Macromolecular crystallization is a complex process, involving a system that typically has five or more components (macromolecules, water, buffer + counterion, and precipitant). Whereas small molecules have only a few contacts in the crystal lattice, macromolecules generally have 10's or even 100's of contacts between molecules. Formation of a consistent, ordered, three-dimensional (3D) structure may be difficult or impossible in the absence of any or presence of too many strong interactions. Further complicating the process is the inherent structural asymmetry of monomeric (single chain) macromolecules. The process of crystal nucleation and growth involves the ordered assembly of growth units into a defined 3D lattice. We propose that tetragonal lysozyme crystal nucleation and growth solutions are highly self-associated and that associated species having 4_3 helix symmetry are the building blocks for the nucleation process. This solution phase self-association carries over into the crystal growth phase, with the aggregated species as the growth units, recapitulating the nucleation process. The symmetry acquired in solution phase self-association facilitates both nucleation and crystal growth. If this model is correct, then fluids and crystal growth models assuming a strictly monodisperse nutrient solution need to be revised. This model has been developed from experimental evidence based upon face growth rate, atomic force microscopy, and fluroescence energy transfer data for the nucleation and growth of tetragonal lysozyme crystals.
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