There and (slowly) back again: entropy-driven hysteresis in a model of DNA overstretching.
When pulled along its axis, double-stranded DNA elongates abruptly at a force of approximately 65 pN. Two physical pictures have been developed to describe this overstretched state. The first proposes that strong forces induce a phase transition to a molten state consisting of unhybridized single strands. The second picture introduces an elongated hybridized phase called S-DNA. Little thermodynamic evidence exists to discriminate directly between these competing pictures. Here we show that within a microscopic model of DNA we can distinguish between the dynamics associated with each. In experiment, considerable hysteresis in a cycle of stretching and shortening develops as temperature is increased. Since there are few possible causes of hysteresis in a system whose extent is appreciable in only one dimension, such behavior offers a discriminating test of the two pictures of overstretching. Most experiments are performed upon nicked DNA, permitting the detachment (unpeeling) of strands. We show that the long-wavelength progression of the unpeeled front generates hysteresis, the character of which agrees with experiment only if we assume the existence of S-DNA. We also show that internal melting can generate hysteresis, the degree of which depends upon the nonextensive loop entropy of single-stranded DNA.
Whitelam
Stephen
S
Department of Chemistry, University of California at Berkeley, and Physical Biosciences and Materials Sciences Divisions, Lawrence Berkeley National Laboratory, Berkeley, California, USA. s.b.whitelam@warwick.ac.uk
Pronk
Sander
S
Geissler
Phillip L
PL
D003198:Computer Simulation; D004247:DNA / Q000737:chemistry* / Q000648:ultrastructure*; D004548:Elasticity; D019277:Entropy; D008846:Micromanipulation; D008956:Models, Chemical; D008958:Models, Molecular; D017711:Nonlinear Dynamics; D009690:Nucleic Acid Conformation; D013314:Stress, Mechanical; D013816:Thermodynamics
D004247:DNA