The effect of simple shear flow on the helix-coil transition of poly-L-lysine

Studies of the helix-to-coil transition in dilute solutions of poly-L-lysine, dissolved in mixtures of water and methanol (MeOH), have been carried under shear flow using flow birefringence and modulated polarimetry. The fraction of helical conformations in a given solution remains independent of shear rate for MeOH concentrations above and below the critical value for the helix-coil transition (i.e., 87.5% MeOH). For the 87.5% MeOH solutions, a shear-induced helix-to-"stretched" coil transition occurs above a critical shear rate. Induction times for the transition show a temperature and shear rate dependence that can be described in terms of an activated jump process. Measurements of circular birefringence on cessation of flow also show that the transition is reversible, with the stretched coil reverting to the helical state on a time scale of several seconds. The activation energy for the jump process is found to be 16.2 kJ/mole.