Screening and optimization of protein crystallization conditions through gradual evaporation using a novel crystallization platform

High-throughput screening of a wide range of different conditions is typically required to obtain X-ray quality crystals of proteins for structure–function studies. The outcomes of individual experiments, i.e. the formation of gels, precipitates, microcrystals, or crystals, guide the search for and optimization of conditions resulting in X-ray diffraction quality crystals. Unfortunately, the protein will remain soluble in a large fraction of the experiments. In this paper, an evaporation-based crystallization platform is reported in which droplets containing protein and precipitant are gradually concentrated through evaporation of solvent until the solvent is completely evaporated. A phase transition is thus ensured for each individual crystallization compartment; hence the number of experiments and the amount of precious protein needed to identify suitable crystallization conditions is reduced. The evaporation-based method also allows for rapid screening of different rates of supersaturation, a parameter known to be important for optimization of crystal growth and quality. The successful implementation of this evaporation-based crystallization platform for identification and especially optimization of crystallization conditions is demonstrated using the model proteins of lysozyme and thaumatin.