The Effects of Salt Concentrations of Crystallization Conditions on Salt Bridges in Crystal Structures

Abstract

X-ray crystallography is the most favored method for obtaining the three-dimensional structures of proteins. This method requires that proteins are crystallized under solutions that sometimes require high concentrations of salt. Salt bridges are suggested to be mostly exposed on the protein surface. Therefore, if high salt concentrations are used in the crystallization conditions, the cations and anions in the solvent may interact with the charged resides that normally interact with each other to form salt bridges, in which case fewer salt bridges might be observed. In order to investigate whether the amount of salt is correlated with the number of salt bridges that are observed in the crystal structures, eight structures of lac repressor and nine structures of CAP grown in crystallization conditions of various ionic strengths were analyzed using PyMOL. The lac repressor data showed a slight negative correlation, and the data for CAP showed no correlation. An unexpected finding was that multiple structures of the same protein showed variable formation of individual salt bridges, uncorrelated with ionic strength