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| - Intramolecular Electron Transfer in Pentaammineruthenium(III)-ModifiedCobaltocytochrome c
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| - The iron in the heme group of horse-heart cytochrome cwas replaced by cobalt according to established methods.The resulting cobalticytochrome c was subsequentlymodified at histidine-33 with a pentaamminerutheniumgroup.Proof of correct derivatization was obtained by atomic absorptionanalysis of cobalt and ruthenium, differentialpulse voltammetry, and enzymatic proteolysis analyzed by diode-arrayHPLC. Cobalt(II)-to-ruthenium(III)intramolecular electron transfer rates were measured as a function oftemperature by electron pulse radiolysis.The azide radical (N3•) was used tooxidize the fully reduced form in order to generate the desiredelectrontransfer precursor. The intramolecular electron transfer rate is1.28 ± 0.04 s-1 at 25 °C(ΔH⧧ = 5.7 ± 0.2kcal/mol, ΔS⧧ = −38.7 ± 0.5 cal/(degmol)) for a driving force of 0.28 ± 0.02 eV. The results arecomparedwith those for analogous pentaammineruthenium-modified, native iron,and zinc-substituted cytochromes c. The0.4 eV increase in driving force for intramolecular electron transferwhen iron is replaced by cobalt is largelycompensated by an increase in reorganization energy.
- Intramolecularcobalt(II)-to-ruthenium(III) electron transfer was studied incobalt-substituted horse-heart cytochrome c modified with(NH3)5Ru. The rate is 1.28 ± 0.04s-1 at 25 °C (ΔG° = 0.28eV). Comparisons are made with analogous ruthenium-modified nativeiron and zinc-substituted cytochromes c. The opposingeffects of increased driving force and reorganization energy in thecobalt case (compared to iron) arequantified.
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