Intramolecular Electron Transfer of Cytochrome c Modified with N,N,N,N",N"-Diethylenetriaminepentaacetatocobaltate (III)

Horse heart cytochrome c (cytc) modified with an N,N,Nʹ,N",N"-diethylenetriaminepentaacetatocobaltate(III) ion, [cytcIII{CoIII(dtpa)}], was prepared and characterized. The major component of singly modified [cytcIII{CoIII(dtpa)}] contains a [CoIII(dtpa)] ion at Lys13, of which the (epsilon)-amino group is linked with a carboxylate ion of dtpa. A one-electron reduced protein, [cytcII{CoIII(dtpa)}], was prepared by reduction with a methylviologen-radical cation, which was produced in situ by a photoreduction using a tris(2,2ʹ-bipyridine)ruthenium(II) ion in the presence of disodium dihydrogen ethylenediaminetetraacetate at 25 °C, pH 7.5 (0.010 mol dm-3 tris(hydroxymethyl)aminomethane-HCl buffer), and an ionic strength of 0.50 mol dm-3 (NaCl). The reaction of [cytcII{CoIII(dtpa)}] to form [cytcIII{CoII (dtpa)}] was very slow, and was followed by conventional spectrophotometry. The reaction was of first order in the protein and the first-order rate constant ((3.1 (plus-minus) 0.5) × 10^-4 s^-1) was independent of the concentration of the protein, indicating that the intramolecular electron-transfer (ET) process is predominant. The reaction mechanism is discussed on a through-bond or through-space pathway. The slow intramolecular ET rate might arise not only from the long distance, but also from the large reorganization energy for the Co(III)/Co(II) couple.