[Spectral studies of binding between DNA bases and magnesium or calcium ions]

Differential spectra (DS) of native and denaturated DNA in the presence of magnesium and calcium ions of 10(-3)--10(-1) M concentrations have been studied. DS of denaturated DNA unlike those of native DNA have pronounced maxima at 290 and approximately nm and approximately 247 nm minimum. Denaturated DNA with the ions studied forms light scattering molecular aggregates precipitating in the course of time. The aggregate dimensions were estimated. The kinetics of spectral variations with temperature and ion or DNA concentration was studied. Comparison of experimental DS with spectra of protonized nucleotides and DNA in the presence of transition metals allows to predict the existence of a binding between megnesium or calcium ions and the N(3) cytosine of single stranded DNA. The constants of ion association with these groups of nucleotides were calculated. The binding of calcium or magnesium with denaturated DNA is shown to be cooperative. This is due to the ion-induced transition of coil DNA strands to an ordered shape.