Electron Transfer in Ruthenium/Zinc Porphyrin Derivatives of Recombinant Human Myoglobins. Analysis of Tunneling Pathways in Myoglobin and Cytochrome c

Site-directed mutants of human myoglobin have been prepared and characterized; each protein has a single surface-modifiable histidine (at position 48, 70, or 83). The proteins were modified by covalent attachment of pentaammineruthenium (a_5Ru) to the surface histidine and substitution of zinc mesoporphyrin IX diacid (ZnP) for the heme. Donor-acceptor separations (edge-edge distances d) in the modified proteins are 9.5 Å, His70; 12.7 Å, His48; and 15.5 Å, His83. Rates of photoinduced electron transfer in these ruthenium-modified myoglobins were measured by transient absorption spectroscopy. The ^(3)ZnP* → Ru^3+ rate constants are 1.6 × 10^7 (His70), 7.2 × 10^4 (His48), and 4.0 × 10^2 s^-1 (His83) (-ΔG°= 0.82 eV); charge-recombination (Ru^2+ → ZnP^+) rates are 1.1 × 10^5 (His48) and 7.3 × 10^2 s^-1 (His83) (-ΔG° = 0.96 eV). Activationless (maximum) rates assuming h = 1.3 eV are 7.2 X lo7 (His70), 3.3 × 10^5 (His48) and 1.8 × 10^3 s^-1 (His83). Distant electronic couplings, which limit the maximum rates in the modified myoglobins, have been analyzed along with data from Ru-modified cytochromes c in terms of a tunneling pathway model. Single dominant pathways adequately describe the electronic couplings in cytochrome c but do not satisfactorily account for the myoglobin couplings. The correlation of electronic coupling with tunneling length for myoglobin is improved significantly by the inclusion of multiple pathways.