Influence of Copper2+ Ions on the Denaturation and Spectral Properties of DNA and Methylated DNA

Further indications have been obtained for the complex binding site of Cu2+ with DNA from difference spectra and titration of methylated and non-methylated DNA-Cu2+ complexes. The ultraviolet spectra of GC-rich DNAs show a shift in their absorption maxima of approx. 3 nm in the presence of Cu2+. A more pronounced displacement of the maxima is observed in the difference spectra of DNA-Cu2+ complexes depending on the GC content. Methylation of DNA at N-7 of guanine reduces the affinity of DNA for Cu2+ association: the destabilizing effect of Cu2+ on melting of the double helix is markedly diminished, and difference spectra are less affected by Cu2+. Spectrophotometric titration of various DNAs indicates binding of Cu2+ to the bases at low ionic strength and at 25°. The Cu2+-induced hyperchromic change depends on the GC-content and is affected by specific methylation of guanine residues and by protonation of the GC pairs. The results are compared with literature data, and can be interpreted in terms of a coordination of Cu2+ with GC pairs: a resonable value for the binding of Cu2+ to DNA at 25° appears to be 1 Cu2+ per GC pair.