Picosecond time-resolved infra-red spectroscopic study of a water-soluble cationic copper-porphyrin with nucleic...

The greater abundance and lower cost of copper make its porphyrin derivatives an attractive alternative to precious-metal based photosensitisers. The Cu(II) complex of 5,10,15,20-meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (CuTMPyP4) is a useful photophysical probe for biomolecules. It is non-luminescent in aqueous solution, as it forms a metastable five-coordinate water complex after photo-excitation. CuTMPyP4 is known to bind to DNA both through the grooves and by intercalation. In this paper picosecond transient absorption (TA) and time-resolved infra-red (TRIR) spectra of CuTMPyP4 in D₂O and in the presence of polydeoxythymidylic acid (poly(dT) and the double-stranded oligonucleotides {d(GC)₅}₂ and {d(CGCAAATTTGCG)}₂ are reported. These spectra show characteristic features depending on whether a five-coordinate transient species or the four-coordinate triplet excited state is formed. Notably, in the case of thymine-containing nucleic acids the TRIR method unambiguously demonstrates the binding of the porphyrin to the C₂O carbonyl group of that nucleobase.