Monitoring crystallization experiments using dynamic light scattering: Assaying and monitoring protein crystallization in solution

Abstract

Various analytical techniques may be valuable as precrystallization assays for monitoring and screening various protein crystal growth conditions. Dynamic light scattering (DLS) offers unique capabilities for such studies. DLS provides a measure of the translational diffusion coefficient, D, of the protein molecules. Assuming a spherical model for the protein, its hydrodynamic diameter, d, can then be estimated. DLS can be used to monitor crystallization events as a function of time, creating the possibility of a dynamically controlled crystal growth experiment under previously characterized solution conditions. Variation of d as a function of particular solution properties such as protein concentration, pH, precipitant concentration, or temperature can also be determined. Early in the evolution of a supersaturated solution of macromolecules, monomers assemble to form aggregates which lead either to crystals or to precipitates. In an obvious terminology it is convenient to refer to the former as craggs (precrystalline aggregates) and the latter as praggs (preprecipitating aggregates). The protein concentration dependence of d should be correlated with the thermodynamic equilibria governing these two types of aggregates and has been suggested as a way to differentiate between praggs and craggs in solution. Although other factors may also lead to a concentration dependence of d, these are in most cases expected to be small, relative to the effect of aggregate size distribution. This capability would allow one to make early predictions regarding the outcome of a particular, unknown crystallization condition on the basis of a quantitative experimental measurement. The protein concentration dependence of d has been used for several proteins to differentiate between the two types of aggregates.