Electron-transfer kinetics of zinc-substituted cytochrome c and its Ru(NH3)5(histidine-33) derivative

The kinetics of the bimolecular electron transfers from triplet-excited zinc-substituted horse heart cytochrome c (Zn-cyt c*) to Ru(NH/sub 3/)/sub 5/L/sup 3 +/ have been measured (L = NH/sub 3/, k(298 K) = 1.4 x 10/sup 7/ M/sup -1/ s/sup -1/; L = histidine, k(298 K) = 2.4 x 10/sup 7/ M/sup -1/ s/sup -1/), along with those of the corresponding thermal back-reactions (L = NH/sub 3/, k(298 K) = 1.5 x 10/sup 8/ M/sup -1/ s/sup -1/; L = histidine, k(298 K) = 3.6 x 10/sup 8/ M/sup -1/ s/sup -1/). The derivatized metalloprotein ruthenium pentaammine histidine-33-zinc-substituted cytochrome c (Ru-Zn-cyt c) has been prepared and characterized by atomic absorption spectroscopy and HPLC analysis of its tryptic digestion fragments. The rate of intraprotein electron transfer from the triplet-excited Zn-porphyrin moiety to the 11.8-A distant Ru(NH/sub 3/)/sub 5/(His-33)/sup 3 +/ residue (k(298 K) = 7.7 x 10/sup 5/ s/sup -1/), as well as that of the thermal back-reaction (k(298 K) = 1.6 x 10/sup 6/ s/sup -1/), has been measured by transient spectroscopy. Intraprotein electron transfer from Ru(II) quenches Zn-cyt c* with a rate constant of 2.4 x 10/sup 2/ s/sup -1/. These kinetics are discussed in terms of the semiclassical theorymore » of electron-transfer reactions.« less