[Contribution of enthalpy to the energetics of complex formation of aromatic ligands with DNA].

The energy contributions of electrostatic, van der Waals interactions, hydrogen bonds, and interactions of charge transfer type to the enthalpy of complex formation of the double-stand DNA with the antitumor antibiotics daunomycin, nogalamycin, and novantron, as well as the mutagens ethidium bromide and proflavine have been calculated. According to the calculations, the van der Waals component (except for nogalamycin) is energetically favorable during complex formation of the antibiotics with DNA, and the contributions of H bonds and electrostatic interactions are unfavorable, with the probability of charge transfer in the complexes being low. It has been shown that the relatively low value of the experimental enthalpy of binding is the sum of components greater in absolute value and different in the sign, which is the cause of large errors in estimating the total enthalpy of complex formation of aromatic ligands with DNA.

[Contribution of changes in translational, rotational, and vibrational degrees of freedom to the energy of complex formation of aromatic ligands with DNA].

A method for the calculation and analysis of the contribution of changes in translational, rotational, and vibrational degrees of freedom to the energy of complex formation of aromatic compounds with DNA duplex has been developed. The results of calculations of the thermodynamic parameters (deltaG, deltaH, deltaS) indicate that changes in the translational and rotational degrees of freedom destabilize, and changes in the vibrational degree of freedom stabilize the complexes, the energetic contribution from the movements under consideration being predominantly determined by the entropy. It was shown that the energetic components of changes in translational, rotational, and vibrational degrees of freedom are in the main comparable with the experimentally determined thermodynamic parameters, which requires the consideration of these components in the energetic analysis of the complex formation of aromatic molecules with DNA. It was found that the total contribution of changes in translational, rotational, and vibrational degrees of freedom to the Gibbs energy of complexing of aromatic molecules with DNA can be assumed to be on the average the same for different ligands and equal to 8.2 kcal/mol.