A novel metallic complex as photoreagent for the DNA guanine bases: Osmium(II) tris(tetraazaphenanthrene)

Abstract

A new Os^II complex, Os(tap)₃²⁺ (tap=1,4,5,8-tetraazaphenanthrene) has been prepared and characterized. Its spectroscopic properties in absorption and luminescence and its electrochemical behaviour are compared with those of Ru(tap)₃²⁺ and Os(phen)₃²⁺. The analysis of the dependence of the luminescence lifetime of Os(tap)₃²⁺ as a function of temperature shows that the ³MC state (metal centred) is not reached at room temperature; therefore the complex is photostable. In the presence of GMP (guanosine 5′-monophosphate), the emission of Os(tap)₃²⁺ is quenched with a rate constant of 6×10⁸ dm3 mol^-1 s^-1. A photoinduced electron transfer, evidenced by laser flash photolysis, is shown to be the origin of this inhibition process. Moreover this primary charge transfer induces the formation of a photoproduct, as observed by absorption spectroscopy and HPLC; this photoproduct corresponds to an adduct of the complex to GMP without loss of ligand. Os(tap)₃²⁺ should thus behave as a new photoreagent for the DNA guanine bases without the disadvantages caused by photoinstability, as observed with Ru(tap)₃²⁺.

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