LUMINESCENCE STUDIES OF THE INTERCALATION OF CU(TMPYP4) INTO DNA

Different types of DNA have been used to investigate binding interactions of Cu(TMpyP4), where TMpyP4 denotes the deprotonated form of meso-tetrakis(4-(N-methylpyridiniumyl))porphyrin. Physical methods employed include electronic absorption and circular dichroism as well as luminescence spectroscopy. Most of the studies have been carried out at mu = 0.2 M in a pH 7.8 Tris buffer at 25-degrees-C. With DNA samples containing both guanine-cytosine and adenine-thymine base pairs, our results confirm that Cu(TMpyP4) can bind at external sites-most likely within one of the grooves of DNA-or by intercalation (Pasternack, R. F.; Gibbs, E. J.; Villafranca, J. J. Biochemistry 1983, 22, 2406-2414). Moreover, our results show that the distribution between the two types of sites depends on the nucleotide-to-copper ratio and that intercalation is favored at moderate ratios. A guanine-cytosine base pair in combination with any other base pair seems sufficient to define an intercalation site. Novel emission with a lifetime of about 20 ns is observed from the tripdoublet and tripquartet excited states of intercalated Cu(TMpyP4). This emission is normally quenched by a mechanism which involves coordination of the solvent at an axial position of the copper center. Deactivation is proposed to occur via a low-energy d-d state of the five-coordinate complex. Solvent-induced quenching occurs when the complex is externally bound but not when the complex is intercalated because the axial coordination sites are blocked. The results are of interest because porphyrins are important DNA-binding agents and because solvent-induced quenching is becoming recognized as an important type of exciplex phenomenon that can occur in coordinatively unsaturated complexes.