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 polynucleot ides, i.e. poly(dG-dC)(2) and poly(dA-dT)(2), and free in a mere phosp hate buffer, have been studied in detail by using time-resolved picose cond transient absorption (TA) and nanosecond resonance Raman (RR) spe ctroscopies. For the Ni(TMpy-P4)-poly(dG-dC)(2) complex, double-expone ntial kinetics of relaxation has been found, with time constants of le ss than or equal to 10 and 350 +/- 20 ps, and absolute absorption spec tra have been reconstructed from experimentally measured difference sp ectra. The long-lived transient species has been assigned to the excit ed intramolecular metal-centered (d,d) state B-3(1g) of the 4-coordina te Ni porphyrin intercalated between G-C base pairs. Transient RR spec tra originating from this state have also been obtained and discussed. A much more complicated process of excitation relaxation has been fou nd for the Ni(TMpy-P4)-poly(dA-dT)(2) complex, where at least four rel axation components can be separated with time constants of less than o r equal to 10, similar to 100, similar to 450 ps, and much greater tha n 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 sho w that a major part of Ni porphyrin molecules yields a photophysical b ehavior 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 phot oprocesses. 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 t he complex with poly(dA-dT)(2), and therefore, axial coordination of o nly one extra-ligand molecule (most probably from the surrounding wate r solution) to the porphyrin central Ni ion is proposed to explain the experimental results. Possible processes of Ni(TMpy-P4) binding to po ly(dA-dT)(2) are discussed on the basis of the current photophysical d ata.