Three-state diagram for DNA.

Experimental phase diagrams (A form, B form, Coil) were built in the coordinates (a, alcohol fraction: T, temperature) for the natural DNAs and poly d(A-T). The main parameter of the B-A transition,-cooperativity length, nu o, was estimated by the slopes of the branches A-B, A-Coil, B-Coil in the vicinity of the triple point: nu o = 10-20 base pairs, which corresponds to the energy for the B/A junction of 1.2-1.8 kcal/mol. We discovered two new effects which are due to the coexistence of the three different conformations in one polymeric molecule: an increase in the melting temperature above that for the 'ideal' triple point (i.e. for the case of the ideal phase transitions); a widening of the melting curve within the B-A transition range.