Formation and reactions of the Wurster's blue radical cation during the reaction of N,N,N',N'-tetramethyl-p-phenylenediamine with oxyhemoglobin.

The aromatic amine N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) reacted directly with oxyhemoglobin in a catalytic reaction resulting in formation of ferrihemoglobin. The second order rate constant of the reaction was found to be 5.5 M-1.s-1. The stable Wurster's blue radical cation produced ferrihemoglobin at rates greater 10(3) M-1.s-1, i.e. more than two orders of magnitude faster than the parent amine. In contrast to the reactions of aminophenols with hemoglobin, free hydrogen peroxide was formed which additionally contributed to ferrihemoglobin formation. Since ferrihemoglobin formation proceeded by two orders of magnitude faster than autoxidation of TMPD, oxyhemoglobin itself acted as an oxidase/peroxidase resulting in electron abstraction from the amino alone pair electrons.