Dependence of apoferritin crystal growth on temperature and cadmium concentration

Static light scattering and multiwell microbatch crystallization experiments have been used to determine how apoferritin crystal growth kinetics and final crystal size depend on temperature, pH, and cadmium concentration. The experiments were conducted on a linear thermal gradient covering a range from 30 to 40 degrees C. Crystal growth was monitored in situ under a microscope without disturbing the thermal environment. A dependence of apoferritin crystal formation and growth on temperature and cadmium concentration was observed. The range of cadmium concentrations that resulted in apoferritin crystal growth was evaluated and found to be consistent with the osmotic second virial coefficients determined via static light scattering. For the domain of conditions where apoferritin crystal formation occurred, the initial molar growth rate of apoferritin crystals increased linearly with temperature. It was observed that increasing cadmium concentration decreased the dependence of the final crystal size on temperature. The measured dependence of growth kinetics on temperature allowed the estimation of an activation energy for crystal growth. By comparing the value of this activation energy with that for nucleation (obtained from the literature), it became possible to explain the observation of larger, but fewer, apoferritin crystals at higher temperatures, which has also been reported in the literature for other proteins.