Facile transition of poly[d(TG)·d(CA)] into a left-handed helix in physiological conditions

The DNA polymer d(GC)n·d(GC)n can undergo a transition from the usual right-handed 10.4 base pairs (bp) per turn B form to a novel left-handed 12 bp per turn Z form in response to altered environmental conditions1–4. Several other alternating purine-pyrimidine DNA polymers with modified bases have been shown to undergo transitions from B to Z conformations, with varying degrees of difficulty5–9. We report here that the unmodified DNA polymer d(TG)n·d(CA)n readily undergoes a transition to a Z conformation when subjected to unwinding torsional stress in ionic conditions that are close to physiological. By using a two-dimensional gel electrophoresis system, we have determined both the critical free energy of supercoiling that is required to initiate the transition and the free energy of super-coiling that is required to maintain this polymer in the Z form.