title: Torsional directed walks, entropic elasticity, and DNA twist stiffness.
abstract: DNA and other biopolymers differ from classical polymers because of their torsional stiffness. This property changes the statistical character of their conformations under tension from a classical random walk to a problem we call the "torsional directed walk." Motivated by a recent experiment on single lambda-DNA molecules [Strick, T. R., Allemand, J.-F., Bensimon, D., Bensimon, A. & Croquette, V. (1996) Science 271, 1835-1837], we formulate the torsional directed walk problem and solve it analytically in the appropriate force regime. Our technique affords a direct physical determination of the microscopic twist stiffness C and twist-stretch coupling D relevant for DNA functionality. The theory quantitatively fits existing experimental data for relative extension as a function of overtwist over a wide range of applied force; fitting to the experimental data yields the numerical values C = 120 nm and D = 50 nm. Future experiments will refine these values. We also predict that the phenomenon of reduction of effective twist stiffness by bend fluctuations should be testable in future single-molecule experiments, and we give its analytic form.
authors.affiliation: Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA 19104, USA.
authors.forename: J D
authors.initials: JD
authors.lastname: Moroz
authors.forename: P
authors.initials: P
authors.lastname: Nelson
mesh_terms: D000818:Animals; D004247:DNA / Q000737:chemistry* / Q000648:ultrastructure; D004548:Elasticity; D006801:Humans; D008958:Models, Molecular; D008962:Models, Theoretical
chemical_list: D004247:DNA