Table 1.

Methods for identifying salt crystals

Properties	Protein crystals	Inorganic crystals	Notes
If identical crystals are in the reservoir	No	Yes	Good control observation
If identical crystals appear in a control experiment that does not contain the protein	No	Yes	A good control experiment to setup
If crystals dehydrate upon exposure to air	Likely	Unlikely	Protein crystals are typically ~30–70% water
If crystals can be readily crushed by mechanical stress	Likely	Unlikely	Protein crystals are held together with weak interactions
If the crystals absorb dye	Likely	Unlikely	Dye can diffuse into protein crystal’s solvent channels
If washed crystals give an SDS-PAGE band at the expected MW	Likely	Unlikely	Useful check to run if enough sample is available
If washed crystals give an SDS-PAGE band at a different MW	Likely	Unlikely	Possible contamination or proteolysis, proceed with caution
If there is weak birefringence under crossed-polarization	Likely	Unlikely	Cubic systems will not show this Inorganic crystals extinguish rapidly
If there is fluorescence at 280nm	Likely	Unlikely	If tryptophan is present, some inorganic chemicals can fluoresce
If there is no X-ray diffraction	Likely	Unlikely	Some inorganic crystals may not diffract X-rays
If X-ray diffraction shows a few well spaced reflections	Unlikely	Likely	Characteristic of salt diffraction