Mechanistic details on Pd(II)/5,10,15,20-tetrakis(1-methyl-4-pyridyl)porphyrin complex formation and reactivity in the presence of DNA

Abstract: Kinetics of coordination of Pd(II) by the macrocyclic porphyrin 5,10,15,20-tetrakis(1-methyl-4-pyridyl)porphyrin (H2P4+) is investigated and confirms quantitative formation of a planar PdP4+complex at room temperature (formation rate 0.19 M-1 s-1 at 25 °C, 0.2 M NaCl, pH 3). Then, the binding ability to DNA of the pre-formed PdP4+complex is analysed. To achieve this aim, spectrophotometry, spectrofluorometry and viscometry are used. Thermodynamic parameters for binding, obtained by the temperature dependence of the equilibrium constants, are deltaH = -71 kJ mol-1 and deltaS = -134 J mol-1 K-1. These values, being both highly negative, agree with full PdP4+intercalation into DNA. Moreover, kinetics of the binding reaction is analysed by the T-jump technique (reaction times in the 1-5 ms range). Experiments on the porphyrin ligand retention on negative SDS and positive DTAC micellar surfaces are also done. Taken altogether, these data provide mechanistic details on complex formation and on DNA binding and relevant energies and driving forces. It is found that interaction between PdP4+and base pairs is very strong (KabsDNA = 8.0E5 M-1 at 25 °C, 1.0 M NaCl), not only owing to the high positive charge borne by the complex, but also to the contribution of high hydrophobicity of the porphyrin ring. In the dye/DNA complex, PdP4+is buried into the helix, as confirmed also by fluorescence quenching tests. Both presence and type of metal ion play a major role, as lower affinity and lower induced helix conformation changes are found in the case of the H2P4+/DNA and CuP4+/DNA systems.