Crystal nucleation is the first stage of the protein crystallization process; it is usually considered the bottleneck in growing high quality protein crystals. Also, it not easy to measure crystal nucleation rates directly. The earlier indirect experimental studies of nucleation were performed using quasi-elastic light scattering (QELS) [246], dynamic light scattering [247–249], and also X-ray and neutron scattering. By using the diffusion approximation, one can extract from the scattering data the distribution of particle (molecules and aggregates) sizes. One peak in the scattering intensity corresponds to the molecule's size. The second peak corresponds to the aggregate size. Observing the second peak can provide some information about the nucleation kinetics. The disadvantage of such methods is that they require various assumptions about particle shapes and the scattering intensity distribution that are difficult to verify.

In this chapter we review several experimental studies of homogeneous nucleation. We note at the onset that it is notoriously difficult to avoid heterogeneous nucleation in experimental measurements. In fact, there is still an ongoing debate as to whether the observed nucleation for lysozyme, discussed in Section 8.1, is homogeneous or heterogeneous. We do not discuss this issue here; an excellent review of heterogeneous nucleation and its possible relevance to lysozyme is given in ref. [619].