Elongational flow studies on DNA in aqueous solution and stress‐induced scission of the double helix

Elongational flow techniques are used to investigate the birefringent response and flow‐induced molecular scission of monodisperse phage‐DNA samples in aqueous solution. A 4‐roll mill apparatus was used to characterize the solutions ac low stain rates, \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \varepsilon $\end{document} ≤ 300 s−1, and the opposed jets apparatus used to study fracture of the DNA molecules at strain rates up to 15 × 103 S−1. The molecular weight values were measured before and after fracture in elongational flow using the high‐resolution technique of pulsed field gel electrophoresis (PFGE). The birefringent response incorporates both rigid and flexible components. The birefringence is nonlocalized and rises gradually to a plateau value, similar to rigid‐rod behavior. In addition a certain minimum value in the strain rate is necessary, an onset value \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \varepsilon $\end{document}0, before the signal appears, indicating a flexible component. This behavior is consistent with a hinged‐rod model and is similar to that observed for the protein collagen molecule at elevated temperature. We propose that this type of behavior is likely for multistrand rope‐like macromolecules where localized separation or partial untwisting of the intertwined chains occurs, creating temporary hinges, in accordance with biochemical evidence for sequence‐specific sites of flexibility. Results are presented on the entanglement effects at high concentrations. We have calculated rotational diffusion rates as a function of concentration and molecular weight. Using PFGE to measure the molecular weight profiles, our fracture studies at high strain rates demonstrate chain halving and quartering in accordance with the predictions of the thermally activated barrier to scission theory for single‐chain polymers.