Hofmeister effects in protein unfolding kinetics: Estimation of changes in surface area upon formation of the transition state

We studied the effect of three electrolytes (LiCl, Na2SO4, GuHCl) on the unfolding reaction of chymopapain, a two-domain protein belonging in the papain family of cysteine proteinases. Due to methodological reasons, these studies were carried out at pH 1.5 where the protein unfolds following biphasic kinetics. We have observed the presence of two different effects of electrolyte concentration on the unfolding reactions. At low ionic strength, the ionic atmosphere brought about an increase in reaction rates, regardless of the type of ions being present; this effect is attributed to a general “electrostatic screening” of charge–charge interactions in the macromolecule. At high ionic strength, each electrolyte exerted a distinctively different effect: both rate constants were largely increased by GuHCl (a well-known protein denaturant), but only slightly by LiCl; in contrast, Na2SO4 (a good precipitant) decreased the value of both unfolding rates. These ion-specific (Hofmeister) effects were further used to estimate changes in accessible surface area (ΔASA) upon formation of the transition states (TS) for unfolding. Results obtained with LiCl and Na2SO4, which we analyzed by means of a parameterization derived from published solubility data of amino acid derivatives, are consistent with ΔASA increments (for each phase) of about 8.0% of the total theoretical ΔASA for complete unfolding of the chymopapain molecule. Results in the presence of GuHCl, which were analyzed by using a previous parameterization of protein unfolding data, gave larger ΔASAs of activation, equivalent to 13 and 16% of the total unfolding ΔASA.