Single-Molecule Thermodynamics of Three Distinct DNA Structural Transitions under Large Tension

Abstract

Torsion unconstrained double-stranded DNA can undergo three structural transitions under large tension. At forces around 65 pN, a well known DNA overstretching transition occurs during which the DNA elongates by about 1.7-fold. Recent experiments have revealed two distinct structural transitions during DNA overstretching: a “strand-unpeeling” of one strand from the other, and a “B-to-S” transition to a mysterious double-stranded “S-DNA”. In addition to DNA overstretching transition, DNA without nicks or free ends can also undergo an internal melting transition to two parallel DNA strands under tension at > 100 pN in physiological solution conditions or under lower tension at sufficiently low salt concentration or high temperature. Here, we report thermodynamics measurements for the entropy and enthalpy changes during these transitions. In addition, the force-responses of these DNA structures are directly determined in Single-molecule experiments. With these experimentally determined thermodynamic and mechanical quantities, the selections of the transitions are summarized in phase diagrams.