What Is the Molecular-Level Role of the Solution Components in Protein Crystallization?

Buffers, organic and inorganic salts, small-molecule organic compounds, and polymers are added to protein solutions to facilitate the formation of protein crystals. To understand the role that these additives play at the molecular level, we analyze several recent results on the thermodynamics and kinetics of protein crystallization and on the interactions between protein molecules in solution. The presented evidence shows that the structuring of the water molecules around the protein molecules in solution is the main component of the thermodynamics of crystallization, has a significant influence on the protein intermolecular interactions in solution, and largely determines the kinetics of growth of the crystals. It appears that an important function of the different additives is similar: they rearrange the structured water to make crystallization thermodynamically feasible and kinetically achievable. On the basis of these insights, we formulate a potential pathway toward rational search for solution compositions leading to crystals of any protein of interest. The proposed pathway consists of identifying the amino acid groups that partake in intermolecular crystal contacts in the known protein structures, understanding the water structuring around the identified amino acid groups, understanding how the typical additives used in protein crystallization modify the water structures, an algorithm leading to a suggestion for crystallization conditions for a protein of interest.