SUBSTITUTED TETRAAMMINERUTHENIUM CYTOCHROME-C DERIVATIVES - CHEMISTRYAND ELECTRON-TRANSFER REACTIONS

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 constant s 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-dou ble 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 dif ferences in the rates are accounted for by variations in the driving f orce and reorganization energies in these ruthenium-modified proteins resulting from the nature of the ligands around the ruthenium center a nd from the different sites of modification on the cytochrome. The ful ly oxidized [L(NH3)(4)Ru(III)cyt c(III)] species undergo slow redox di sproportionation reactions (k = 35 M(-1) s(-1), pH 7.0) which have bee n studied by optical and electrochemical methods. The Ru(IV) species t hus 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.