Ruthenium-modified myoglobin

A similar study was performed on ruthenium-modified myoglobins, in which AG variations were obtained by changing the nature of the ruthenium complex covalently bound to the protein, and by substituting a porphyrin to the heme [137]. It is gratifying to observe that, in spite of the rather heterogeneous character of this series, the study leads to an estimation of 1.9 to 2.4 eV for A which is consistent with the value 2.3 eV derived in section 3.2.1 from temperature dependent experiments. Satisfactory agreement between the results given by the two methods is also observed in the case of ruthenium-modified cytochrome c [138]. 

Fig. 8. A view into the interior of a ruthenium modified myoglobin where the amino acids in the vicinity of Trp-14 are shown. The dots correspond to the statistieal density Pn i,(r) of (discretized) tunneling path vertices (rj in Eq. 26) from 500,000 tunneling paths [19], The (r) is clustered in a cylindrical zone centered on the average path, shown as the light line appearing in the center and emerging toward the viewer. The computation modeled paths of electron transfer in Ru(His-12) myoglobin studied experimentally by Gray and coworkers [88]...

Sugiyama Y, Takahashi S, Ishimori K, Morishima I. Pressure effects on electron transfer rates in zinc/ruthenium modified myoglobin. J Am Chem Soc 1997 119 9582-3. 

The long-range electron transfer reactions in ruthenium-modified myoglobin, in which the labile heme unit has been replaced by various metalloporphyrins, have been reviewed. The reductions of the [Ru(NH3)5] moiety, attached at His-48, by Pd- and Pt-substituted hemes in myoglobin proceed at rates of 9 1 x 1(P and 1.2x lO s", respectively. The difference in rates for electron transfer between Fe (heme) and Mg or Zn(porphyrin) centers in [a(Fe(II)P),j3(M T)] hemoglobin hybrids indicates a direct process as opposed to the involvement of a conformational gate. Using [Co(NH3)5Cl] to quench the Zn state, a rate constant of 2.4 x 10 s has been measured for back electron transfer within [a(Zn- -P)i8(Fe(III)CN)]. ... 

Figure 8 Four quantum paths (depicted in red, yellow, orange, and green) for the tunneling electron in ruthenium-modified myoglobin sampled from 5 x 1(P paths of a Monte Carlo run. The protein is in light blue. The heme and ruthenium redox centers are separated by 28.2 A center-to-center. Adapted from Ref. 53...

Figure 2.8. The Gibbs energy optimized ET rate vs. edge-to-edge distance relationship for intraprotein electron transfer. The bacteria RC rate constants are shown as circles and excited heme-ruthenium ET in modified myoglobin and cytochrome c are shown as triangles (Moser and Dutton, 1992). Reproduced with permission.

Conformational changes could control ET reactions in proteins. The rates of such changes often are in the same range as ET rates for example, the T-R transition in hemoglobin occurs at a rate of approximately 2 X 10 s (112). Hoffman and Ratner (66,67) have pointed out that a way to test for conformational control of an ET reaction is to measure the reaction rate at different driving forces. If the rate stays the same, the ET reaction is conformationally controlled. If it does not, it is not confor-mationally controlled. No evidence for conformation control exists for ET in ruthenium-modified proteins on this basis. Data from both ruthenated His-33 in horse heart cytochrome c (126) and ruthenated His-48 in myoglobin (103) show that the rate changes with AG° in a manner consistent with Marcus theory. 

A variety of physical methods has been used to ascertain whether or not surface ruthenation alters the structure of a protein. UV-vis, CD, EPR, and resonance Raman spectroscopies have demonstrated that myoglobin [14, 18], cytochrome c [5, 16, 19, 21], and azurin [13] are not perturbed structurally by the attachment of a ruthenium complex to a surface histidine. The reduction potential of the metal redox center of a protein and its temperature dependence are indicators of protein structure as well. Cyclic voltammetry [5, 13], differential pulse polarography [14,21], and spectroelectrochemistry [12,14,22] are commonly used for the determination of the ruthenium and protein redox center potentials in modified proteins. 

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