Mechanism of a Pd(II)/porphyrin complex formation and studies on its interaction with polynucleotides

Interactions of synthetic and natural nucleic acids with porphyrins have been intensively investigated since the discovery in 1979 that these compounds are able to intercalate into DNA. The flat aromatic structure of porphyrins, both free and in form of metal complexes, coupled with their high absorptivity, make these substances especially suitable for investigations on the mechanisms of drug binding to nucleic acids; moreover, introduction of a metal ion converts a porphyrin into a metallo-intercalator with additional properties. In recent years the field of porphyrin and metal-porphyrin chemistry has expanded significantly, mainly in connection with the use of these molecules in biology and for light harvesting and energy conversion. DNA/porphyrin systems have been analysed using very different methods but many details of the binding still need to be elucidated. We present here a combined thermodynamic and kinetic analysis of the formation of the Pd(II) complex of 5,10,15,20-tetrakis(1-methyl-4-pyridiyl)-porphyne and of its interaction with DNA. The kinetic analysis of the binding of the Pd(II) to 5,10,15,20-tetrakis(1-methyl-4-confirms 1:1 binding stoichiometry and quantitative reaction. As concerns the binding of the complex to DNA, fluorescence and absorbance titrations under different conditions of temperature and salt content concur in indicating that the binding is strong. Kinetic and equilibrium parameters for the complex interaction with the nucleic acid are obtained and the binding mechanism is discussed. An intercalative binding mode is found to be operative.