Excited states of water-soluble metal porphyrins as microenvironmental probes for DNA and DNA-model compounds: Time-resolved transient absorption and resonance Raman studies of Ni(TMpy-P4) in [Poly(dG-dC)](2) and [Poly(dA-dT)](2)

The dynamics and mechanisms of photoexcitation relaxation of the water-soluble cationic metalloporphyrin nickel(II) 5,10,15,20-tetrakis[4-(N-methylpyridyl)]porphyrin (Ni(TMpy-P4)) bound to DNA-model polynucleotides, i.e. poly(dG-dC)(2) and poly(dA-dT)(2), and free in a mere phosphate buffer, have been studied in detail by using time-resolved picosecond transient absorption (TA) and nanosecond resonance Raman (RR) spectroscopies. For the Ni(TMpy-P4)-poly(dG-dC)(2) complex, double-exponential kinetics of relaxation has been found, with time constants of less than or equal to 10 and 350 +/- 20 ps, and absolute absorption spectra have been reconstructed from experimentally measured difference spectra. The long-lived transient species has been assigned to the excited intramolecular metal-centered (d,d) state B-3(1g) of the 4-coordinate Ni porphyrin intercalated between G-C base pairs. Transient RR spectra originating from this state have also been obtained and discussed. A much more complicated process of excitation relaxation has been found for the Ni(TMpy-P4)-poly(dA-dT)(2) complex, where at least four relaxation components can be separated with time constants of less than or equal to 10, similar to 100, similar to 450 ps, and much greater than 1 ns. Our studies support the existence of at least two types of Ni(TMpy-P4) interaction with poly(dA-dT)(2), each having its own kinetics of TA decay and transient RR spectra. Both TA and RR sets of data show that a major part of Ni porphyrin molecules yields a photophysical behavior typical for a 4-coordinate species, the excited (d,d) state B-3(1g) playing the key role in relaxation processes, while a minor part of Ni(TMpy-P4) also participates in axial ligand binding/release photoprocesses. Comparative analysis of transient RR spectra of Ni(TMpy-P4) bound to the A-T sequence and free in a phosphate buffer shows that no 6-coordinate B-3(1g)(L)(2) transient species is photogenerated in the complex with poly(dA-dT)(2), and therefore, axial coordination of only one extra-ligand molecule (most probably from the surrounding water solution) to the porphyrin central Ni ion is proposed to explain the experimental results. Possible processes of Ni(TMpy-P4) binding to poly(dA-dT)(2) are discussed on the basis of the current photophysical data.