Analysis and statistics of crystallisation success increase by composition modification of protein and precipitant mixing ratio.

The nucleation zone has to be reached for any crystal to grow, and the search for crystallization conditions of new proteins is a trial and error process. Here a convenient screening strategy is studied in detail that varies the volume ratio of protein sample to the reservoir solution in the drop to initiate crystallization that is named "composition modification". It is applied after the first screen and has been studied with twelve proteins. Statistical analysis shows a significant improvement in screening using this strategy. The average improvement of "hits" at different temperatures is between 32 and 42%, for examples, 41.8% ± 14.0% and 35.7% ± 12.4% (± standard deviation) at 288 K and 300 K, respectively. Remarkably, some new crystals were found by composition modification which increased the probability of reaching the nucleation zone to initiate crystallization. This was confirmed by a phase diagram study. It is also demonstrated that composition modification can further increase crystallisation success significantly (1.3 times) after the improvement of "hits" by temperature screening. The trajectories of different composition modifications during vapour diffusion were plotted, further demonstrating that protein crystallizability can be increased by hitting more parts of the nucleation zone. It was also found to facilitate the finding of initial crystals for proteins of low solubility. These proteins gradually become more concentrated during the vapour diffusion process starting from a larger protein solution ratio in the initial mixture.