MES, BES, CHES and PIPES buffers which possess a sulfonylethyl group, can contain a contaminant, oligo (vinylsulfonic acid), that is a side product in the chemical synthesis of these buffers. This polyanionic contaminant mimics RNA and at low salt (0.05 M Sodium chloride (NaCl) binds to and inhibits the enzyme with a Ki of 11 pM. This has implications to anyone studying nucleic acid-binding proteins.

Reference
Smith, B. D., Soellner, M. B., and Raines, R. T. (2003). Potent inhibition of ribonuclease A by oligo(vinylsulfonic acid). J. Biol. Chem. 278, 20934–20938.