Phenylthiohydantoin-proline is primarily used in the field of peptide and protein sequencing. It plays a critical role in the Edman degradation process, a method for stepwise removal and identification of amino acids from the N-terminus of a polypeptide. This chemical helps in the efficient cleavage and identification of proline residues, which are often challenging to analyze due to their unique ring structure. Its application is essential in biochemical research, particularly in understanding protein structure, function, and interactions. Additionally, it is utilized in the development of diagnostic tools and therapeutic agents by enabling precise analysis of protein sequences.