Interactions of Electronically Excited Copper(II)-Porphyrin with DNA: Resonance Raman Evidence for the Exciplex Formation with Adenine and Cytosine Residues Peter Mojzes ˇ,* ,² Sergei G. Kruglik, ‡ Vladimı ´r Baumruk, ² and Pierre-Yves Turpin § Institute of Physics, Charles UniVersity, Ke KarloVu 5, CZ-12116 Prague 2, Czech Republic, B. I. StepanoV Institute of Physics, National Academy of Sciences of Belarus, 70 F. Skaryna AVenue, Minsk 220072, Belarus, and L.P.B.C., UniVersite ´ Pierre et Marie Curie, mailbox 138, 4 Place Jussieu, F-75252 Paris 5, France ReceiVed: March 17, 2003; In Final Form: June 5, 2003 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 CdO group which was presumed to participate in the CuP*CdO exciplex formation, and consequently one of its basic endocyclic nitrogens (N 1 ,N 3 , or N 7 ) 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 C 6 dO, N 3 , and N 7 potential ligands. Introduction Water-soluble cationic (metallo)porphyrins complexed with nucleic acids (DNA, RNA) are widely investigated owing to their possible use in photodynamic therapy of cancer. To elucidate their binding modes to nucleic acids, investigation of porphyrins complexed with monomer nucleoside/nucleotide building units, 1,2 single-stranded 3-5 and double-stranded syn- thetic oligo/polynucleotides, and natural DNAs and RNAs (for a review, see refs 6-8) is of major importance. Among various metalloporphyrins studied in complexes with nucleic acids, the copper(II) derivative of 5,10,15,20-tetrakis- [4-(N-methylpyridyl)]porphyrin (CuP) attracted particular inter- est owing to its ability to form in its excited electronic state a transient axially coordinated complex, the so-called exciplex, with a convenient ligand in its close proximity. 9-15 CuP exciplex formation can be conveniently monitored by resonance Raman spectroscopy (RRS), because, under high-power pulsed excita- tion, specific exciplex Raman bands appear at ∼1346 (ν 4 *) and ∼1550 (ν 2 *) cm -1 in pairs with their ground-state counterparts located at ∼1366 (ν 4 ) and ∼1570 (ν 2 ) cm -1 , respectively. 9 By analogy with (d,d)-excited copper(II) porphyrins (CuOEP, CuTPP) that were shown to form exciplexes with various oxygen-containing solvent molecules, 16,17 exocyclic keto sub- stituents (C 2 dO and/or C 4 dO) of thymine (or uracil in RNA) have been proposed as the best candidates for axial ligation. 12,13 This is plausible since, in addition to double-stranded poly(dA- dT) 2 , considerable yield of the CuP*CdO exciplex was found in CuP complexes with single-stranded poly(dT) and poly(rU). 12 The presence of a convenient ligand is a necessary but not sufficient requirement for exciplex formation: the proper position of the porphyrin Cu center with respect to the potential ligand, conditioned by the porphyrin binding mode and by the overall secondary structure of the polymer, is also required for effective axial coordination. External groove-binding of CuP to runs of (at least four) alternating AT base pairs seems to provide the most favorable conditions for obtaining the exciplex, 11 while intercalation between base pairs disfavors or even fully hinders exciplex formation. For example, in contrast to strong exciplex RRS signals from complexes of outside-bound CuP with poly- (dA-dT) 2 , only weak exciplex signals were detected with poly- (dA)‚poly(dT). 11,12 Intercalation between GC base pairs (e.g. in poly(dG-dC) 2 ) completely prohibits exciplex formation, while weak but noticeable exciplex markers were found for CuP (hemi)intercalated into poly(dA-dC)‚poly(dG-dT). 12,14 Re- cently it was shown 15 that exciplex formation is not restricted to regular polynucleotide structures, since a high yield of exciplex was observed in CuP molecular complexes with thymine mononucleotides (e.g. 5′-dTMP) in a large mononucle- otide excess. Besides the CuP*CdO species, a CuP*H 2 O exciplex having a water molecule as its fifth axial ligand has been found in aqueous solutions containing free CuP molecules 13,14,18-20 Both exciplex species exhibit identical shifts of their RRS marker bands, but they can be distinguished by their relaxation kinetics in picosecond time-resolved RR (psTR 3 ) experiments, 14 since their lifetimes differ by about 3 orders of magnitude (∼1-3 ns and <10 ps, respectively). 14 In the present study the question is addressed whether other bases, besides thymine and uracil, can give rise to the long- lived CuP exciplex species. Complexes with DNA(RNA)- modeling single-stranded homo-oligo/polynucleotides have been studied, since, on one hand, homo-oligo/polynucleotides may provide a regular polymer structure convenient for CuP binding and, on the other, they are free from complications arising from the presence of two different bases as in double-stranded he- lices. * To whom correspondence should be addressed. Telephone: (+420) 221 911 471. Fax: (+420) 224 922 797. E-mail: mojzes@karlov.mff.cuni.cz. ² Charles University. ‡ National Academy of Sciences of Belarus. E-mail: sergei.kruglik@poly- technique.fr. § Universite ´ Pierre et Marie Curie. E-mail: turpin@lpbc.jussieu.fr. 7532 J. Phys. Chem. B 2003, 107, 7532-7535 10.1021/jp034677v CCC: $25.00 © 2003 American Chemical Society Published on Web 07/08/2003