Kinetics and mechanism of reduction of ferrylmyoglobin by ascorbate and D-isoascorbate

Reduction of iron(IV) in ferrylmyoglobin by ascorbate (and D-isoascorbate) is fast; at neutral or moderately acidic pH there are three parallel reactions: (i) a bimolecular reaction with 2.7 +/- 0.8 M(-1). s(-1), Delta H# = 45 +/- 13 kJ . mol(-1), and Delta S# = -85 +/- 46 J . mol(-1). K-1; (ii) a bimolecular reaction between ascorbate and protonated ferrylmyoglobin (28 +/- 5 M(-1). s(-1), Delta H# = 31 +/- 8 kJ . mol(-1), and Delta S# = 113 +/- 28 J . mol(-1). K-1); and (iii) an intramolecular electron transfer from ascorbate bound to protonated ferrylmyoglobin (7.5 +/- 0.1 s(-1), Delta H# = 89.6 +/- 0.9 kJ . mol(-1), Delta S# = 72 +/- 3 J . mol(-1). K-1); rate constants and activation parameters are for aqueous 0.16 M NaCl solutions at 25.0 degrees C. Binding of ascorbate has Delta H- = -43 +/- 3 kJ . mol(-1) and Delta S- = -100 +/- 9 J . mol(-1). K-1, and dissociation of protonated ferrylmyoglobin pK(a) = 5.34 +/- 0.02 (Delta H- = 11 +/- 2 kJ . mol(-1), Delta S- = -66 +/- 7 J . mol(-1). K-1). One-electron transfer dominates to yield iron(III) metmyoglobin, and ascorbyl radical was detected by ESR spectroscopy. D-Isoascorbic acid was found to be slightly more acidic (pK(a) = 3.706 +/- 0.003, Delta H- = 5.7 +/- 0.5 kJ . mol(-1), Delta S- = -52 +/- 2 J . mol(-1). K-1) than ascorbic acid (pK(a) = 3.772 +/- 0.003, Delta H- = 5.8 +/- 0.4 kJ . mol(-1), Delta S- = -53 +/- 1 J . mol(-1). K-1), while D-isoascorbate had reducing properties similar to those of ascorbate (cyclic voltammetry and stopped-flow spectroscopy).