During protein crystallization screening, we encountered several types of proteins grew yellow stick crystals with a reservoir containing 0.1 M BIS-TRIS pH 6.5, 2.0 M Ammonium sulfate. However, X-ray diffraction proved that those yellow/brown crystals were salt crystal. Protein was purified in lysis buffer (150 mM NaCl, 25 mM Tris-Hcl pH=8.0, 5 mM DTT). We use sitting drop vapor diffusion method (1 ul protein plus 1 ul reservoir) at 18◦C. My question is "What is the yellow/brown salt crystal",as I know ammonium sulfate is white color crystal.
Hi,
According to your description, the crystal you obtained before is salt crystal (the X-ray diffraction data proved), then I doubt whether the reservoir contaminated during the process of crystallization, especially in the screening of crystals using 96-well sitting drop crystal plate. At the time, due to improper operation or poor sealing, it is very likely that the reservoir will contaminate each other. It is recommended that you use a hanging drop method (24-well plate) and increase the drop volume.
Good luck!
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