Precrystallization clusters of holoferritin and apoferritin at low temperature.

The formation of small nanosized clusters of the proteins holoferritin and apoferritin at low temperature was studied using small angle x-ray scattering. A strikingly large temperature dependence for the average molecular spacing in the clusters was observed. Calculations of the scattered intensity for various cluster models were performed. Comparison of the data with the simulations revealed the presence of crystalline order in the clusters of size ranging from a few molecules to a few hundred molecules. The crystalline order was found to be preserved with the lattice spacing varying with temperature by up to 20%. The small clusters were observed to grow into large micron-sized crystals when they were annealed and under certain conditions, the small clusters were found to coexist with the large crystals. This suggests that these clusters are closely related to critical nucleation. The data are consistent with an isotropic nucleation pathway, but cannot completely rule out a smaller presence of planar nucleation.