An Effective Method for Isolating Ergothioneine Analogues for Use in Peptide Synthesis: 2‐Thiohistidine and Fmoc‐2‐Thiohistidine

Here, we report on an improved synthesis of the novel amino acid 2-thiohistidine and its Fmoc-derivative through the addition of sodium chloride to the isolation steps of each synthetic procedure. These compounds must be synthesized in either water or a mixture of water/organic solvent and have proven difficult to isolate by traditional methods. We serendipitously discovered that the addition of sodium chloride caused each compound to precipitate from the solution during the workup. Since 2-thiohistidine is the amino acid analogue of the natural product ergothioneine, we hypothesize that sodium cations mimic the binding of other biologically relevant metal cations to the thioimidazole moiety. Ergothioneine is known to bind Cu⁺, Zn²⁺, and Hg²⁺, and it may bind other metals. Our fortuitous discovery is not only useful for the isolation and purification of 2-thiohistidine and its derivatives; peptides that contain this unique amino acid may be designed to take advantage of binding to Group I cations and other metals.