cis-diamminedichloroplatinum(II) (cis-DDP) form covalent adduct with DNA both in vivo and in vitro. In the present study, the UV-spectroscopic helix-coil transition method is used to analyze the changes of the high-molecular DNA double helix occurring due to the formation of complexes with cis-DDP at the concentrations of cis-DDP 0.00003 < r_b < 0.003, where r_b is the number of moles of bound DDP per mole of nucleotide.

The characteristic behavior of the high molecular DNA melting temperature T_m on concentration of cis-DDP shows that there exist three precisely expressed areas of concentration of ligand on efficiency of influence of cis-DDP on DNA:

1. the area of monotonous increase of DNA melting temperature at 4-5 degrees (10^-5 < r_b < 10^-4)
2. the area of monotonous downturn of DNA melting temperature (10^-4 < r_b < 10^-3)
3. the area of sharp fall of DNA melting temperature of (10^-3 < r_b).

The character of T_m changes in the first area of cis-DDP concentration seems rather strange. The observed changes of parameters of helix-coil transition cannot be explained in terms of local alterations of the double helix caused by local, kinetically inert, bidentate and monodentate platinum binding to DNA sequences. It is obvious good agreement of experimental data for ring structures formation with appropriate theoretical calculations.