BCS PEG Smears
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 from SGC, Oxford and sold by Molecular Dimensions overcomes the impracticality of sampling all PEG varieties that has led to a few variants predominating in the most common screens (right).
PEG Smears consist of a mixture of PEG compounds. The BCS Screen utilises four different Smears (see right) - low molecular weight, medium molecular weight, high molecular weight and broad range of molecular weights. In each case equal volumes of individual stock solutions at equal concentrations are mixed to produce the final solution.
PEG Smears are designed primarily for efficient screening of chemical space. It is generally thought that well-defined, single PEG species produce the best diffracting crystals. Thus, optimization might involve identifying the single PEG compound that produces the best crystal for structural studies.
HOWEVER, in some cases the use of a PEG smear from The BCS Screen produced better quality data than using any single PEG species. On the left, you can clearly see that MMAA crystals grown with a LMW smear diffract better than those grown with a single PEG species.
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.
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