Kinetics and mechanism of electron transfer between meso-tetra sulphonated phenyl porphyrin iron(III)-apomyoglobin and Fe(EDTA)2- complexes

The kinetics of electron transfer between Fe(EDTA)2- and meso-tetra sulphonated phenyl porphyrin iron(III)-apomyoglobin have been studied by applying stopped-flow mixing and monitoring photometric changes at soret band (429 nm). The studies were carried out at pH's 6, 6.5, 7, 7.5, and 8 and at temperature between 10 and 40 degrees C. The mechanism proposed on the basis of the dependence of kobsd on Fe(EDTA)2- concentrations at various pH's, followed the rate equation: kobsd = ka[H+] + Kakb/[H+] + Ka.[Fe(EDTA)2-] The values of rate parameters calculated using a weighted non-linear least-squares analysis were: ka, 528 +/- 2 sec-1; kb, 25 +/- 1 sec-1; and Ka, 2.0 +/- 0.1 microM at 25 degrees C and 0.5 M sodium phosphate, and those of thermodynamic parameters calculated by the Eyring equation were: delta H*, 8.1 +/- 0.3 kcal mole-1 and delta S*, -23.4 +/- 1.1 eu at pH 7 and 0.5 M sodium phosphate.