Evaluating theoretical rate constants for bimolecular antioxidant processes involving ionic species under physiological conditions

The rate constant for the reaction of CO ₃ ^∙- with anionic species could be influenced by Columbic forces and ionic strength of the cellular environment. We investigated the reaction of CO ₃ ^∙- with urate, ascorbate and caffeate at the M06-2X/6–311 + G(d,p) level in an aqueous environment. The thermal rate constant was adjusted using the Debye-Smoluchowski and Brønsted-Bjerrum equations. Electrostatic interactions moderately reduced the rate constant for single electron transfer from urate and caffeate to CO ₃ ^∙-, but were negligible for the reaction with ascorbate. However, when we took into account the primary kinetic salt effect, the Debye-Smoluchowski rate constants were increased to values close to those predicted without inclusion of the Debye factor. Columbic repulsion and ionic strength have a modest impact on the rate constant for reactions involving an ionic radical. However, if the reactions involved ions of opposite charge, the influence of the Debye function would become more significant.