The Surprise Impurity From Acetylating Peptides

Summary. Process chemists at Ferring Pharmaceuticals in Denmark reported in Organic Process Research & Development, published online on 8 October, previously unreported N-terminal hydantoin and N-acetyl hydantoin impurities that formed while capping peptide N-termini with acetic anhydride under commonly used base and solvent conditions. N-terminal acetylation is a routine step in peptide synthesis, so an unnoticed side product at this stage can carry through to the final material. The team isolated the impurities and determined their structures by high-resolution mass spectrometry and NMR spectroscopy.

Research context (RUO). Systematic studies showed that formation of the byproducts depends strongly on the solvent and on the peptide sequence, and the authors identified conditions that suppressed them. Isotope-labelling experiments ruled out acetic anhydride as the source of the carbonyl group built into the hydantoin ring and supported participation of the solvent in the side reaction. The authors describe the finding as an unexpected side reaction in widely used acetylation chemistry and offer practical guidance for impurity control in peptide synthesis and process development. The study is analytical and synthetic chemistry and involves no biological testing. For laboratories that make or analyse N-terminally acetylated peptides, it flags a family of impurities worth looking for in mass spectra and chromatograms, and shows that solvent choice is one lever for keeping them in check.