POSSIBILITY OF TRUE PHASE-TRANSITION IN HETEROGENEOUS DNA DURING THERMAL-DENATURATION UNDER CONDITIONS CLOSE TO EQUAL STABILITY OF A+T AND G+C PAIRS

The DNA helix–coil transition has been studied in the presence of high concentrations of manganese ions (about 10−3M), which corresponds to the conditions close to equal stability of the A+T and G+C pairs, at the ionic strengths of 10−1, 10−2, and 1.6 × 10−3M Na+. With the Mn2+ ion effect, the transition range is significantly reduced to not more than 0.2°C at 1.2 × 10−3M Mn2+ and 1.6 × 10−3M Na+. The melting curves display a sharp kink at the end of the helix–coil transition, which is interpreted as an indication of the second‐order phase transition. It is shown that the melting curves obtained can be approximated by a simple analytical expression 1 – θ = exp[–a(tc ‐ t)], where θ is the DNA helix fraction, tc is the phase transition temperature, and a is an empirical parameter characterizing the breadth of the melting range and responsible for the magnitude of a jump of the helicity derivative with respect to the temperature at the phase transition point. Copyright © 1979 John Wiley & Sons, Inc.