BIS-(DIPICOLINATE) COMPLEXES OF COBALT(III) AND IRON(II) AS NEW PROBES OF METALLOPROTEIN ELECTRON-TRANSFER REACTIVITY - ANALYSIS OF REACTIONS INVOLVING CYTOCHROME-C AND CYTOCHROME-C551

The electron-transfer reactions of horse heart cytochrome c and Pseudomonas aeruginosa cytochrome c551 with bis(dipicolinato)cobaltate(III) (Co(dipic)2-) and bis(dipicolinato)ferrate(II) (Fe(dipic)22-) have been studied and found to demonstrate second-order kinetic behavior. The rate constants (M-1 s-1) [25 °C, µ = 0.2 M, pH 7.0 (phosphate)], ΔH╪ (kcal/mol), and ΔS╪ (eu) for these reactions are: 1.11 (4) X 104, 4.4 (8), and -25 (3) for ferrocytochrome c and Co(dipic)2-; 1.5 (3) X 104, 7.2 (3), and -15.4 (1.2) for ferricytochrome f and Fe(dipic)22-; 5.8 (1) X 102, 3.6 (6), and -34 (2) for ferrocytochrome c551 and Co(dipic)2; and 3.21 (4) X 103. 9.6 (6), and -10(2) for ferricytochrome c551 and Fe(dipic)?2 2-. The calculated electrostatics-corrected protein self-exchange rate constants (k11corr) based on the cytochrome/Fe(dipic)22- kinetic data are over an order of magnitude greater than those previously obtained based on Fe(EDTA)2-, consistent with the conclusion that the presence of the hydrophobic, β-conducting pyridine ligand in the complex promotes electron transfer by permitting reagent penetration toward the metal center in the protein. The k11corr values based on the oxidant Co(dipic)2- are substantially larger than those based on Fe(dipic)22-, suggesting that the electron-transfer reactivity of Co(dipic)2- is enhanced by protein-reagent interactions. © 1979, American Chemical Society. All rights reserved.