Underwound DNA under tension: L-DNA vs. plectoneme

In many biological processes DNA experiences force in the pN range and torque that underwinds it. Magnetic tweezers experiments show that the superhelicity(σ)-extension curve, the so-called bell curve, is asymmetric with respect to the inversion of σ. We study the case of underwound DNA which was not addressed theoretically before. While the case of overwound DNA is fully explained by the formation of supercoil, the extension of underwound DNA reveals nontrivial tension dependence. We show that plectonemic coils form at moderate tension, whereas left-handed DNA, so-called DNA, prevails at high tension (above ≈ 0.5 pN). In a narrow but physiologically relevant crossover range of tension, that is between 0.4 pN and 0.7 pN, extra unwinding turns are statistically distributed to either plectoneme or L-DNA. In this regime the states of a torsionally stressed DNA should be most sensitive to external mechanical stimuli.