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[Study of cooperative transitions in DNA using phase diagrams].

The study of the triple equilibria (A, B, Coil) and (A, B, Z) makes it possible to obtain interconnection between thermodynamic parameters of different co-operative transitions in DNA. Melting of DNA of poly[d(A--T)] was performed in a broad interval of trifluoroethanol (TFE) concentrations including those of the B to A transition range. A branch, which characterizes the helix-coil equilibrium, has a universal shape for different DNAs and the polynucleotide: melting temperature is minimum near 30% TFE and increases sharply within the region of the A form stability (greater than 65% TFE). The B-A equilibrium depends on temperature only slightly. The slope angles of the branches in the vicinity of the triple point (A, B, Coil) allow to estimate the co-operativity length of the B-A transition: it proves to be of the order of 20 base pairs for DNA and the polynucleotides. Due to the finite transition widths a peculiar situation, not studied up to now, is realized in our cases, at which three different conformations may be present in one polymeric molecule. This results in some new effects, predicted by a theory, such as stabilization by the B/A junctions of the helical state and the increase in width of the melting curves within the B-A transition interval. It thus appears that the phase diagram method may become a very informative approach to the studies of DNA metamorphosis in solution.