Crystallization adjutant

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A crystallization adjutant is a material used to promote crystallization, normally in a context where a material does not crystallize naturally from a pure solution. It can either interact directly with the protein, and end up in a fixed position in each unit cell of the resulting crystal, or interact more with the disordered parts of the solvent.

Typical adjutants are polyethylene glycols of various molecular weights, usually rather higher than the ones used for lubrication, and buffers such as citrate ion to enforce the rather precise pH requirements for crystallization that many proteins have. Small inorganic ionic compounds such as calcium sulfate are also used, though often this is because of a biological requirement for Ca²⁺ for the protein to fold into the correct form. For membrane proteins, a precisely-calibrated concentration of a [detergent] is often necessary. Alcohols and amines are also commonly used; the hope there is that the functional groups will arrange themselves rigidly between functional groups in the protein and stabilize a region around them.

Finding the precise combination of adjutants that promotes crystallization is a "black art"; careful experimental design can produce results more efficiently than exhaustive search, but the final optimization is almost always a matter of running tens to hundreds of experiments and waiting for one to produce crystals.