Figure 4.

Vapor-diffusion method. A drop containing unsaturated precipitant and protein solution is placed in a well containing a reservoir with precipitant in higher concentrations. The well is hermetically sealed to prevent droplet evaporation and to allow vapor equilibration of the droplet and the reservoir. Equilibration of water vapor from the protein-containing droplet to the reservoir solution causes the protein solution to reach a supersaturation level, where nucleation and initial growth occur. (A) Hanging drop technique. The droplet is placed on a siliconized glass that is used to close the well; (B) Sitting drop technique. The droplet is placed on a small bridge inside the well; (C) Sandwich drop technique. The droplet is placed between two cover slips, one of which closes the well.