α-Helix unfolding in simple shear flow

Abstract

The unfolding dynamics of the α-helical poly-L-lysine (α-PLL) in Couette flow is reported. Real-time circular dichroism measurements for a range of molecular weights and shear rates have been made. The PLL molecules show a time- and shear rate-dependent unfolding in simple shear flow with a critical strain (t_c) value of ∼10⁵. This strain value is found to be independent of the chain-length of the α-helices. The extent of unfolding is less pronounced with increasing molecular weight (M) for a given strain, showing a linear dependence of the remaining helix, α, on M: α ≈ M. Furthermore, the helix content, α, is found to show a power law dependence with strain: α ≈ (t)^−1/2. A shear-induced rapid unfolding of short chain α-PLL molecules in the flow field occurs. The shear-stability of the larger molecular weights is due to the cohesive forces stabilizing the helix, combined with the associated hydrodynamic screening of helical segments from the full effect of the drag in the flow field. The data are compared with recent molecular dynamics simulations of the dynamics of dilute polymer solutions in shear flow and scaling arguments are used to interpret the trends in the data.