Horse-heart (hh) cytochrome c, modified at His-33, and Candida krusei (Ck) cytochrome c, modified at His-39, with a series of cis- and trans-[L(NH3)(4)Ru(III)cyt c] derivatives, where L is isonicotinamide (isn) or pyridine (py), have been prepared and characterized. Rate constants for intramolecular electron transfer from the heme(II) to Ru(III) in the [L(NH3)(4)Ru(III)cyt c(II)] intermediates generated by oxidative pulse radiolysis of the fully reduced modified protein species were as follows (k (s(-1)), Delta H-double dagger (kcal mol(-1)), Delta S-double dagger (cal deg(-1)mol(-1)), -Delta G degrees (eV)) : 440, 7.3, -22, 0.18 for L = trans-isn (hh); 440, 6.2, -26, 0.18 for L = cis-isn (hh); 126, 8.8, -19, 0.11 for L = trans-py (hh); 220, 6.4, -27, 0.13 for L = trans-isn (Ck); 154, 2.3, -41, 0.18 for L = NH3 (Ck). Relative differences in the rates are accounted for by variations in the driving force and reorganization energies in these ruthenium-modified proteins resulting from the nature of the ligands around the ruthenium center and from the different sites of modification on the cytochrome. The fully oxidized [L(NH3)(4)Ru(III)cyt c(III)] species undergo slow redox disproportionation reactions (k = 35 M(-1) s(-1), pH 7.0) which have been studied by optical and electrochemical methods. The Ru(IV) species thus created subsequently rearranges in an irreversible manner. In the presence of excess oxidant, all of the bound ruthenium is converted to the rearranged form. To avoid this problem, the [L(NH3)(4)Ru(III)cyt c(II)] intermediates for intramolecular electron-transfer studies were generated from the stable [L(NH3)(4)Ru(II)cyt c(II)] form.