Diorganotin(IV) and triorganotin(IV) complexes of meso tetra(4 sulfonatophenyl)porphine: do they bind DNA?

It was observed that organometallic porphyrin systems, where the Sn(IV) residue is in side chains ,coordinated via sulphonatophenyl groups of porphyrin, show interesting and peculiar in vitro activity, in agreement with the anti-tumour activity of organotin complexes. In particular, it has been shown that dibutyl- and tributyl-tin(IV) derivatives of meso-tetra-(4-sulfonatophenyl)porphine are cytotoxic and induce apoptosis in A375 human melanoma cells. Moreover, anionic porphyrins are selective G-quadruplex binders with important applications in studies on telomerase inhibition. To contribute to enlighten the possible mechanism of action of these organometallic species, we analysed the interaction of the diorgano- and triorgano-tin(IV) derivatives of meso-tetra-(4-sulfonatophenyl)porphine to natural (calf thymus) DNA. Free meso-tetra-(4- sulfonatophenyl)porphine was also studied for comparison purposes. Fluorescence and absorbance titrations with relevant calculation of binding constants and analysis of their temperature dependence, fluorescence quenching and viscometric studies indicate that the drug binding to DNA occurs indeed, this having principally the features of external binding (also in case of the less hindered species). On the other hand, interestingly, the binding produces non negligible viscosity variations that might be related to polynucleotide conformational changes induced by the organometallic derivative.