Resonance Raman Study on Photoinduced Interaction of Water-Soluble Cu-Porphyrin with Short Oligothymidylates

Abstract

It is known that water-soluble copper(II) porphyrin Cu(TMpy-P4) interacts in a specific manner with DNA and some DNA-model poly- and oligonucleotides creating a photoinduced exciplex in the excited state with CO group of thymine [1]. However, the questions remained to be solved about the minimal length of nucleotide and possible role of a secondary structure of polymer required for the exciplex formation. To bring more light into the problem, spectroscopic studies on CuP in interaction with a series of short oligothymidylates d(pT)n, n=1.¨4, has been performed. Picosecond transient absorption (TA) study of these complexes has been reported previously [2, 3]. In this work, we present results of Raman characterization of photoinduced intermolecular interactions between CuP and short oligothymidylates, as well as comparative study of CuP in water and in interaction with poly(dA-dT)2. Analysis of transient Raman spectra (Figure 1), together with TA data obtained previously, revealed the existence of several different types of exciplex formed after photoexcitation. We can distinguish excited CuP complexes with water and CO group of thymine which, in general, can coexist in a solution. Exciplex species were detected in transient RR spectra as new bands shifted in the low-frequency region relatively to their ground state counterparts. In the case of CuP-water interaction (Figure 1, curve A) intensity of v2 * and v4* exciplex bands at 1552 and 1348 cm−1, respectively, is not more than 10% of v2 and v4 ground state ones. For CuP-d(pT)n complex substantial increase of the intensity of transient bands is observed in comparison with CuP-water exciplex (Figure 1, curves B-E). Exciplex bands grows continuously with increasing of the number of thymine residues in oligonucleotides. As for CuP in poly(dA-dT), excited complex bands dramatically increase in intensity and become dominate in transient RR spectra (Figure 1, curve F).