Next, we investigated the optical properties and stability of the S-aryl linkage in the two labeled peptides P1-SNBD and cP1-SNBD. We examined their chemical stability after incubation in phosphate buffered saline (PBS, pH 7.0) at 25°C and cell culture media at 37°C (Figures and) as well as different biologically relevant reducing environments (Figure ), and found no major degradation in any of these conditions. Finally, we measured the emission spectra of P1-SNBD and cP1-SNBD and confirmed that the two peptides retained the fluorogenic behavior of benzodiazoles (Figure ). Altogether, these results indicate that Pd-mediated S-arylation provides a rapid means to site-specifically label unprotected peptides with good yields and purities, and that SNBD-labeled peptides exhibit suitable properties for live-cell imaging under physiological conditions. SNBD-labeled peptides maintain secondary structure and physicochemical properties of peptides.