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 (a5Ru) 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 3ZnP*→ Ru3+ rate constants are 1.6 × 107 (His70), 7.2 × 104 (His48), and 4.0 × 102 s−1 (His83) (−ΔG° = 0.82 eV); charge-recombination (Ru2+→ZnP+) rates are 1.1 × 105 (His48) and 7.3 × 102 s−1 (His83) (−ΔG° = 0.96 eV). Activationless (maximum) rates assuming λ = 1.3 eV are 7.2 × 107 (His70), 3.3 × 105 (His48), and 1.8 × 103 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.
ASJC Scopus subject areas
- Colloid and Surface Chemistry