A copper-poprhyrin system as nucleic acid intercalator: Kinetic and thermodynamic studies on complex formation and dna interaction

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 in DNA. The binding of porphyrins to nucleic acids and the structures of the porphyrin-DNA complexes are being investigated using a variety of physical techniques(NMR, equilibrium dialysis, flow dichroism and viscometry). In contrast, kinetic studies on cationic porphyrins systems are still scarce and rather incomplete. The system investigated in this research is 5,10,15,20-TETRAKIS(1-METHYL-4-PYRIDYL)-porphyne copper(II) complex to natural DNA. The mechanism of the binding reaction of Cu2+ to the porphyrin ring was also and first of all investigated by both spectrophotometry and kinetic studies and the reaction parameters are obtained. The mechanism of the prevailing binding step involve fast loss of two protons prior to the rate determining step which consists in the penetration of copper into the porphyrin ring. Concerning interaction with DNA, spectrophotometric titrations indicated intercalation of the copper-porphyrin complex between DNA base pairs do occur and both the value of the equilibrium constant and of the binding site size for such an interaction were obtained. The kinetics of the interaction have been investigated using the T-jump method. According to the observed mechanism a very fast formation of an external pre-intercalated complex occurs first. Then, this external complex converts to an intermediate intercalated form, and finally, through the last step, the final stable formation of the intercalate is achieved.Keywords: Cu(II)-Porphyrin complexes, Complex formation, DNA, Intercalation, Reaction Mechanism.