[Effect of changes in the stoichiometry of DNA--ligand complexes during heat denaturation of DNA on helix-coil transition parameters].

The influence of a change in DNA-ligand complex stoichiometry in the process of denaturation on temperature and temperature range width of helix-coil transition (Tm, delta T) has been considered. The stoichiometry is characterized by two parameters: by the number of base pairs covered with one ligand (m) and by the number of base pairs per one binding centre (r). It is shown that the change of r and m during denaturation strongly influences the DNA melting process. If the product rm doesn't change during denaturation, the melting temperature shift caused by a ligand concentration growth has a finite limit. If r1m1 not equal to r2m2 (index 1 is related to the coil DNA from and index 2--to the helix DNA form), the melting temperature during concentration growth aspires to + infinity, when r1m1 > r2m2, and to absolute zero, when r1m1 < r2m2. The sign of the effect (stabilization or destabilization) is independent of a binding constant and a factor of cooperativity of DNA-ligand interaction. Dependences of Tm and delta T on ligand concentration for different r1, r2, m1, m2 are analysed.