Photo-reduction of polyazaaromatic Ru(II) complexes by biomolecules and possible applications

An important property of the Ru(II) complexes that contain polyazaaromatic ligands such as TAP (1,4,5,8-tetraazaphenanthrene) and HAT (1,4,5,8,9,12-hexaazatriphenylene) is the very high oxidation potential of the 3MLCT (Metal to Ligand Charge Transfer) state. Therefore, under illumination, they are able to abstract an electron even from rather poor electron donors. In this work, we show how it is possible to take advantage of this property for applications of photo-induced electron transfer between these excited complexes and biopolymers such as DNA, oligonucleotides and oligopeptides.One of the consequences of the charge transfer processes is photocleavage of plasmid DNA, demonstrated some years ago by agarose gel electrophoresis and more recently detected and quantified by AFM. Another effect of the photo-induced electron transfer is the formation of photo-adduct(s) of the metal complex with the guanine units of oligonucleotides or with the tryptophan moieties of oligopeptides. In the photo-adducts with the guanine nucleobases, a covalent bond is formed between one of the polyazaaromatic ligands and the guanine base without destruction of the ligands sphere around the metal ion. Interestingly, the formation of these photo-adducts can be exploited in order to photo-crosslink two single-stranded oligonucleotides. Such photo-crosslinking via the metal complexes could lead to applications in biological diagnostic tests and possibly in vivo in the area of gene phototherapy.