Thioimidate Solutions to Thioamide Problems during Peptide Synthesis

Thioamides have structural and chemical similarity to peptide bonds, and therefore offer valuable insights when probing peptide backbone interactions, including hydrogen bonding, stereoelectronic, and hydrophobicity effects. There is a perception that methods to install thioamides within peptides are sufficient, yet anecdotal reports indicate that many labs have sought to employ thioamides in a variety of studies but the results of many synthetic campaigns do not yield the intended products, leading researchers to abandon such projects and any information these structural probes would provide. We catalogue and provide evidence for the major pitfalls associated with current methods to synthesize thioamide-containing peptides during each stage of solid-phase peptide synthesis (SPPS), including (A) thioamide coupling, (B) peptide elongation, and (C) peptide cleavage from resin. We then demonstrate the utility of thioimidate protecting groups as a means to side-step each of these problematic synthetic difficulties. Our approach is generally applicable to all peptides and ultimately permits access to an important benchmark $\alpha$-helical peptide that had previously eluded synthesis and isolation. With the process of thionopeptide synthesis demystified, a broader range of researchers should find it easier to employ thioamides in the study of peptide-based biomolecules.