Observation of a “blue” intermediate in the reaction between ferrylmyoglobin and thiocyanate. Deactivation of the hypervalent meat pigment as a step-wise process with slow intramolecular electron transfer

A “blue” reaction intermediate (absorption maximum at 835 nm) is observed when ferrylmyoglobin is reacted with excess thiocyanate in neutral aqueous solution. The intermediate is formed in a bimolecular process (first-order in thiocyanate and ferrylmyoglobin) with a rate constant (0.119±0.006 M−1 s−1 at 25°C) showing moderate temperature dependence (enthalpy of activation ΔH#=45±6 kJ mol−1, entropy of activation ΔS#=−113±19 J mol−1 K−1), but decays in a first-order, highly temperature-dependent reaction (1.53±0.05×10−4 s−1 with ΔH#=99±1 kJ mol−1 and ΔS#=14±3 J mol−1 K−1). Hypothiocyanite is not detected as an intermediate (as for peroxidase-catalyzed oxidation of thiocyanate), and the “blue” intermediate is proposed to be a protein radical with thiocyanate bound and which decays by intramolecular electron-transfer. ©