Quasi-planar nucleus structure in apoferritin crystallization

First-order phase transitions of matter, such as condensation and crystalli zation, proceed through the formation and subsequent growth of 'critical nu clei' of the new phase. The thermodynamics and kinetics of the formation of these critical nuclei depend on their structure, which is often assumed to be a compact, three-dimensional arrangement of the constituent molecules o r atoms(5,6). Recent molecular dynamics simulations have predicted compact nucleus structures for matter made up of building blocks with a spherical i nteraction field(7,8), whereas strongly anisotropic, dipolar molecules may form nuclei consisting of single chains of molecules(9). Here we show, usin g direct atomic force microscopy observations, that the near-critical-size clusters formed during the crystallization of apoferritin, a quasi-spherica l protein, and which are representative of the critical nucleus of this sys tem, consist of planar arrays of one or two monomolecular layers that conta in 5-10 rods of up to 7 molecules each. We rnd that these clusters contain between 20 and 50 molecules each, and that the arrangement of the constitue nt molecules is identical to that found in apoferritin crystals. We anticip ate that similarly unexpected critical nucleus structures may be quite comm on, particularly with anisotropic molecules, suggesting that advanced nucle ation theories should treat the critical nucleus structure as a variable.