Intramolecular Electron-Transfer Pathway for Deoxy and Zinc Myoglobins Modified with N,N,N,N",N"Diethylenetriaminepentaacetatocobaltate(III)

Horse heart metmyoglobin (metMb), whose N-terminal Gly1 is linked by an N,N,Nʹ,N",N"-diethylenetriaminepentaacetatocobaltate(III) ion ([CoIII(dtpa)]) with an amide bond, was prepared and characterized. A one-electron reduced protein, [deoxyMb{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 (a 0.010 mol dm^-3 tris(hydroxymethyl)aminomethane–HCl buffer), and an ionic strength of 0.50 mol dm^-3 (NaCl). The reaction of [deoxyMb{CoIII(dtpa)}] to form [metMb{CoII(dtpa)}] obeyed a first-order rate law on the protein concentration. The first-order rate constant was dependent on the concentration of the protein, indicating that both intra- and intermolecular electron-transfer (ET) processes simultaneously occur. The latter is the reaction with excess [metMb{CoIII(dtpa)}] remaining. Both intra- and intermolecular ET rates could be explained by the Marcus theory, including the distance dependence of the rate of the reaction, and might arise mainly from the very large reorganization energy for the Co(III)/Co(II) couple. Zinc-substituted myoglobin, [ZnMb{CoIII(dtpa)}], was also prepared and the photoinduced ET reaction from the excited triplet state of zinc myoglobin to the Co(III) moiety was examined. The observed ET quenching rate constant is reasonably explained by the Marcus theory through the same ET pathway as that for [deoxyMb{CoIII(dtpa)}]. It is suggested that the ET occurs with through-bond and van der Waals interactions from the heme edge to the Co(III) edge via Phe138 and Leu137 over 18.8 A (1 A = 1 × 10^-10 m) both in deoxy and zinc myoglobins.