Cation and strong co-solute effects on protein kinetic stability.

Degradation of proteins is important for the operating life of biocatalysts and the shelf life of protein pharmaceuticals. We have previously found that the deactivating effects of salts on proteins can be correlated to an indicator of ion hydration, the B-viscosity coefficient of the anion in solution. Here, we test the influence of cations on protein kinetic stability by observing deactivation of mRFP (monomeric red fluorescent protein) in ammonium, caesium and chloride salt solutions, and we find that mRFP deactivation does not depend on cation hydration. We also measure mRFP deactivation in solutions containing denaturants (guanidinium chloride or urea) or stabilizing co-solutes (glycerol or sucrose) frequently encountered in many protein formulations to test whether hydration of these co-solutes can be used to indicate their relative effects on protein kinetic stability. We find that mRFP deactivation in solutions containing kosmotropic salts or stabilizers reaches a limiting rate and that hydration of denaturants is not an indicator of their denaturing strength.