The Mechanism of Cryoprotection of Proteins by Solutes

We have tested the capacity of 28 different compounds to protect lactate dehydrogenase from damage during freeze-thawing. These solutes come from very dissimilar chemical classes including sugars, polyols, amino acids, methylamines, and lyotropic salts. All the compounds tested, except NaCl, protected the enzyme, to varying degrees, from inactivation. The only characteristic that these compounds have in common, as a group, is that they have all been shown to be preferentially excluded from contact with the surface of proteins in aqueous solution. It has been demonstrated previously (via thermodynamic arguments) that this interaction of solutes with proteins leads to the stabilization of proteins in nonfrozen, aqueous systems. Conversely, those solutes, e.g., urea and guanidine HCl, that bind to proteins destabilize proteins in solution, and we have found that they also enhanced the inactivation of lactate dehydrogenase during freeze-thawing. Based on the results of our freeze-thawing experiments and a review of the theory of protein stabilization in nonfrozen, aqueous solution we propose that the cryoprotection afforded to isolated proteins by solutes can be accounted for by the fact that these solutes are preferentially excluded from contact with the protein's surface.