Several ionic liquids have been shown to act as suppressors of protein aggregation and to effectively promote the in vitro refolding of denatured proteins. In this work, the influence of the anion of these organic salts on their performance as refolding enhancers was tested, using a series of N-ethyl-N'-methyl imidazolium (EMIM) salts. Variation of the anion had a profound effect on the renaturation of the recombinant plasminogen activator rPA, which served as a model protein. The effect was roughly correlated with the stability of proteins in the tested aqueous ionic liquid solutions. Strongly destabilizing anions with higher alkyl substitutions like, e.g., hexyl sulfate were unable to promote protein refolding. However, the dimethyl and diethyl phosphate salts, which are known to be quite compatible with protein stability, also effectively suppressed renaturation, while alkylated sulfate anions with the same influence on protein stability did promote in vitro refolding. Only ionic liquid co-solvent systems with an intermediate capacity for solubilizing the proteinogenic amino acid tryptophan were found to permit effective renaturation of the model protein rPA. The most effective refolding enhancer among the tested ionic liquids was EMIM chloride.
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