Contrasting the denaturing effect of guanidinium chloride with the stabilizing effect of guanidinium sulfate

Abstract

Guanidinium chloride, GdmCl, is a strong denaturing agent of globular proteins, whereas guanidinium sulfate, Gdm₂SO₄, is a stabilizing agent of globular proteins. The stabilizing activity of Gdm₂SO₄ is unexpected because the denaturant capability of GdmCl is due to direct interactions of Gdm⁺ ions with protein surface groups. It is shown that the statistical thermodynamic approach devised to explain the molecular origin of cold denaturation [G. Graziano, Phys. Chem. Chem. Phys., 2010, 12, 14245–14252] can provide a rationalization of the different behaviour of GdmCl and Gdm₂SO₄ towards globular proteins. The fundamental quantity is the reversible work to create in the aqueous solution a cavity suitable to host the D-state and a cavity suitable to host the N-state. In aqueous GdmCl solutions, this contribution is not large enough to overwhelm the conformational entropy gain upon unfolding and the direct attractions between Gdm⁺ ions and protein surface groups; in aqueous Gdm₂SO₄ solutions, it is so large that it overwhelms the two destabilizing contributions. Sulfate ions, due to their high charge density, interact strongly with water molecules producing a number density increase, that, in turn, renders the cavity creation process very costly, reversing the denaturing power of Gdm⁺ ions and stabilizing the N-state of globular proteins.