Acid-catalysed autoreduction of ferrylmyoglobin in aqueous solution studied by freeze quenching and ESR spectroscopy

Decay of the hypervalent muscle pigment ferrylmyoglobin, formed by activation of metmyoglobin by hydrogen peroxide, was found, when studied by a combination of ESR and UV/VIS spectroscopy in aqueous solution at physiological pH, to proceed by parallel second- and first-order kinetics. At pH below 6.5 a sharp ESR signal (g = 2.003) with an increasing intensity for decreasing pH were observed in solutions frozen in liquid nitrogen, and a broad signal (g = 2.005) was seen throughout the studied pH range also in frozen solutions. The g = 2.005 signal is suggested to arise from an intermediate formed in an intramolecular rate-determining electron-transfer in ferrylmyoglobin, whereas the g 2.003 signal is caused by a radical formed in a proton-assisted electron-transfer initiating the specific acid-catalysed autoreduction.