This is the continuation of a target agnostic survey of often used protein crystallization reagents, based on data obtained from the Biological Macromolecule Crystallization Database (BMCD ver. 4.03). The question I'm trying to address is: which buffers and salts should you inventory?
Covering protein crystallization space with PEGs seemed a simple affair: a set of only 12 different Polyethyleneglycols is sufficient to formulate ca. 88% of all PEG-based protein crystallization conditions.
The situation is much less clear cut for buffers and salts that are relevant to protein crystallization. Shy of half of all protein crystallizations listed in the (BMCD ver. 4.03), 45%, can be carried out with 8 different buffers (see Fig. below). Tris buffer seems to be the champion. I interpret this as a result of investigator bias rather than there being a solid scientific reason for this buffer to play such an important role. My explanation is that neutral pH Tris buffers dominate the lab bench, and researchers take what they find first… If this is in fact true, it could support the notion that the nature of the buffer is of somewhat low importance for many protein crystallizations.
Salts are much more interesting since they can have a dramatic effect on water properties and protein surface decoration, both affecting the ordered association of protein molecules into a crystal. Ammonium sulfate, the classic protein precipitation reagent is the clear winner. Curiously, several of the salts, such as citrates, phosphates and acetates, - the ones that provide both high ion strength and pH buffer capacity are fairly high ranked.
Popular protein crystallization buffers and salts as extracted from the Biological Macromolecule Crystallization Database (BMCD ver. 4.03)
When it comes to warehousing stock solutions for simple and quick preparation of optimization screens, these buffers: CHES, CAPS, Bicine, Tris, Hepes, Imidazole, Bis-Tris, MES
and these salts: magnesium acetate, lithium nitrate, calcium chloride, zinc acetate, potassium/sodium tartrate, sodium citrate, sodium chloride, sodium phosphate, potassium citrate, magnesium sulfate, lithium chloride, calcium acetate, ammonium phosphate, ammonium sulfate are a good start.