Relative Solvent Exposure of the Alpha-Helix and Beta-Sheet in Water Determines the Initial Stages of Urea and Guanidinium Chloride-Induced Denaturation of Alpha/Beta Proteins.

Among several denaturants, urea and guanidinium chloride (GdmCl) are the two strong and extensively used denaturants in unfolding experiments. However, the sequences of events in terms of secondary structure melting of several proteins in these two solvents have generated diverse results. A clear molecular understanding has still not been drawn to address the differential secondary structure specificity between urea and GdmCl. Here, we present a way to predict the possible unfolding route of α/β proteins, in terms of secondary structure melting, based on the observation of relative solvent exposure of the alpha-helix and beta-sheet of protein in water. We find that while beta-sheet always melts first in urea, a diverse preference is evident for GdmCl. Larger solvent exposure of the backbone of the beta-sheet helps the beta-sheet to make favorable hydrogen bonds with urea which in turn causes the initial melting of the beta-sheet of alpha/beta proteins in urea. On the other hand, because GdmCl has hydrophobic weakening ability through preferential interaction, the region which has comparatively more hydrophobic solvent exposure melts first in GdmCl. Urea- and GdmCl-induced initial unfolding pathways of the alpha/beta protein is thus determined by the relative solvent exposure of the alpha-helix and beta-sheet of protein in water. Therefore, detailed knowledge of relative solvent exposures in water can provide a hint of the possible unfolding pathway provided the mode of action of the solvent is known.