Physics – Condensed Matter – Soft Condensed Matter
Scientific paper
1997-08-21
Proc. Natl. Acad. Sci. USA 94(1997)14418-14422
Physics
Condensed Matter
Soft Condensed Matter
Plain TeX, harvmac, epsf; postscript available at http://dept.physics.upenn.edu/~nelson/index.shtml
Scientific paper
10.1073/pnas.94.26.14418
DNA and other biopolymers differ from classical polymers due to 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 et al., Science 271 (1996) 1835], 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=120nm and D=50nm. 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.
Moroz David J.
Nelson Philip
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