DNA-Binding and Physical Studies of Pt(4′-NR₂-trpy)CN⁺ Systems (trpy = 2,2′:6′,2′′-Terpyridine)

This paper focuses on DNA-binding interactions exhibited by Pt(dma-T)CN⁺, where dma-T denotes 4′-dimethylamino-2,2′:6′,2′′-terpyridine, and includes complementary studies of the corresponding pyrr-T complex, where pyrr-T denotes 4′-(N-pyrrolidinyl)-2,2′:6′,2′′-terpyridine. The chromophores are useful for understanding the interesting and rather intricate DNA-binding interactions exhibited by these and related systems. One reason is that the terpyridine ligands employed provide intense visible absorption and enhanced photoluminescence signals. Incorporating cyanide as a coligand further aids analysis by suppressing covalent binding. Physical methods utilized include X-ray crystallography for structures of the individual inorganic complexes. Viscometry as well as spectral studies of the absorbance, emission, and circular dichroism (CD) yield information about interactions with a variety of DNA hosts. Although there is no sign of covalent binding under the conditions used, most hosts exhibit two phases of uptake. Under conditions of high loading (low base-pair-to-platinum ratios), the dma-T complex preferentially binds externally and aggregates on the surface of the host, except for the comparatively rigid host [poly(dG-dC)]₂. Characteristic signs of the aggregated form include a bisignate CD signal in the charge-transfer region of the spectrum and strongly bathochromically shifted emission. When excess DNA is present, however, the complex shifts to intercalative binding, preferentially next to GC base pairs if available. Once the complex internalizes into DNA it becomes virtually immune to quenching by O₂ or solvent, and the emission lifetime extends to 11 μs when [poly(dI-dC)]₂ is the host. On the other hand, the host itself becomes a potent quenching agent when GC base pairs are present because of the reducing strength of guanine residues.