Group I metal ions of increasing atomic weight suppress the growth of crystalline aggregates and enhance the growth of large, single crystals of T7 RNA polymerase

Crystals of bacteriophage T7 RNA polymerase grown with ammonium phosphate or ammonium sulfate as precipitant and glycerol as a structure stabilizing agent exhibit a strong tendency to grow as aggregates. To identify an agent that would suppress this behavior a large number of metal salts were screened as additives to the crystallization solution. The singly charged group I metal ions (with the exception of lithium) were found to suppress the growth of crystalline aggregates and promote the growth of large, single crystals. The degree to which the growth of aggregates was suppressed and the growth of single crystals was enhanced was clearly and reproducibly correlated with the atomic weight of the metal. Thus, ordered according to their ability to suppress the growth of aggregates and enhance the growth of single crystals, the metal ions were Cs > Rb > K > Na.