Deformation of λ‐phage DNA molecules in an elongational flow field

The response of λ‐phage DNA molecules to a well‐defined elongational flow field generated by a Taylor four‐roller mill was investigated by observing the flow birefringence, Δ n. Δ n in the center of the four rollers, near the stagnation point, was localized at the mill exit symmetry plane. The intensity gradually increased from the off‐symmetrical plane to the center of the mill, and at the exit symmetrical plane, the intensity was maximum. Δ n also gradually increases with the strain rate, \documentclass{article}\pagestyle{empty}\begin{document}$\dot \varepsilon$\end{document} . These observations indicate that DNA molecules in the solution would be free draining in nature. From the decay of λ n at each point in the mill after a sudden stop of the mill operation at 24 s −1 , the rotational diffusion coefficient of molecules, D r , at each point in the mill space was estimated, where the relaxation time of the decay of λ n was considered to be related to the molecular disorienting process. It is concluded that at 24 s −1 λ‐phage DNA molecular coils near the stagnation point, which was assumed to be a prolate spheroid as a whole, was so deformed that the aspect ratio p (= b/a ≦ 1, where a and b are, respectively, the longer and shorter axes) would be \documentclass{article}\pagestyle{empty}\begin{document}${\textstyle{1 \over {12}}}$\end{document} of that of the DNA molecule which has just entered the mill space. This result suggests that there is a possibility for the DNA molecule to be in a stretched conformation at a higher strain rate. © 1996 John Wiley & Sons, Inc.