Phenylthiohydantoin-tryptophan (PTH-Trp) is the stable derivative formed from the N-terminal tryptophan residue during Edman degradation in protein sequencing. It is commonly used as a reference standard for identifying and quantifying tryptophan derivatives via high-performance liquid chromatography (HPLC) or other chromatographic methods. This compound's distinct retention time and UV absorbance properties facilitate precise analysis of peptide sequences and protein structures in biochemical and proteomics research.