Denaturants Stabilizers
 
The structures of globular proteins (and other biological macromolecules) are not very stable. Therefore, it is often necessary to increase the stability by addition of inert compounds in order to maintain a biochemical activity or to preserve an assembled structure, such as various organelles or multisubunit enzymes. The stabilizing compounds must characteristically be added at high concentration (>1 M), since their interactions with proteins are weak (see Stabilization And Destabilization By Co-Solvents). Strongly interacting molecules, although effective in particular cases, generally tend to alter some properties of the protein molecules.
The most frequently used stabilizing agents are 1 M sucrose, other sugars (trehalose, glucose), 3 to 4 M glycerol, neutral amino acids at 1 M levels (glycine, proline, alanine), methyl amines (betaine, sarcosine, trimethyl amine N oxide), polyols (sorbitol, inositol, mannitol), some salts [Na₂SO₄, (NH₄)₂SO₄] (1). All have the property of being preferentially excluded from proteins, which imparts stabilization and precludes excessive surface contacts that could alter the local protein structure. Most stabilizers used in the laboratory are also cryoprotectants and osmolytes, in that they are chosen by nature to maintain high osmotic pressure in, eg, amphibians, and to protect against freezing (2).
 Conversely, a number of compounds are used to denature proteins when so desired. These again act at high concentration and are characterized by their preferential binding to proteins. The most common denaturants are urea, guanidinium chloride, sodium dodecyl sulfate (SDS), trichloroacetic acid, CaCl₂, LiBr, and other salts on the denaturant side of the Hofmeister series (3).
References
1. S. N. Timasheff (1992) In Stability of Protein Pharmaceuticals (T. J. Ahern and M. C. Manning, eds.), Plenum, New York, pp. 265, 286. 
2. P. H. Yancey, M. E. Clark, S. C. Hand, R. D. Bowlus, and G. N. Somero (1982) Science 217, 1214-1222 .
3. P. H. von Hippel and T. Schleich (1969) In Structure and Stability of Biological Macromolecules (S. N. Timasheff and G. D. Fasman, eds.), Marcel Dekker, New York, Chap. "6".