Pre-Crystallization State of Ferritin at Low Temperature

In the course of a systematic exploration of the crystallization kinetics and conditions of the protein ferritin using x-rays, we discovered an unexpected new state of aggregation of the protein at low temperature. This new state was found to form reversibly below the freezing point of the solution. Using Small Angle X-ray Scattering (SAXS), we studied the properties of solutions of ferritin upon cooling and found that ferritin molecules form clusters of varying size and structures depending on the temperature and the thermal history of the sample. Simulations of the SAXS patterns were made using various cluster structures and these show that clusters of roughly 50 molecules form upon freezing. The structure was found to be similar to the FCC structure of macroscopic ferritin crystals which leads us to the conclusion that these clusters may be precursor states to the crystallization of ferritin.