Protein binding in deactivation of ferrylmyoglobin by chlorogenate and ascorbate

Kinetics of reduction of iron(IV) in ferrylmyoglobin by chlorogenate in neutral or moderately acidic aqueous solutions (0.16 M NaCl) to yield metmyoglobin was studied using stopped flow absorption spectroscopy. The reaction occurs by direct bimolecular electron transfer with (2.7 +/- 0.3) x 10(3) M-1.s(-1) at 25.0 degrees C (Delta H-# = 59 +/- 6 kJ mol(-1), Delta S-# = 15 +/- 22 J.mol(-1) K-1) for protonated ferrylmyoglobin (pK(a) = 4.95) and with 216 +/- 50 M-1 s(-1) (Delta H-# = 73 +/- 8 kJ . mol(-1), Delta S-# = 41 +/- 30 J.mol(-1).K-1) for nonprotonated ferrylmyoglobin in parallel with reduction of a chlorogenate/ferrylmyoglobin complex by a second chlorogenate molecule with (8.6 +/- 1.1) x 10(2) M-1 s(-1) (Delta S-# = 74 +/- 8 kJ mol-l, Delta S-# = 59 +/- 28 J mol(-1) K-1) for protonated ferrylmyoglobin and with 61 +/- 9 M-1.s(-1) (Delta H-# = 82 +/- 12 kJ.mol(-1), Delta S-# = 63 +/- 41 J mol(-1) K-1) for nonprotonated ferrylmyoglobin, Previously published data on ascorbate reduction of ferrylmyoglobin are reevaluated according to a similar mechanism. For both protonated and nonprotonated ferrylmyoglobin the binding constant of chlorogenate is similar to 300 M-1, and the modulation of ferrylmyoglobin as an oxidant by chlorogenate (or ascorbate)leads to a novel antioxidant interaction for reduction of ferrylmyoglobin by ascorbate in mixtures with chlorogenate.