KCS

Fluorescent labeling has been used as an important tool for detection of functional molecules in the biological system. Various fluorescent substances have been studied, however, limited biocompatibility and relatively high cost of fluorescent substances still need improvement for the application in biosystem. Therefore, it is important to develop fluorescent labeling method for simple and low-cost. In a previous research, it was reported that 5-oxo-2,3-dihydro-5H-[1,3]thiazolo[3,2-a]pyridine-3,7-dicarboxylic acid (TPA), a blue fluorescent substance with a high-quantum yield, could be synthesized from citrate and cysteine via one-step reaction. It is very promising for the biocompatible and low-cost fluorescent substance because both citrate and cysteine are highly-biocompatible and common substances in biological systems. However, the reaction has low yield and very high-temperature condition around 150 °C, which is difficult to be applied for the direct labeling onto the biological samples such as proteins or peptides. Therefore, in this study, inspired by the fact that TPA is formed from citrate and cysteine through two amide bonds, we developed a new method for simultaneous TPA formation and direct TPA labeling onto peptides in mild reaction conditions that could be applied to typical solid phase peptide synthesis (SPPS). TPA can be synthesized from citrate and cysteine in presence of a peptide-coupling reagent, which is confirmed by NMR and MS. The synthetic yield is greatly enhanced by the coupling reagent even at low temperature. Moreover, citrate treatment can form a TPA-labeled peptide from a cysteine-terminated peptide on the SPPS resin in the same reaction condition. Finally, we observed the blue fluorescence of the TPA-labeled peptide inside HeLa cells.