The role of CO2 in cobalt-catalyzed peroxidations

Augmentation, by CO2/HCO3−, of Co(II)-catalyzed peroxidations was explored to clarify whether the rate enhancement was due to CO2 or to HCO3−. The rate of oxidation of NADH by Co(II) plus H2O2, in Tris or phosphate, was markedly enhanced by CO2/HCO3−. Phosphate was seen to inhibit the Co(II)-catalyzed peroxidation, probably due to its sequestration of the Co(II). When CO2 was used, there was an initial burst of NADH oxidation followed by a slower linear rate. The presence of carbonic anhydrase eliminated this initial burst; establishing that CO2 rather than HCO3− was the species responsible for the observed rate enhancements. Both kinetic and spectral data indicated that Co(II) was converted by H2O2 into a less active form from which Co(II) could be regenerated. This less active form absorbed in both the UV and visible regions, and is assumed to be a peroxy bridged binuclear complex. The rate of formation of this absorbing form was increased by HCO3−/CO2. A minimal mechanism consistent with these observations is proposed.