The intercalation to DNA of bipyridyl complexes of platinum(II) with thioureas

The non-covalent interaction of the complexes [Pt(bpy)(R,R′NCSNR″,R‴)2]Cl2 (bpy = 2,2′-bipyridine; R = R′ = R″ = R‴ = H; R = Me, R′ = R″ = R‴ = H; R = n-Bu, R′ = R″ = R‴ = H; R = p-tolyl, R′ = R″ = R‴ = H; R = Et, R′ = H, R″ = Et, R‴ = H) with calf thymus DNA has been studied at pH 7 and 25 °C. The processes give rise to: (i) reversible bathochromic shifts and strong hypochromicity of the absorption bands of the complexes, (ii) induced circular dichroism and (iii) an increase both in the melting temperature and viscosity of the DNA comparable to that observed for other well known metallointercalators. The binding constants, KB, have been determined spectrophotometrically using the McGhee von Hippel equation. Plot of logKB vs −log[Na+] for the complex with unsubstituted thiourea gives a straight line with a slope value close to that expected for a dicationic intercalator. The binding affinity of the various complexes for DNA is independent of the thiourea nature; this suggests that the intercalation occurs through stacking of the bpy moiety while the ancillary ligands lie outside the nucleobases far away from the sugar phosphate backbone. The data show also that the electronic effects of the ligand substituents are not transmitted to the intercalating unit.