Enhanced conformational changes in DNA in the presence of mercury(II), cadmium(II) and lead(II) porphyrins

Abstract

The interactions of the metalloporphyrins of tetrakis (1-methylpyridinium-4yl)porphyrin ([M(TMPyP)] 4+) where M=Hg(II), Cd(II) and Pb(II)) with pBluescript II plasmid DNA have been studied by the measurement of circular dichroism (CD), UV–visible and fluorescence spectra at 0.1 M NaNO 3, pH 7.5 and 25 °C. The CD spectra of the DNA changed quite significantly, with the conformational changes in the presence of the metalloporphyrins being much more enhanced compared to that of their free metal ion counterparts. The conformational changes in DNA upon binding to the Hg(II) porphyrin and Hg(II) were, however, different from those of the Cd(II) porphyrin, Pb(II) porphyrin, Pb(II), Cd(II) and H 2(TMPyP) 4+. In the concentration range of 0−2.30×10 −5 M of DNA, the absorption spectra of H 2(TMPyP) 4+ showed substantial hypochromicity at 423 nm and a red shift of Δ λ=16 nm in the presence of DNA whereas the Hg(II)-, Pb(II)- and Cd(II) porphyrins showed blue shifts of absorption maximum wavelengths of Δ λ=−17 nm, Δ λ=−35 nm and Δ λ=−4.5 nm, respectively. Furthermore, the shifted absorption maximum wavelengths/nm of the porphyrins in excess amount of DNA were comparable; 438, 439, 440 and 440 for H 2(TMPyP) 4+, Hg(II)-, Pb(II)- and Cd(II) porphyrins, respectively. The changes in absorption spectra for Hg(II)-, Pb(II)- and Cd(II) porphyrins revealed that these metalloporphyrins dissociated upon binding to DNA which was confirmed by CD as well as fluorescence spectra. The CD results, UV–Vis and fluorescence data indicate that the metalloporphyrins interact differently with DNA based on their binding modes. And the enhanced changes in conformation of DNA in the presence of the metalloporphyrins are due to the synergistic effects of the simultaneous binding of the metal ions and the free base porphyrin to DNA compared to their free metal ion counterparts: [M(TMPyP)] 4++DNA+2H +⇌[M(II)(DNA)H 2(TMPyP) 4+]. The detailed equilibrium reactions have been described along with suggestions of possible applications in the medical and biological fields.