Kinetics and mechanisms of protein crystallization at the molecular level

This chapter focuses on the processes by which a protein molecule in a supersaturated solution joins a protein crystal. The pair of proteins ferritin/apoferritin is used as an example. The most commonly used technique in such investigations has been atomic force microscopy. I discuss the modifications and tests of the atomic force microscope necessary to obtain real-time, in situ molecular resolution imaging. Then, I review tests that establish a quantitative correspondence between the continuous models of crystal growth and the discrete nature of the processes at the molecular level. I address the issue of whether the incorporation of a molecule from a solution into a growth site on the crystal surface is limited by the slow rate of decay of an Eyring-type transition state. The conclusion is that, on the contrary, a scenario, envisioned by Smoluchowski and Debye, is followed in which the kinetics is only limited by the rate diffusion over the free-energy barrier of interaction between two molecules, or an incoming molecule and a surface. Review of the data for many other protein and nonprotein systems suggests that this conclusion is valid not only for the crystallization of ferritin/apoferritin, but also for many other protein and small-molecule crystallization systems. Finally, I review results establishing that the pathway that a molecule from a solution takes on its way to an incorporation site on the crystal surface is indirect: it includes adsorption on the surface and two-dimensional diffusion toward the incorporation site. These results will likely contribute to the understanding at the molecular level not only of the processes of crystallization of proteins and small molecules, but also of the deposition of layers of proteins and other soft materials on substrates, and of other processes of self-assembly in solution.