Thermodynamics of the binding of the three water-soluble porphyrins with DNA

Abstract

Interactions of three water soluble porphyrins, tetra (p-trimethyl) ammonium phenyl porphyrin iodide (TAPP) as a cationic porphyrin, tetra sodium meso-tetrakis (p-sulphonato phenyl) porphyrin (TSPP) as an anionic porphyrin, and manganese tetrakis (p-sulphonato phenyl) porphyinato acetate (MnTSPP) as a metal porphyrin, with calf-thymus DNA were comprehensively studied at 1 mM phosphate buffer, pH 7.0, and at various temperatures, using UV–Vis absorption spectroscopy. The binding constant and stoichiometry were determined by analysis of optical absorption spectra of porphyrin at various DNA concentrations using SQUAD software. The results show that the best fitting corresponds to 1 : 1 complex model between base pair of DNA and porphyrin. All the thermodynamic parameters were calculated by van’t Hoff equation at various temperatures. The results show that the process is essentially entropy driven. The MnTSSP has the highest affinity respect to TSPP and TAPP that can represent the formation of an axial bond between the phosphate group of nucleotide and Mn in the central core of MnTSSP. However, the higher affinity of TAPP as a cationic porphyrin respect to TSPP as an anionic can be related to the role of electrostatic interactions.