Spectroscopic study on the interaction of three water-soluble porphyrins with calf thymus DNA

Porophyrins and their metal derivatives are strong DNA binders with association constant of  to  . Some of these compounds have been used for the radiations sensitization therapy of cancer and are targeted to interact with cellular DNA. Binding of porphyrins to DNA changes the expectral and other physic chemical characteristic of porphyrins, The mode of binding can be extracted from inspection of spectral changes of porphyrine. Hence, Uv-V is spectroscopy is one of the most usual techniques for such investigations. Moreover, measuring of these spectral changes at various molar ratio of DNA/porphyrin can led to calculate the binding constant and stoichiometry. In the present work, the interaction of three water soluble porphyrins, tetra (ptrimethyle) ammonium phenyl porphyrin iodide (TAPP) as a cationic porphyrin, tetra sodium meso-tetrakis (p-sulphonato phenyl) porphyrin (TSPP)as an anionic porphyrine and manganese tetrakis (p-sulphonato phenyl) porphyinato acetate (MnTSPP) as a metal porphyrine with calf thymus DNA have been comprehensively studied at 1mM phosphate buffer, pH 7.0 and various temperatures using Uv-Vis absorption spectroscopy. The results represent a bath chromic effect without any shift in maximum wavelengths, which can be related to the external binding mode without staching. The absorption data at all wavelength regions were analysis at various molar ratios of DNA/porphyrin for estimation of binding characteristic using SQUAD software. The result confirms the formation of 1:1complex between DNA and porphyrins. The MnTSSP has the highest affinity respect to TSPP and TAPP that can be represent the formation of axial bond between 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. The results represent that the process is endothermic and essentially entropy driven.