempts to oxidize formate with simple NAD⁺ analogues have been unsuccessful but the reduction potential of 10-methylacridinium ion is -240 mv larger than that of NAD⁺ which should facilitate the reaction, according to Cleland's point 1.).Further, in refluxing formic acid solvent, formate has been shown to reduce 10-methylacridinium ion.We now report that formate is oxidized by 10-methylacridinium ion at measurable rates in both isopropanol (IPA) - water (4 : 1 by volume) and dimethylformamide (DMF) - water (4 :1 by volume) at 50 deg or 25 deg C. (2)HCOO^- and H(13)COO^- isotope effects have been measured and compared etc...............