title: Revealing the competition between peeled ssDNA, melting bubbles, and S-DNA during DNA overstretching by single-molecule calorimetry.
abstract: Double-stranded DNA (dsDNA) unconstrained by torsion undergoes an overstretching transition at about 65 pN, elongating the DNA to about 1.7-fold. Three possible structural transitions have been debated for the nature of DNA overstretching: (i) "peeling" apart of dsDNA to produce a peeled ssDNA strand under tension while the other strand coils, (ii) "inside-strand separation" of dsDNA to two parallel ssDNA strands that share tension (melting bubbles), and (iii) "B-to-S" transition to a novel dsDNA, termed S-DNA. Here we overstretched an end-opened DNA (with one open end to allow peeling) and an end-closed (i.e., both ends of the linear DNA are covalently closed to prohibit peeling) and torsion-unconstrained DNA. We report that all three structural transitions exist depending on experimental conditions. For the end-opened DNA, the peeling transition and the B-to-S transition were observed; for the end-closed DNA, the inside-strand separation and the B-to-S transition were observed. The peeling transition and the inside-strand separation are hysteretic and have an entropy change of approximately 17 cal/(K⋅mol), whereas the B-to-S transition is nonhysteretic and has an entropy change of approximately -2 cal/(K⋅mol). The force-extension curves of peeled ssDNA, melting bubbles, and S-DNA were characterized by experiments. Our results provide experimental evidence for the formation of DNA melting bubbles driven by high tension and prove the existence of nonmelted S-DNA. Our findings afford a full understanding of three possible force-driven structural transitions of torsion-unconstrained DNA and the resulting three overstretched DNA structures.
authors: BioSystems and Micromechanics Interdisciplinary Research Group, Singapore-MIT Alliance for Research and Technology, National University of Singapore, Republic of Singapore 138602.
authors: Xinghua
authors: X
authors: Zhang
authors: Hu
authors: H
authors: Chen
authors: Shimin
authors: S
authors: Le
authors: Ioulia
authors: I
authors: Rouzina
authors: Patrick S
authors: PS
authors: Doyle
authors: Jie
authors: J
authors: Yan
mesh_terms: D001667:Binding, Competitive; D055592:Biophysical Phenomena; D002151:Calorimetry; D004247:DNA / Q000737:chemistry / Q000378:metabolism; D053960:DNA Breaks; D004277:DNA, Single-Stranded / Q000737:chemistry* / Q000378:metabolism; D007700:Kinetics; D008958:Models, Molecular; D009690:Nucleic Acid Conformation; D009691:Nucleic Acid Denaturation; D052898:Optical Tweezers; D013314:Stress, Mechanical; D013816:Thermodynamics; D044366:Transition Temperature
chemical_list: D004277:DNA, Single-Stranded; D004247:DNA