An improved chemical approach toward the C‐terminal sequence analysis of proteins containing all natural amino acids

An improved chemical method, capable of derivatizing all natural amino acids to their corresponding thiohydantoins, is described. This involves activation by acetyl chloride in TFA followed by derivatization with ammonium thiocyanate. Possible interference of reactive side chains was investigated by reacting N‐acetylamino acids as well as several peptides with propionyl chloride instead of acetyl chloride. The products were characterized by PDMS mass spectrometry and 1H‐NMR. This chemical method allows, for the first time, complete derivatization of N‐acetylproline to proline thiohydantoin. Applying this chemistry to peptides with a C‐terminal proline, the yields for formation of proline thiohydantoin were found to be up to 60%, depending on the peptide sequence. The previous inability to derivatize C‐terminal proline to thiohydantoin was thought to stem from the fact that proline cannot form the oxazolonium ion required for efficient reaction with the thiocyanate ion. However, we have found mass spectrometric evidence for the existence of a proline oxazolonium ion, under basic as well as under acidic conditions. This improvement in derivatization of C‐terminal amino acids including proline is a major step forward in the development of a general chemical C‐terminal sequencing method that permits the C‐terminal sequence analysis of proteins of any amino acid composition.