| Abstract: | The binding of cupric ion (Cu++) to DNA was followed by spectrophotometry, melting profiles, and hydrodynamic techniques, in 0. 1M NaClO4 and at pH 5. 6. A small amount of Cu++ is bound specifically to bases (about 1 Cu++ per 20 nucleotides), in agreement with polarographic and EPR data. A preferential stabilization of G–C pairs and only a slight increase of the flexibility of the molecule were observed. In 5 × 10?3M NaClO4, a higher number of nonhomogeneous binding sites is found by spectrophotometry. It is concluded that at least two types of sites are available for Cu++. The first one, where Cu++ is chelating N7 of purines to phosphate, is observed only at low ionic strength and destabilizes the double helix. The second exists mainly at 0, 1M or higher ionic strength. All the sites are identical and could be attributed to two successive guanine residues in the same strand. Similar behavior was found for other divalent cations, e. g., Fe++, Mn++, and Co++. |
