Direct observation of nucleus structure and nucleation pathways in apoferritin crystallization

Using atomic force microscopy (AFM) in situ during the crystallization of the protein apoferritin from its solution, we imaged the arrangement of the molecules in near-critical clusters, larger or smaller than the crystal nucleus, that are representative of the nucleus structure. At supersaturations Delta mu/k(B)T of 1.1 -- 1.6 -- 2.3, the nuclei contain about 50 -- 20 -- 10 molecules. The molecular arrangement within the nuclei is similar to that in the crystal bulk. Contrary to the general belief, the observed nuclei are not compact molecular clusters, but are planar arrays of several rods of 4--7 molecules set in one or two monomolecular layers. Similarly unexpected nuclei structures might be common, especially for anisotropic molecules. Hence, the nucleus structure should be considered as a variable by advanced theoretical treatments.