Department of Chemistry, University of Washington , Box 351700, Seattle, Washington 98195, United States.
J Phys Chem B. 2017 Jun 15;121(23):5709-5717. doi: 10.1021/acs.jpcb.7b01869. Epub 2017 Jun 6.
Deforming an intrinsically straight elastic rod into a circle is shown to introduce an axial tension that acts to extend the rod, much like an externally applied force. The response of a circular DNA to such axial tension was reckoned using a previously suggested model of a force-dependent cooperative transition between a shorter torsionally softer a conformation and a longer torsionally stiffer b conformation. Each of three earlier reported optimal sets of parameters, that well-fitted both relative extension vs force and torsion elastic constant vs force data on single DNAs under tension, was applied here to predict torsion elastic constants of the effective springs between base pairs for both linear and circular 181 bp and ∼210.8 bp DNAs under their respective conditions. Predicted values for both linear and circular species agreed well with their corresponding experimental values, which strongly suggests that the observed 1.4- to 1.5-fold enhancement of the torsion elastic constants upon circularization arises from such axial tension. Experimental torsion elastic constants lie in the range (6.4-6.6) × 10 J for these linear DNAs and in the range (9.1-9.9) × 10 J for the corresponding circles, significantly below the limiting value ∼12 × 10 J at tensions exceeding 4 pN.
将原本笔直的弹性棒变形为圆形会引入轴向张力，这种张力会像外部施加的力一样使棒伸长。使用先前提出的力依赖的协同构象转变模型来计算圆形 DNA 对这种轴向张力的响应，该模型涉及较短的扭转较软的 a 构象和较长的扭转较硬的 b 构象之间的转变。三个较早报道的最佳参数集，这些参数集都很好地拟合了拉伸下单 DNA 的相对伸长与力以及扭转弹性常数与力的数据，被应用于预测在各自条件下线性和圆形 181bp 和 ∼210.8bp DNA 之间碱基对之间有效弹簧的扭转弹性常数。线性和圆形两种物种的预测值与相应的实验值吻合较好，这强烈表明，在圆形化过程中观察到的扭转弹性常数提高了 1.4 到 1.5 倍是由于这种轴向张力所致。对于这些线性 DNA，实验得到的扭转弹性常数在（6.4-6.6）×10 J 范围内，对于相应的圆形 DNA，扭转弹性常数在（9.1-9.9）×10 J 范围内，显著低于超过 4 pN 的张力下约 12×10 J 的极限值。
