Fast preparation of a desiccant array for the gradual desiccation method in protein crystallization screening

The gradual desiccation method (GDM) is a modification of the vapor diffusion method for protein crystallization screening. This method can dramatically increase the chances of obtaining protein crystals and is therefore potentially useful for practical protein crystallization screening. However, it is troublesome to prepare the desiccant for the GDM because each of the 96 desiccants must be of the same mass. Repeated manual weighing of the desiccant (at least 96 times for one plate) to obtain the same amount is required, and manual distribution of the weighed desiccants to the respective reservoir wells is also necessary. These procedures require a considerable amount of labor and thus lower the efficiency of the screening process. Additionally, they reduce the applicability of this method in routine protein crystallization screening. To solve this problem, a high-throughput method is proposed, which involves dispensing an aqueous solution of salts (a combination of CoCl₂ and AlCl₃) into a droplet array (8 × 12, corresponding to the arrangement in a standard crystallization plate) on a piece of tape, then drying this array to obtain the final desiccant array. Simply covering and sealing this desiccant array over the crystallization droplets in the crystallization plate can give a perfect vapor diffusion screen. With this method, the labor and automation requirements of the GDM will be comparable to those of the conventional vapor diffusion method; furthermore, the amount of the desiccant can be easily and accurately controlled, allowing the GDM to be applied in daily protein crystallization screening.