Fmoc-Arg(Mtr)-Opfp is a protected derivative of the amino acid arginine, serving as a key building block in peptide synthesis, especially solid-phase peptide synthesis (SPPS). The Fmoc (9-Fluorenylmethyloxycarbonyl) group protects the α-amino group, removable under mild basic conditions. The Mtr (4-Methoxy-2,3,6-trimethylbenzenesulfonyl) group safeguards the guanidino side chain of arginine, deprotected under stronger acidic conditions. This orthogonal protection enables selective, sequential peptide assembly with high control and purity. The pentafluorophenyl (Opfp) ester activates the carboxyl group for efficient coupling, reducing side reactions. It is widely used in producing therapeutic peptides, research peptides, and biologically active compounds.