Thermodynamic investigation of a new water-soluble porphyrin with calf thymus DNA

The equilibrium binding of a new water-soluble tetra-cationic porphyrin, 5-(1-(4-carboxybutyl) pyridinium-4-yl)10,15,20-tris(1-methylpyridinium-4-yl) porphyrin (5-CBPyP) with calf thymus DNA in comparison with meso-tetrakis(4-N-methyl pyridinium)porphyrin (TMPyP) has been studied in 7.5 mM phosphate buffer, pH=7.2; and at various temperatures by UV-Vis absorption, fluorescence spectroscopies and viscosity measurement. The thermodynamic parameters were calculated by van't Hoff equation at various temperatures. The values of -137.13±1.22 kJ/mol and -337.21±4.75 J/mol.K for 5-CBPyP and -159.12±1.22 kJ/mol and -406.11±4.45 J/mol.K for TMPyP, were estimated for enthalpy and entropy changes of interaction, respectively. The data indicate that the process is exothermic and enthalpy driven suggesting that electrostatic forces play a considerable role in the interaction process. The results of spectroscopic techniques and viscosity measurement represent the intercalation mode of binding for both porphyrins and higher binding affinity of TMPyP respect to 5-CBPyP.