Kinetics of reduction of hypervalent iron in myoglobin by crocin in aqueous solution

Crocin in aqueous solution is oxidized by ferrylmyoglobin, MbFe(IV) = O, in a second order reaction with k = 183 l.mol-1.s-1, delta H++298 = 55.0 kJ.mol-1, and delta S++298 = -17 J.mol-1.K-1 (pH = 6.8, ionic strength 0.16 (NaCl), 25 degrees C), as studied by stopped-flow spectroscopy. The reaction has 1:1 stoichiometry to yield metmyoglobin, MbFe(III), and has delta G theta = -11 kJ.mol-1, as calculated from the literature value E0 = +0.85 V (pH = 7.4) vs. NHE for MbFe(IV)=O/MbFe(III) and from the half-peak potential +0.74 V (vs. NHE in aqueous 0.16 NaCl, pH = 7.4) determined by cyclic voltammetry for the one-electron oxidation product of crocin, for which a cation radical structure is proposed and which has a half-peak potential of +0.89 V for its formation from the two-electron oxidation product of crocin. The ferrylmyoglobin protein-radical, MbFe(IV)=O, reacts with crocin with 2:1 stoichiometry to yield MbFe(IV)=O, as determined by ESR spectroscopy, in a reaction faster than the second order protein-radical generating reaction between H2O2 and MbFe(III), for which latter reaction k = 137 l.mol-1.s-1, delta H++298 = 51.5 kJ.mol-1, and delta S++298 = -31 J.mol-1.K-1 (pH = 6.8, ionic strength = 0.16 (NaCl), 25 degrees C) was determined. Based on the difference between the stoichiometry for the reaction between crocin and each of the two hypervalent forms of myoglobin, it is concluded in agreement with the determined half peak reduction potentials, that the crocin cation radical is less reducing compared to crocin, as the cation radical can reduce the protein radical but not the iron(IV) centre in hypervalent myoglobin.