The chirality of DNA: elasticity cross-terms at base-pair level including A-tracts and the influence of ionic strength.

A systematic analysis of B-DNA elasticity cross-terms was performed using molecular dynamics simulations of three different duplexes designed to contain all dinucleotide steps including a 6-mer A-tract. The influence of ionic strength was also evaluated by several trajectories of each molecule with different NaCl concentrations at physiological rank. Simulations show DNA flexibility is independent of salt, in agreement with the Odijk-Skolnick-Fixman model. In addition, our results demonstrate DNA asymmetry at this scale is more complex than predicted by long-scale DNA models, with the cross-terms relating twist, slide, roll, and twist on the one hand and tilt and shift on the other being most essential. We find the rest of the coupling terms can be generally discarded due to their lack of correlation, with the exception of purine-purine's rise-tilt and shift-tilt. More specifically, A-tracts do not present any specific features in terms of their flexibility and chirality properties within those expected for purine-purine steps. Finally, some hints about coupling mechanisms are provided; we suggest cup deformation is mostly responsible for the positive twist-rise correlation at step level, whereas roll-rise and tilt-rise correlations can be understood via changes in stagger