Kinetics and mechanisms of electron transfer between blue copper proteins and electronically excited chromium and ruthenium polypyridine complexes

The kinetics of the quenching of the long-lived excited states of CrL_3^(3+) and RuL_3^(2+) complexes (L is 1,10-phenanthroline and 2,2’-bipyridine or substituted derivatives) by the copper proteins plastocyanin, azurin, and stellacyanin have been studied in aqueous solution. The rate constants for quenching by the Cu(I) proteins approach a limiting value of ~10^(6) s^(-1) at high protein concentration. The kinetic behavior for plastocyanin is discussed in terms of a model in which the metal complex binds at a remote site 10-12 Å from the copper center. The model allows for electron transfer both from this remote site and by attack of the metal complex adjacent to the copper center. The results show that at low protein concentration the adjacent pathway is about 10 times faster than the remote pathway. The rate constant for the intramolecular electron transfer from the remote site is consistent with the value expected on the basis of theoretical calculations