Urea promotes polyproline II helix formation: Implications for protein denatured states

It is commonly assumed that urea denatures proteins by promoting backbone disorder, resulting in random-coil behavior. Indeed, it has been demonstrated that highly denatured proteins obey random-coil statistics. However, the random-coil model is specified by the global geometric properties of a polymeric chain and does not preclude locally ordered backbone structure. While urea clearly disfavors a compact native structure, it is not clear that the resulting backbone conformations are disordered. Using circular dichroism (CD) spectroscopy, we demonstrate that urea promotes formation of left-handed polyproline II (P_II) helical structures in both short peptides and denatured proteins. The observed increase in P_II content is sequence-dependent. These data indicate that denatured states possess significant amounts of locally ordered backbone structure. It is time for the formulation of new denatured-state models that take into account the presence of significant local backbone structure. Criteria for such models are outlined.