INTRAMOLECULAR ELECTRON-TRANSFER IN PENTAAMMINERUTHENIUM(III)-MODIFIED COBALTOCYTOCHROME-C

The iron in the heme group of horse-heart cytochrome c was replaced by cobalt according to established methods. The resulting cobalticytochr ome c was subsequently modified at histidine-33 with a pentaammineruth enium group. Proof of correct derivatization was obtained by atomic ab sorption analysis of cobalt and ruthenium, differential pulse voltamme try, and enzymatic proteolysis analyzed by diode-array HPLC. Cobalt(II )-to-ruthenium(III) intramolecular electron transfer rates were measur ed as a function of temperature by electron pulse radiolysis. The azid e radical (N-3(.)) was used to oxidize the fully reduced form in order to generate the desired electron transfer precursor. The intramolecul ar electron transfer rate is 1.28 +/- 0.04 s(-1) at 25 degrees C (Delt a H double dagger = 5.7 +/- 0.2 kcal/mol, Delta S double dagger = -38. 7 +/- 0.5 cal/(deg mel)) for a driving force of 0.28 +/- 0.02 eV. The results are compared with those for analogous pentaammineruthenium-mod ified, native iron, and zinc-substituted cytochromes c. The 0.4 eV inc rease in driving force for intramolecular electron transfer when iron is replaced by cobalt is largely compensated by an increase in reorgan ization energy.