Observation of two-mode binding to DNA by bipyridyl-(ethylenediamine)platinum(II): Isothermal titrational calorimetry and midinfrared absorption studies

The binding of bipyridyl-(ethylenediamine)platinum(II) to calf-thymus DNA has been studied in solution via isothermal titration calorimetry and in unoriented films via midinfrared spectroscopy. The calorimetric data reveal that the ligand binds to DNA at two different sites with the first binding site being filled by about one ligand for every five base pairs. The binding is entropically driven: +25 cal/mol K for the first site and +22 cal/mol K for the second site. Midinfrared absorption data (400--1800 cm^-1) from unoriented films show no dependence on the ligand content at relative humidities of 23% and 80%.