Schurr J Michael
Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195, United States.
J Phys Chem B. 2015 May 28;119(21):6389-400. doi: 10.1021/acs.jpcb.5b03174. Epub 2015 May 13.
The measured effective torsional rigidities of single twisted DNAs under various tensions conflict with theoretical predictions of Moroz and Nelson (MN) at low forces in the 0.25-2.0 pN range. However, MN theory was recently shown to agree well with effective torsional rigidities obtained from simulations, indicating that MN theory is valid down to 0.25 pN for a filament with a constant intrinsic torsional rigidity. Here MN theory is used with an assumed persistence length, 50 nm, to obtain the force-dependent intrinsic torsional rigidity of the filament at each force from its measured effective torsional rigidity. The resulting values rise ∼1.8-fold with increasing force from 0.25 to 2.0 pN. Unexpected behavior of the relative extensions of the untwisted DNAs of Mosconi et al. is noted, and ascribed to a small increase in contour length with force over the 0.18-2.0 pN range. The variations of both the intrinsic torsional rigidity and rise per base pair (bp) with force are suggested to arise from a force-induced shift of a cooperative equilibrium between two conformations with different rises per bp. A two-state nearest-neighbor model is formulated, and ranges of optimal parameters are determined by fitting the model to the experimental differences in rise per bp as a function of force. Optimal adjustment of the torsion elastic constants of the two states enables the same optimal model(s) with fixed parameters to provide reasonably good fits of the experimental torsion elastic constant data. The results reconcile single-molecule measurements on DNAs under tension with numerous results from fluorescence polarization anisotropy, topoisomer distributions, X-ray scattering of DNAs with attached gold colloids, and other kinds of measurements.
在0.25 - 2.0 pN范围内的低力条件下，测量得到的单链扭曲DNA在各种张力下的有效扭转刚性与Moroz和Nelson（MN）的理论预测相冲突。然而，最近研究表明MN理论与模拟得到的有效扭转刚性吻合良好，这表明对于具有恒定固有扭转刚性的细丝，MN理论在低至0.25 pN时仍然有效。在此，MN理论结合假定的持久长度50 nm，根据测量得到的有效扭转刚性来获取细丝在每个力下的力依赖型固有扭转刚性。结果表明，随着力从0.25 pN增加到2.0 pN，所得值增加了约1.8倍。我们注意到Mosconi等人未扭曲DNA的相对延伸存在意外行为，并将其归因于在0.18 - 2.0 pN范围内轮廓长度随力的小幅增加。固有扭转刚性和每碱基对（bp）上升随力的变化被认为是由于每bp上升不同的两种构象之间的协同平衡发生了力诱导的转变。我们构建了一个两态最近邻模型，并通过将该模型拟合到每bp上升随力变化的实验差异来确定最佳参数范围。对两种状态的扭转弹性常数进行最佳调整，使得具有固定参数的相同最佳模型能够合理地拟合实验扭转弹性常数数据。这些结果使在张力下对DNA的单分子测量结果与来自荧光偏振各向异性、拓扑异构体分布、附着金胶体的DNA的X射线散射以及其他测量类型的众多结果相协调。
