Crystal growth studies in microgravity with the APCF .1. Computer simulation of transport dynamics

A computer program has been designed to simulate the pre-nucleation history of protein crystal growth experiments in the Advanced Protein Crystallization Facility developed by ESA to conduct crystallization experiments under microgravity conditions. The program was fed with the initial conditions of a set of experiments performed by the ESA Diagnostic Group during the SpaceHab-01 mission. The time histories of the salt and protein concentrations and supersaturation at any point inside the reactor were obtained. The differences between Dialysis and Free Interface Diffusion techniques are discussed. Both DIA and FID experiments performed in the short protein chamber of the APCF are basically very slow mixing batch experiments although differences exist in the equilibration behavior. The kinetics of protein nucleation coupled with transport phenomena are analyzed, showing that the supersaturation rate modifies the supersaturation value at which nucleation occurs as well as waiting times for nucleation. Reactor geometry and initial conditions can be optimized. As a general rule, higher protein concentrations will produce crystals that nucleate at lower supersaturation and supersaturation rates, provided that the growth chamber is long enough to implement a true diffusive set-up. When these conditions are fulfilled, the growth system becomes self regulated and crystals nucleate under different supersaturation and supersaturation rate conditions, performing a kind of automatic, single-reactor screening experiment.