Multistep Crystallization Pathways for Protein Crystals and Colloidal Assemblies

While much is now known about the extraordinary complexity and structural diversity possible for crystals of proteins, nanoparticles and colloids, far less is known about the process by which these crystals form. Simple "classical" models of nucleation and growth may hold for the self-assembly of simple crystal structures, but what of crystals with large unit cells? We present recent results from computer simulation demonstrating that crystallization pathways in protein solutions or entropic colloidal fluids may be as diverse and complex as the resulting crystals, with crystallization following a remarkable variety of multistep pathways that include fluid-fluid transitions. We explore the role of shape, entropy, and, in the case of proteins, both specific and nonspecific interactions in selecting the pathway.