Electron transfer in ruthenium/zinc porphyrin derivatives of recombinant human myoglobins. Analysis of tunneling pathways in myoglobin and cytochrome c

Abstract

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 A, His70; 12.7 A, His48; and 15.5 A, 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 X IO' (His'lO), 7.2 X lo4 (His48), and 4.0 X lo2 s-I (His83) (-AGO = 0.82 eV); charge-recombination (Ru2+ -ZnP') rates are 1.1 X lo5 (His48) and 7.3 X lo2 s-I (His83) (-AGO = 0.96 eV).Activationless (maximum) rates assuming h = 1.3 eV are 7.2 X lo7 (His70), 3.3 X lo5 (His48). and 1.8 X lo3 s-l (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.

Electron Tunneling in Proteins: Coupling Through a β Strand

“…This pathway can be described in terms of an effective covalent tunneling length (uI), and ET rates should drop exponentially with cr, rather than with the direct distance between redox sites. A coupling decay of 0.6 per covalent link results in an exponential decay constant of 0.73 A-' (8).…”

“…We measured ET rates in eight different Ru(bpy)2(im)(HisX)-modified cytochromes c (cyt c) (8,14). However, in contrast to the case with Ru-modified azurin, there is no uniform secondary structure type separating the heme and the seven Ru-binding sites (Fig.…”

Multistep Electron Transfer in Oligopeptides: Direct Observation of Radical Cation Intermediates

“…The rate constants k intra (Table 1) decrease by a factor of only about two when the spacer between the aromatic redox centers is elongated by addition of two proline units (compare 1 b with 1 c and 1 c with 1 d ). For an electron transfer through the peptide bonds one would expect a rate decrease of about 500 for two additional proline units 18. This indicates that the electron transfer does not proceed by a through‐bond mechanism.…”

Coupling of Electron Transfer and Protein Dynamics

“…In the subsequent paper [28] this model has been improved by considering multiple parallel pathways. But the experimental curves for mb and cyt-c in the coordinates logKemax -a1 are described by a considerably different dependence.…”