We have shown since many years that the illumination of Ru-TAP complexes (TAP = 1, 4, 5, 8-tetraazaphenanthrene) in presence of guanine (G) bases either belonging to mononucleotides or DNA gives rise to formation of an irreversible covalent adduct between the nitrogen 2 of guanine and the carbone in ortho position of the non chelated nitrogen of TAP. The structure of this photo-adduct was determined previously by NMR. Several biological applications more particularly in gene-silencing, have been developed with these Ru-TAP complexes tethered to oligonucleotides. Photo-bridging can be obtained between such Ru-conjugates and G containing DNA or oligonucleotides. In the frame of these investigations, we discovered that photo-bridging can also be observed between two G bases of complementary oligonucleotides or DNA strands via a simple Ru-TAP complex (not tethered to an oligonucleotide). This means that not only a mono-adduct of a G base can be produced, but also a second addition of a second G base can take place on the same Ru complex. This very interesting photo-induced bridging process was a bit intriguing. Indeed when the first photo-adduct was isolated for its characterization, it revealed no luminescence, in contrast to the starting Ru-TAP complex, which is a good emitter. Therefore how would it be possible to add a second G moiety under illumination of the mono-adduct when this latter is very short-lived in the excited state. The study of the behaviour of the isolated mono-adduct will be presented and discussed in view of explaining its photo-induced oligonucleotide and DNA crosslinking observed by gel electrophoresis and AFM (Atomic Force Microscopy). A mechanism will be proposed and discussed as a function of the solution pH, based on kinetic studies under steady state and pulsed laser illumination from the femto and picosecond time domain to the nano and microsecond time-scale.

Covalent photo-adducts of Ru-TAP complexes with DNA guanine bases: Their applications and mechanisms of formation

249th National Meeting (2015)

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