Protein crystallization nucleation mechanism

Many smdies have contributed to the understanding of protein crystallization however, there is no unified approach that can yet fully explain its mechanism at a molecular level. In the following, specified notions of the factors governing protein crystal nucleation and growth are presented. 

The lipidic cubic phase has recently been demonstrated as a new system in which to crystallize membrane proteins [143, 144], and several examples [143, 145, 146] have been reported. The molecular mechanism for such crystallization is not yet clear, but the interfacial water and transport are believed to play an important role in nucleation and crystal growth [146, 147], Using a related model system of reverse micelles, drastic differences in water behavior were observed both experimentally [112, 127, 128, 133-135] and theoretically [117, 148, 149]. In contrast to the ultrafast motions of bulk water that occurs in less than several picoseconds, significantly slower water dynamics were observed in hundreds of picoseconds, which indicates a well-ordered water structure in these confinements. 

Two early observations may have some relevance to the mechanism by which antifreeze proteins act. Polymer gels of highly ramified, net-like structures have been shown to depress the freezing point of water and cause the ice crystals to assume a dominant growth orientation (93). It is also noteworthy, perhaps, that certain amino acids are good ice nucleators whereas others are not (94). 

Ten Wolde and Frenkel [171] have made the very interesting observation that this hidden transition can nonetheless profoundly influence the crystallization behavior of the system. Fluids that are in the vicinity of this submerged critical point display substantial density fluctuations, just as they do when near a usual critical point. The crystallization mechanism in these instances proceeds by a route in which the fluid fluctuates to a solid-like density before arranging itself into a crystal form. This is in contrast to a mechanism in which the crystal first nucleates into a very small crystal, which then grows as it encounters additional fluid molecules. This understanding can contribute to the difficult art of crystallizing proteins. In fact, successful crystallizations have been known to be associated with a fluid opalescence that previously was not considered to be in any way related to the same effect seen in critical fluids. Density-functional approaches have since been applied and found to support the ten Wolde-Frenkel hypothesis [172]. 

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