Reactions of Sulfur-Containing Organic Compounds and Peptides in 1-Ethyl-3-methyl-imidazolium Acetate

Abstract

The neat ionic liquid (IL) [Cmim][OAc] is not just capable of dissolving thiol- and disulfide-containing compounds, but is able to chemically react with them without addition of any catalytic reagent. Through the analysis of four small organic molecules and a cysteine-containing peptide we could postulate a general reaction mechanism. Here, the imidazolium-carbenes preferentially react with the disulfide bond, but not thiol group. Moreover, the imidazole moiety was found to abstract the sulfur atom from the cysteine residue, providing an alternative way to transform Cys residues, which were artificially inserted into a peptide sequence in order to perform native chemical ligation (NCL) of two peptide fragments. Finally, the chemical reaction of [Cmim][OAc] with a cysteine-containing biomolecules can be tuned or even suppressed through the addition of at least 30% of water to the reaction mixture.

Unexpected Intrinsic Lability of Thiol-Functionalized Carboxylate Imidazolium Ionic Liquids

“…It has been recently reported [25] that neat 1-ethyl-3-methyl-imidazolium acetate is able to react with disulfide species to form the same imidazole 2-thione observed by us. The evolution of the reaction was followed by 1 H-NMR and it was proposed that the reaction proceeded through the formation of a carbene, by the abstraction of the H-2 proton from the imidazolium promoted by the acetate, followed by a nucleophilic substitution on the disulfide.…”

Challenges and Perspectives in Chemical Synthesis of Highly Hydrophobic Peptides

“…However, N-heterocyclic carbenes (NHC) which are present in the neat [C 2 mim][OAc], generated by subtraction of C2 proton by relatively basic acetate anion, initiated formation of a high percentage of by-products and therefore prevented the breakthrough of this method. In 2017, we identified those NHC-induced side-products and studied conditions where the formation of those side-products can be entirely suppressed, therefore enabling using [C 2 mim][OAc] as a solvent during NCL (Baumruck et al, 2017). Additionally, Wehofsky et al (2008) reported protease-catalyzed ligation in the presence of 60% of 3-methylimidazolium dimethylphosphate in buffer solution with high turnover rates.…”

“…A further promising approach offers ionic liquids as possible media for peptide desulfurization. Studies performed in [C 2 mim][OAc] gave first evidence that Cys can be desulfurized to Ala within the sequence of unprotected peptides (Baumruck et al, 2017). When applying desulfurization conditions, all other present cysteines need to be orthogonally protected.…”

“…With great interest to solve solubility problems during NCL, but also handling of peptides, ionic liquids seem to have enormous potential (Tietze et al, 2012;Baumruck et al, 2017). Besides SPPS and NCL, purification of these "difficult sequences" is not trivial.…”

NHC in Imidazolium Acetate Ionic Liquids: Actual or Potential Presence?

“…The demonstration of the potential presence of NHC in imidazolium acetates was obtained by many groups, carrying out successfully catalyzed or stoichiometric reactions (Lambert et al, 2016; Baumruck et al, 2017; Binks et al, 2018; Pandolfi et al, 2018), but in no case any attempt to evidence the actual presence of NHC was done. This last topic was faced in a few papers.…”