During B-Z transition there is no large scale breakage of Watson-Crick base pairs. A direct demonstration using 500 MHz 1H NMR spectroscopy.

Monitoring of the Watson-Crick GNH1 proton in poly(dG-dC).poly(dG-dC) at 500 MHz in 90% H2O:10% D2O at 30 degrees C as a function of NaCl concentration (1.5 to 3.6 M), demonstrates that the bases retain Watson-Crick pairing throughout the transition. This observation unequivocally demonstrates that during the B-Z transition there is no large scale and detectable base pair opening and that macroscopically the phenomenon can be described as a direct helix to helix transition. We present frame by frame, an energetically sound stereodynamical trajectory for this transfiguration from right-handed B-DNA to left-handed Z-DNA.