Ruthenated histidine

Figure 13. Schematic representation of a5Ru(His-33)-modified horse heart cytochrome c, showing the ruthenated histidine and the heme with its axial ligands (His-18 and Met-80). The ET distance (edge-edge) is 11.7 A (118).

Figure 14. Schematic representation of sperm whale myoglobin showing the ruthenated histidines 12, 48, 81, and 116, and the heme with its axial histidine. The edge-edge ET distances are shown for each histidine (118).

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. 

The Ru(NH3)j+ moiety can be attached to histidine-83 on the azurin surface. It can then be oxidized to Ru(III) without altering the conformation of the protein. This ruthenated protein is mixed with Ru(bpy)3+ and laser irradiated. The sequence of events which occurs is shown in the scheme... 

It is possible to introduce new redox groups into proteins using chemical and/ or biosynthetic methods. Attachment of mthenium to specific histidine residues on various electron transfer proteins has provided a method for the characterization of intramolecular electron transfer processes in proteins (77, 78). Studies of intramolecular electron transfer in ruthenated proteins are described in more detail in Section IV. 

Ruthenation appears to occur without perturbing the structure of the protein, except for perhaps local changes in the vicinity of the modified histidine. The electrochemical properties of the protein are only slightly altered by m-thenation. The E° value for the intrinsic redox cofactor of a protein typically increases by about 20 mV, consistent with the addition of a positively charged group to the protein. 

Figure 9.20 Pathways for election transfer in a modified cytochrome c. The diagram shows the structure of a ruthenated His-62 mutant yeast iso-1-cytochrome c, including the positions of some of the aminoacid residues (histidine, etc.). The His-62 group is ruthenated and electron transfer takes place from Ru. to the haem group (the position of the metal atom is marked by a cross). Possible pathways are shown by dashed and dotted lines. From Ref. [34,f. ...

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