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, Δ H # = 45 ± 13 kJ·mol - 1, and Δ S # = −85 ± 46 J·mol - 1 ·K - 1; (ii) a bimolecular reaction between ascorbate and protonated ferrylmyoglobin (28 ± 5 M - 1 ·s - 1, Δ H # = 31 ± 8 kJ·mol - 1, and Δ 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, Δ H # = 89.6 ± 0.9 kJ·mol - 1, Δ S # = 72 ± 3 J·mol - 1 ·K - 1 ); rate constants and activation parameters are for aqueous 0.16 M NaCl solutions at 25.0 °C. Binding of ascorbate has Δ H ⊖ = −43 ± 3 kJ·mol - 1 and Δ S ⊖ = −100 ± 9 J·mol - 1 ·K - 1, and dissociation of protonated ferrylmyoglobin p K a = 5.34 ± 0.02 (Δ H ⊖ = 11 ± 2 kJ·mol - 1, Δ 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 (p K a = 3.706 ± 0.003, Δ H ⊖ = 5.7 ± 0.5 kJ·mol - 1, Δ S ⊖ = −52 ± 2 J·mol - 1 ·K - 1 ) than ascorbic acid (p K a = 3.772 ± 0.003, Δ H ⊖ = 5.8 ± 0.4 kJ·mol - 1, Δ 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).