Time-resolved resonance raman study of the exciplex formed between excitedCu-porphyrin and DNA

The photoinduced reversible process of exciplex formation and decay between the water-soluble cationic metalloporphyrin 5,10,15,20-tetrakis[4-(N-methy lpyridyl)] (Cu(T4MPyP)) and calf-thymus DNA has been studied by a picosecon d time-resolved resonance Raman (ps-TR3) technique. For a detailed analysis of the exciplex properties, the following model compounds have also been i nvestigated: double-stranded polynucleotides poly(dA-dT)(2), poly(dG-dC)(2) , and poly(dA-dC). poly(dG-dT), single-stranded poly(dT), and the 32-mer d[ (Gr)(7)ATAT(GC)(7)](2). Additional Raman measurements have also been done i n using cw and 20-ns laser sources. It is shown that this reversible excipl ex is formed, with a yield depending on the nucleic base sequence, in less than 2 ps after photoexcitation, between photoexcited ru(T4MPyP) and C=O gr oups of thymine residues in all thymine-containing sequences of nucleic aci ds. Such a rapid exciplex building process implies that it involves porphyr in molecules initially located, in the steady state of this interaction, at AT sites of the nucleic acids. This has two main consequences, which contr adict previously reported assumptions (Strahan et al,, J. Phys. Chem. 1992, 96, 6450): (i) although the binding mode of the porphyrin actually depends on the base sequence, there is no preferential binding of ru(T4MPyP) to th e various sites of DNA, and (ii) there is no photoinduced ultrafast porphyr in translocation from GC to AT sites of DNA. In addition, it is shown that with surrounding water molecules an exciplex can also be formed in similar to1 ps, whose spectral characteristics are not distinguishable from those f ormed with thymine residues. However, these two exciplex species can be dis tinguished from each other by their relaxation kinetics: the lifetime of th e exciplex formed with water lies in the 3-12 ps range, while that of the e xciplex formed with nucleic acids lies in the nanosecond time domain (1-3 n s). A set of possible routes is discussed for each of the exciplex building /decay processes.