Interaction between the DNA model base 9-ethylguanine and a group of ruthenium polypyridyl complexes: Kinetics and conformational temperature dependence

The binding capability of three ruthenium polypyridyl compounds of structural formula [Ru(apy)(tpy)Ln-](ClO4)((2-n)) [1a-c; apy = 2,2'-azobis(pyridine), tpy = 2,2':6',2 ''-terpyridine, L = Cl, H2O, CH3CN] to a fragment of DNA was studied. The interaction between each of these complexes and the DNA model base 9-ethylguanine (9-EtGua) was followed by means of H-1 NMR studies. Density functional theory calculations were carried out to explore the preferential ways of coordination between the ruthenium complexes and guanine. The ruthenium-9-EtGua adduct formed was isolated and fully characterized using different techniques. A variable-temperature H-1 NMR experiment was carried out that showed that while the 9-EtGua fragment was rotating fast at high temperature, a loss of symmetry was suffered by the model base adduct as the temperature was lowered, indicating restricted rotation of the guanine residue.