Physics – Condensed Matter – Statistical Mechanics
Scientific paper
2007-11-21
Physics
Condensed Matter
Statistical Mechanics
21 pages, 11 figures, contribution to the symposium "Lattices and Trajectories", celebrating the careers of Stu Whittington an
Scientific paper
We study the conformations of polymer chains in a poor solvent, with and without bending rigidity, by means of a simple statistical mechanics model. This model can be exactly solved for chains of length up to N=55 using exact enumeration techniques. We analyze in details the differences between the constant force and constant distance ensembles for large but finite N. At low temperatures, and in the constant force ensemble, the force-extension curve shows multiple plateaus (intermediate states), in contrast with the abrupt transition to an extended state prevailing in the $N \to \infty$ limit. In the constant distance ensemble, the same curve provides a unified response to pulling and compressing forces, and agrees qualitatively with recent experimental results. We identify a cross-over length, proportional to $N$, below which the critical force of unfolding decreases with temperature, while above, it increases wiyh temperature. Finally, the force-extension curve for stiff chains exhibits "saw-tooth" like behavior, as observed in protein unfolding experiments.
Guttmann Anthony J.
Jacobsen Jesper L.
Jensen Iwan
Kumar Sanjay
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