Can pulling cause right- to left-handed structural transitions in negatively supercoiled DNA double-helix?

The folding angle distribution of stretched and negatively supercoiled DNA double-helix is investigated based on a theoretical model we proposed earlier [H. Zhou et al., Phys. Rev. Lett. 82, 4560 (1999)]. It is shown that pulling can transit a negatively supercoiled DNA double-helix from the right-handed B-form to a left-handed configuration which resembles DNA Z-form in some important respects. The energetics of this possible transition is calculated and the comparison with recent experimental observations are qualitatively discussed.