Do proteins really unfold in a shear flow?

Many protein structures unfold (denature) when subjected to extremes of heat, cold, pH, solvent composition, or mechanical stress. One might expect that shearing forces induced by a nonuniform fluid flow would also destabilize proteins, as when a protein solution flows rapidly through a narrow channel. However, although we find many references to shear denaturation in the protein literature, we find no quantitative demonstration of the phenomenon. Therefore we have investigated whether a high shear can destabilize a protein to any measureable extent. We study a small globular protein (horse cytochrome $c$, 104 amino acids) whose fluorescence increases sharply upon unfolding. We pump the sample through a silica capillary (180 $\mu $m ID) at speeds $\sim $ 10 m/s to create a simple shear \textit{dv}$_{z}$/\textit{dx} $\sim $ 5 $\times $ 10$^{5}$ s$^{-1}$, under UV laser illumination. We can detect unfolding of as little as 1{\%} of the sample, or (under favorable conditions) a reduction of $\sim $0.05 kJ/mol in the protein's stability. We will discuss preliminary results along with a simple theoretical perspective on shear denaturation.