Ruthenium complexes histidine

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. 

In contrast to ferrocenes, osmium and ruthenium complexes are capable of forming coordinative bonds with donor centers of GO including histidine imidazoles. There are therefore two ways of bringing coordinated transition metals onto enzyme surfaces, i.e., via natural and artificial donor sites. Artificial centers are commonly made of functionalized pyridines or imidazoles, which must be covalently attached to GO followed by the complexation of an osmium or... 

Issues related to the preferred pathways and the distance dependence of electron transfer in biological systems have been addressed by covalently linking electron-transfer donors or acceptors (e.g. a ruthenium complex) to specific sites (e.g. a histidine) of a protein or an enzyme.The distance dependence of the electron-transfer rate constants is generally fitted to equation (44), with most values of for proteins falling in the range of 1.0 to 1.3 A The protein in... 

Intramolecular oxidation and reduction in cytochrome c complexes covalently modified was studied by several groups (for review see 190). Histidines (191, 192, 193) and cysteines (194) were used to attach covalently Ruthenium complexes to Fe- or Zn-substituted cytochrome c. Most of the experiments were done using laser lash photolysis. In each series of experiments, the distance was considered as constant and determined by molecular modelling. The free energies span between 0.5 to 1.4V. The L T rate constants do vary with the driving force as expected. However the reactions proceed with rate constants lower than those expected on the basis of results obtained on peptides. Results were all analyzed using Marcus theory. X and Hab were considered as adjustable parameters. Each series of experimental data was fitted separately (3 to 6 points). In all these papers, X values go from 1.15 to 1.22 eV and Hab vary from 0.1 to 0.24 cm l. Activation volumes were also measured (195). It seems that the transition state is more compact than the reactant state in both intra- and inter-molecular steps. 

In 1988, Gray and co workers reported electron transferrates between redox centers bound in large macromolecules as a function of the distance between the redox centers [8,35]. Electron transfer between ruthenium complexes [Ru(NH3)5Hist], for example, where Hist is a histidine group in a protein, was found to increase by a factor of 10 when the complexes were 2.1 A closer. This corresponds to a = 0.91 in Equation 9.9. 

Forward and reverse rate constants and activation parameters have been determined for the intramolecular electron transfer (13.0 A) in several ruthenium-modified (histidine 39) zinc cytochrome c complexes.These rate constants are 3 times as large as the values for the Ru(His-33) analogs (13.3 A), with slightly greater electronic coupling between the metal centers. The temperature dependence of the rate constant for electron transfer in the Ru(His-33) modified cytochrome c complex has been measured over the 10-200 K range.The rate is independent of temperature (3.6 s" ) between 10 and 100 K, above which two regions of Arrhenius-like behavior (transition at about 150 K) are observed. 

Histidine residues are, however, generally regarded as major possible binding sites for ruthenium-arene complexes in proteins. To model this interaction, we also studied the reaction of [RuCl(en)(rj6-bip)]+ (10) with L-histidine at 310 K in aqueous solution (91). The reaction was quite sluggish and did not reach equilibrium until 24 h at 310 K, by which time only about 22% of the complex had reacted. Two isomeric imidazole-bound histidine adducts could be discerned, with more or less equal binding of Ne... 

The standard ruthenium modification procedure involves the reaction of aquopentaammineruthenium(II) (ajRu " ) with the imidazole of a surface histidine of a protein [5, 13, 14]. The a5Ru(histidine)-modified proteins are stable in both the Ru(II) and the Ru(III) oxidation states and, although ajRu slowly dissociates from surface histidines [15], the ajRu complex stays attached for at least two months under appropriate conditions [16]. 

This general approach has, however, serious limitations. The position of the site for attack (and therefore the electron transfer distance involved) is very conjectural. In addition, the vexing possibility, which we have encountered several times, of a dead-end mechanism (Sec. 1.6.4) is always present. One way to circumvent this difficulty, is to bind a metal complex to the protein at a specific site, with a known (usually crystallographic) relationship to the metal site. The strategy then is to create a metastable state, which can only be alleviated by a discernable electron transfer between the labelled and natural site. It is important to establish that the modification does not radically alter the structure of the protein. A favorite technique is to attach (NH3)5Ru to a histidine imidazole near the surface of a protein. Exposure of this modified protein to a deficiency of a powerful reducing agent, will give a eon-current (partial) reduction of the ruthenium(III) and the site metal ion e.g. iron(III) heme in cytochrome c... 

Treatment with the nickel(II) complex of the tripeptide glycine-glycine-histidine in the presence of magnesium monoperoxyphthalate Visible light irradiation in the presence of tris(bipyridyl)ruthenium(II) dication and ammonium persulfate Ethylmercury phosphate Fluorescein... 

Different strategies have been used to attach transition metal complexes to proteins for example, the imidazole moiety of histidine can be coordinated to metal centres such as ruthenium(II), osmium(II) and rhenium(I). Many studies have utilised the imidazole of a histidine residue as a ligand for... 

Wang F, Bella J, Parkinson JA, Sadler PJ (2005) Competitive reactions of a ruthenium arene anticancer complex with histidine, cytochrome c and an oligonucleotide. J Biol Inorg Chem 10 147-155... 

Electron transport between cubane [Fe4S4] clusters, cytochrome c, or Cu + centers in blue copper proteins " and the periphery of the proteins has been examined by complexing ruthenium species to surface histidines. In the case of the iron sulfur cubane in Chromatium vinosum, four surface histidines served as points of ruthenium attachment. The rates of electron transport from the Fe4S4 core to ruthenium varied over two orders of magnitude and were used to diagnose the preferred channel for electron transport. Cysteine and lysine residues have also been used as binding sites in studies of cytochrome c and cytochrome P450 cam proteins. 

Applications. A biotinylated GOX-based biosensor was developed based on a new electropolymerized material consisting of a pol3rp3uidyl complex of ruthenium(II) functionalized with a pyrrole group [90]. Because histidine, lysine and arginine functions also coordinate Os /Os , biosensors based on co-electrodeposited GOX, HRP, soybean peroxidase (SBP) and laccase with redox Os /Os polymer have been developed [89]. A metal chelate formed by nickel and nitrilotriacetic acid was used to modify a screen-printed electrode surface. The functionalized support allowed stable attachment of acetylcholinesterase and the resulting biosensor was used for sensitive detection of organophosphorus insecticides [91]. This method is attractive because it ensures a controlled and oriented enzyme immobilization, considerably improving the sensitivity and the detection limit. 

A method for the study of ET from a protein metal center to a surface ruthenium is given in Scheme IV (103). In this method, [Ru(bpy)3] " acts as an oxidant, selectively removing an electron from a surface a5Ru(II)(histidine). A Ni-RBr scavenger system [Ni(II)hexamethyl-tetraazacyclododecane and an alkyl bromide] oxidizes the [Ru(bpy)3] before it can back react with the a5Ru(III)(histidine) complex. Electron transfer from the reduced protein metal center to the oxidized ruthenium can be monitored spectroscopically. 

---