Rate of Nucleation of a Z-Form in a B-Form DNA

The kinetics of salt-induced conformational changes of a long (about 900 bp) poly[d(G-C)·poly[d(G-C)], in the B → Z transitions, have been examined by ultraviolet and infrared spectroscopy. In general, Z → B reaction was found to be much faster than the B → Z reaction. At the midpoint ([NaCl] = 2.2 M) of the transition, the conformational changes are extremely slow. Here again, the Z → B reaction (on [NaCl] = 3 → 2.2 M) is faster (rate constant k = 0.02 sec -1 at 25 °C) by three orders of magnitude than the B → Z reaction (k = 0.00002 sec -1 on [NaCl] = 1.5 → 2.2 M at 25 °C). This fact was interpreted as indicating that the rate of nucleation of the Z-form in the B-form chain is much lower than the rate of nucleation of the B-form in the Z-form chain.