Direct observation of nucleus structure and nucleation pathways in apoferritin crystallization

Using atomic force microscopy (AFM) in situ during the crystallization of t he protein apoferritin from its solution, we imaged the arrangement of the molecules in near-critical clusters, larger or smaller than the crystal nuc leus, 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 i n 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 n uclei structures might be common, especially for anisotropic molecules. Hen ce, the nucleus structure should be considered as a variable by advanced th eoretical treatments.