We examined the site-selective reactivity of Pd-SNBD by synthesizing the linear model nonapeptide P1 (sequence: KLYACSRYL, Figure ), which included one Cys residue as well as other reactive amino acids (e.g., Lys, Arg, Tyr). The unprotected peptide P1 was synthesized using standard Fmoc/ t Bu SPPS and subsequently coupled to Pd-SNBD under different reaction conditions, including variable temperatures, buffers and molar ratios (Figure ). Time-course HPLC analysis of the coupling reactions confirmed the effective installation of the benzodiazole fluorogen to the peptide P1 using 5 equivalents of Pd-SNBD in aqueous buffers and DMF as a co-solvent. Remarkably, the optimization of the reaction conditions led to the clean formation of the linear fluorogenic peptide P1-SNBD as well as its head-totail cyclic analogue cP1-SNBD (Figure ) in less than 60 min with good conversions (e.g., 50-90%) and purities >90% after HPLC purification (Figures and). These findings confirmed the applicability of the Pd-SNBD complex as a straightforward fluorogenic reagent to effectively label Cys residues in unprotected linear and cyclic peptides.