Deactivation of hypervalent meat pigments. Kinetics of reduction of ferrylmyoglobin by nitrite and iodide

The hypervalent heme pigment ferrylmyoglobin, MbFe(IV)O, a potential pro-oxidant in meat products, is deactivated efficiently by the antioxidant nitrite to a nitrite complex of metmyoglobin, MbFe(III). The second-order rate constant for direct reduction in aqueous 0.16 M NaCl with pH=7.4 was found to have the value k2=13.9±0.4 M−1 s−1 at 25°C with the activation parameters ΔH#=30±2 kJ mol−1 and ΔS#=−123±7 J mol−1×K−1. Iodide, an example of a simple spheric electron donor, is less efficient with k2=0.34±0.01 M−1 s−1 as the result of a higher enthalpy of activation (ΔH#=43±3 kJ mol−1 and ΔS#=−109±8 J mol−1 K−1). Although nitrite, like iodide, thus belongs to a class of deactivators, for which intra-molecular electron transfer to form a cation radical +·MbFe(III)O− is rate-determining and characterized by a negative entropy of activation, it differs from the spheric iodide, by inducing conformation changes in the protein to lower the barrier of activation.