Improving protein crystal quality by decoupling nucleation and growth in vapor diffusion

A simple method for growing protein crystals in the metastable zone using the vapor diffusion technique is described. The coverslips holding the hanging drops are transferred, after being incubated for some time at conditions normally giving many small crystals, over reservoirs at concentrations that normally yield clear drops. Fewer, much larger and better diffracting crystals are obtained, compared with conventional crystallization at similar conditions. To our knowledge, this is the first report of a significant crystal improvement due to "backing off" from nucleation conditions, using the hanging drop method. A correlation of the transfer time with published results for vapor diffusion equilibration of poly(ethylene glycol) solutions is also presented.