A Ruthenium complex as a modifier of protein
1,10-Phenanthroline-5,6-dione (pdon) is a chelate ligand containing an o-quinoid moiety which has many interesting characteristics. Owing to its redox activity, pdon in a metal-free state and in complexes with transition metals (ruthenium, cobalt, osmium, iron, and nickel) shows strong electrocatalytic activity for the oxidation of NADH. Since pdon can interact via a diiminic binding site and through an o-quinoid group, it acts as a bridging ligand to construct binuclear or multinuclear complexes. In these complexes, an o-quinoid moiety coordinates directly with different metal centers. The carbon atom of pdon is liable to be attacked by a nucleophile. Lysine and arginine have a nucleophilic component in their side chains, thus, the amino group and guanidino group of a protein might be allowed to react with the quinone moiety of pdon as a nucleophile. Transition metal complexes, especially ruthenium complexes, have many attractive properties as chemical modifiers of proteins. A series of ruthenium-modified proteins, have been synthesized for the study of the electron transfer process in proteins and chemical modification using the ruthenium complex might be applicable to other unknown proteins .
The complex, [Ru(pdon)(bpy)2](ClO4)2 (bpy = 2,2'-bipyridyl) ( (ClO4)2 (Fig 1), was reacted with cytochrome c (cyt. c) as a model protein.
Fig. 1. [Ru(pdon)(bpy)2](ClO4)2
The guanidino or amino group of cyt. c might attack the carbon atom of the quinone moiety of 1, subsequently forming the adduct of 1 and cyt. c (Fig 2) .
Fig. 2. Adduct formation
In conclusion we described the reactivity of a quinone moiety of the ruthenium complex (ClO4)2 as a facile modifier of proteins. The modification of the 1 cation to cytochrome c was confirmed by mass spectroscopy. The increase in reactivity at higher pH can account for the deprotonation of amino groups or guanidino groups of cyt. c. The ratio of cyt. c to 1 in the product reacted at pH 9 was estimated to be 1:0.8 based on UV–vis spectroscopy. These results suggest that 1 might adapt to a simple modification of a metal complex on a protein surface under mild conditions .
Fig. 3. UV–vis spectra of 1 (dashed line), cyt. c (dashed and dotted line), and 1-modified cyt. c (solid line).