PEG is an effective crystallization agent due to its protein-protein interaction promoting properties. The Biological Macromolecular Crystallization database records at least 20 different PEG compounds in frequent use as macromolecular precipitants. The BCS Screen overcomes the impracticality of sampling all PEG varieties that has led to a few variants predominating in the most common screens (right).
The bottom line is that in tests at the SGC with 191 human proteins, The BCS Screen had a 42% hit rate with only 96 conditions. Many of these proteins had failed to crystallize in other screens.
The first 24 conditions in The BCS Screen form a grid screen of the four PEG Smears against pH (range 4.5-9.5). The remaining 72 conditions consist of a sparse matrix screen in which the precipitant is always one of the PEG Smears but with a variety of additives and buffers. The additives include some of the more unusual crystal-promoting agents such as rubidium chloride (also useful for phasing) and ammonium nitrate. More common additives including sodium bromide (useful for phasing) and glycerol are also used in the screen.
Reference
Chaikuad, A, Knapp, S and von Delft, K (2015). Defined PEG smears as an alternative approach to enhance the search for crystallization conditions and crystal-quality improvement in reduced screens. Acta Crystallographica F71: 1627-1639.
