Interactions of Electronically Excited Copper(II)−Porphyrin with DNA: Resonance Raman Evidence for the Exciplex Formation with Adenine and Cytosine Residues

Molecular complexes between the water-soluble cationic metalloporphyrin copper(II) 5,10,15,20-tetrakis[4-(N-methylpyridyl)]porphyrin (CuP) and a series of DNA-model single-stranded homopolynucleotides have been studied by resonance Raman spectroscopy as concerns their ability to form exciplexes under high-power nanosecond laser irradiation. Highly efficient exciplex formation is found for adenine-containing poly(dA) and cytosine-containing oligo(dC)9. In contrast to thymine, uracil, and cytosine, adenine has no exocyclic CO group which was presumed to participate in the CuP*CO exciplex formation, and consequently one of its basic endocyclic nitrogens (N1, N3, or N7) is proposed here to interact as an axial ligand with electronically excited CuP to form CuP*N exciplexes. The importance of an adequate geometry for CuP fixation to the polynucleotide for exciplex formation is also confirmed, since the exciplex is barely observed with poly(rA) and poly(rC), and even less in guanine-containing poly(dG) and poly(rG) despite the presence of C6O, N3, and N7 potential ligands.