The kinetics of oxyferryl (Fe-IV=O) heme reduction in horst heart myoglobin (Mb) by a(4)LRu(II) (a = NH3; L = NH3, pyridine, isonicotinamide) bound at the surface His48 were investigated with pulse radiolysis. The observed first-order rate constants (k(obs1)) decreased with increasing pH and reduction potential for the a(4)LRu centers (E degrees/ Ru-III/II = 77, 330, and 400 mV for L = NH3, Pyr, and Isn). Rate-pD data obtained in D2O for the a(4)LRu derivative revealed the presence of an equilibrium isotope effect, and a pK(a) of 5.7 (6.2 in D2O) was obtained for the acid-base group, which is assigned to the distal His64. A mechanism where protonation precedes ET provides a good fit of the kinetic data for the three a(4)LRu derivatives. Marcus theory analysis of the k(ET) (0.74, 1.8, and 3.6 s(-1) for L = Isn. Pyr, and NH3) extracted from the k(obs1) values yielded a reorganization energy (lambda) of 1.8 for Ru-II --> Fe-IV=O ET in the a(4)LRu derivatives but a lambda of 2.1 eV for the a(5)Ru derivative, From the latter, it is concluded that ET is strongly gated in the a(5)Ru derivative, and this is assumed to be the major reason for the low reactivity of Fe-IV=O in Mb at high -Delta G degrees.