1143 ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2017, Vol. 91, No. 6, pp. 1143–1145. © Pleiades Publishing, Ltd., 2017. Original Russian Text © P.O. Vardevanyan, M.A. Parsadanyan, A.P. Antonyan, S.N. Hakobyan, 2017, published in Zhurnal Fizicheskoi Khimii, 2017, Vol. 91, No. 6, pp. 1071–1073. Thermodynamic Parameters Analysis of Ethidium Bromide and Mitoxantrone Binding with DNA by Adsorption Isotherms 1 P. O. Vardevanyan a, *, M. A. Parsadanyan a , A. P. Antonyan a , and S. N. Hakobyan b a Yerevan State University, Faculty of Biology, Yerevan, Armenia b State Engineering University of Armenia, Yerevan, Armenia *e-mail: p.vardevanyan@ysu.am Received March 3, 2016 Abstract—Analysis of the adsorption isotherms by virtue of experimentally obtained data of anti-tumorous compound mitoxantrone and ethidium bromide binding with DNA has been carried out. The obtained data showed that throughout the comparatively simple linear isotherm, the more precise values of the binding constant K and number of nucleotides n, per binding site were received. Based on the values of K, those for the enthalpy changes at complex-formation of these ligands with DNA were obtained. Keywords: adsorption isotherm, mitoxantrone, ethidium bromide, DNA, complex-formation DOI: 10.1134/S0036024417060280 Different models are used in studies of reversible binding of ligands with nucleic acids (NA) [1–5], though for the analysis of experimental data it is important to gain such a state, when all binding sites on NA are entirely occupied by ligand molecules at the existence of only one binding mode. However, it is necessary to note that low-molecular compounds mainly bind to NA in more than one mode, since many of them usually are in ionic (mainly cationic) state in solution [2, 3]. Despite this fact, the adsorption isotherm analysis in Scatchard’s coordinates is usually carried out by formula suggested by McGhee and von Hippel. In this case NA is observed as linear lattice with infinite length, consistent of identical and non-interacting binding centers. At filling of such a structure, in the case of non-cooperative interaction, the model of excluded centers of filling (potential centers neighbor- ing with the binding site become excluded for other molecules of ligand) is taken into consideration that is why adsorption isotherms are described by the follow- ing equation [4]: , (1) where K is the binding constant of ligand by one type, n is the number of bases of NA per binding site, C f is concentration of free molecules of ligand, C b is con- centration of bound molecules of ligand, C p is concen- tration of phosphate groups of NA; . In the case, when interactions between ligand bound molecules (cooperative binding) are taken into account, a respective parameter of cooperativeness – ω is inserted. Within the frames of cooperative model “excluded binding sites”, the adsorption isotherm is described by the following equation [5] (2) where . Parameter ω can exceed of 1 (cooperative binding) or less be than 1 (anti-cooperative binding). At ligand cooperative binding with NA the adsorption isotherm in the region of small fillings is obtained to be crowned and non-linear at anti-cooperative binding (hyper- bolic). It should be noted that in the majority of cases to achieve a totally bound state high concentrations of NA are needed [6], that is why the adsorption iso- therms are analyzed in the region of small fillings. However, in this case the results will be reliable if all binding sites by one mode are totally saturated by ligand molecules, which, nevertheless is not always realized. On the other hand, as far as the majority of ligands bind with NA by more than one mode, the 1 The article was translated by the authors. ( ) ( ) [ ] - - = - - 1 f 1 1 1 n n nr r K C n r = b p / r C C ( ) ( )( ) ( )( ) ( ) ( ) - = -   ω- - + - ×   ω- -     - + + ×   -   f 1 2 1 2 1 1 2 1 1 1 1 , 21 n r K nr C nr r R nr n r R nr ( ) [ ] ( ) ( ) = - + + ω - 1/2 2 1 1 4 1 R n r r nr SHORT COMMUNICATIONS