INTERACTION OF ELECTRONICALLY EXCITED COPPER(II) PORPHYRIN WITH OLIGONUCLEOTIDES AND POLYNUCLEOTIDES - EXCIPLEX BUILDING PROCESS BY PHOTOINITIATED AXIAL LIGATION OF PORPHYRIN TO THYMINE AND URACIL RESIDUES

Complexes of a water-soluble metalloporphyrin Cu(TMpy-P4) (CuP) with a variety of synthetic oligonucleotides, single- and double-stranded po lynucleotides of different base composition and sequences, have been t ested by resonance Raman spectroscopy for the possibility to form an e xciplex under high-power pulsed laser excitation at 424 nm, i.e., in t he Soret band of the porphyrin. Such a photoinduced exciplex had been previously observed in the CuP interaction with poly(dA-dT), calf thym us DNA, and double-stranded oligomers containing at least four alterna ting A-T base pairs: it was associated with an A-T-specific stabilizat ion of an electronically excited CuP. The present results show that th e presence of A-T sites in the duplexes is not exclusive of other para meters, since the exciplex is also observed in CuP-poly(dT) and CuP-po ly(rU) complexes as well. This demonstrates that either thymine or ura cil residues are necessary for forming the exciplex. However, the exci plex does not occur in CuP complexes with poly(dA).poly(dT), poly(rA). poly(rU), dTMP, UpU, and d(pT)2: this suggests that the presence of th ymine or uracil residues in solution is, in and of itself, not suffici ent, but the fixation mode of CuP on polynucleotide and the proper sec ondary structure of the polymer also play important roles in the proce ss of exciplex formation. An explanation of exciplex building is propo sed in terms of a short-lived five-coordinated complex of a (d-d) exci ted state of CuP with thymine or uracil carbonyl groups as the fifth l igands. The structural properties of thymine- (uracil-) containing pol ynucleotides required for an effective axial coordination are discusse d. A model of porphyrin-DNA interaction is proposed, allowing the lack of exciplex formation in CuP-poly(dA).poly(dT) and CuP-poly(rA).poly( rU) complexes to be explained.