Binding, Electrochemical Activation, and Cleavage of DNA by Cobalt(II) Tetrakis-N-methylpyridyl Porphyrin and Its β-Pyrrole Brominated Derivative

The binding of nucleic acids by water-soluble cobalt(II) tetrakis-N-methylpyridyl porphyrin, (TMPyP)Co, and its highly electron-deficient derivative cobalt(II) tetrakis-N-methyl pyridyl-β-octabromoporphyrin, (Br₈TMPyP)Co, was investigated by UV−visible absorption, circular dichroism (CD), and electrochemical and gel electrophoresis methods. The changes of the absorption spectra during the titration of these complexes with polynucleotides revealed a shift in the absorption maxima and a hypochromicity of the porphyrin Soret bands. The intrinsic binding constants were found to be in the range of 10⁵−10⁶ M^-1. These values were higher for the more electron-deficient (Br₈TMPyP)Co. Induced CD bands were noticed in the Soret region of the complexes due to the interaction of these complexes with different polynucleotides, and an analysis of the CD spectra supported a mainly external mode of binding. Electrochemical studies revealed the cleavage of polynucleotides by (TMPyP)Co and (Br₈TMPyP)Co in the presence of oxygen preferentially at the A−T base pair region. Gel electrophoresis experiments further supported the cleavage of nucleic acids. The results indicate that the β-pyrrole brominated porphyrin, (Br₈TMPyP)Co, binds strongly and cleaves nucleic acids efficiently as compared with (TMPyP)Co. This electrolytic procedure offers a unique tool in biotechnology for cleaving double-stranded DNA with specificity at the A−T regions.