Moroz, J. DavidNelson, Philip C2023-05-232023-05-231998-06-012017-04-26https://repository.upenn.edu/handle/20.500.14332/43240We investigate the statistical mechanics of a torsionally constrained polymer. The polymer is modeled as a fluctuating rod with bend stiffness AkBT and twist stiffness CkBT. In such a model, thermal bend fluctuations couple geometrically to an applied torque through the relation Lk = Tw+Wr. We explore this coupling and find agreement between the predictions of our model and recent experimental results on single λ-DNA molecules. This analysis affords an experimental determination of the microscopic twist stiffness (averaged over a helix repeat). Quantitative agreement between theory and experiment is obtained using C = 109 nm (i.e. twist rigidity CkBT =4.5 × 10−19 erg cm). The theory further predicts a thermal reduction of the effective twist rigidity induced by bend fluctuations. Finally, we find a small reflection of molecular chirality in the experimental data and interpret it in terms of a twist-stretch coupling of the DNA duplex. PACS: 87.15.-v, 87.10.+e, 87.15.By.This document is the unedited Author’s version of a Submitted Work that was subsequently accepted for publication in Macromolecules, copyright © 1998 American Chemical Society after peer review.Physical Sciences and MathematicsPhysicsArticle