Z-Phe-ONp, or N-(Benzyloxycarbonyl)-L-phenylalanyl 4-nitrophenyl ester, is a protected and activated derivative of phenylalanine used as a building block in peptide synthesis. It enables efficient coupling of the phenylalanine residue to the N-terminus of another amino acid via the reactive p-nitrophenyl ester, which acts as a good leaving group to facilitate peptide bond formation. This reagent is essential in research laboratories for synthesizing complex peptides and studying their structures and functions. It is also widely applied in pharmaceutical development for creating peptide-based drugs through precise sequence assembly.