Effect of divalent cations on the daunomycin-deoxyribonucleic acid complex

Studies have been carried out on the interaction of Cu²⁺ and Mg²⁺ with complexes formed between daunomycin and DNA. In the visible region, at pH 5·2, the addition of Cu²⁺ (10^?4M) to a denatured DNA-daunomycin complex causes a spectral shift from 505 nm to 540 and 582 nm. These peaks disappear at pH values above and below 5·2. Mg²⁺ (10^?1M), at pH 6·9, causes the appearance of peaks similar to those obtained with Cu²⁺, but this occurs with either native or denatured DNA—daunomycin complexes. Daunomycin shows a fluorescence emission peak at 555 nm. In the presence of Cu²⁺, there is a reduction in the fluorescence intensity of the DNA—daunomycin complex, at pH values above 6·0. Below this pH, Cu²⁺ has no effect. Mg²⁺, on the other hand, has a marked quenching effect at pH 7·8. As the pH drops below 7·0, this effect gradually disappears. Below pH 3·0, where DNA has no effect on fluorescence, both Cu²⁺ and Mg²⁺ are without effect.