Click Chemistry to Fluorescent Amino Esters: Synthesis and Spectroscopic Studies

A new series of fluorescent amino esters have successfully been prepared by click chemistry by introducing different fluorophores (fluorescein, dansyl, 4-nitro-2,1,3-benzoxadiazole, coumarin and benzothiadiazole) into the side-chain of either serine, lysine or phenylalanine. The newly synthesized fluorescent amino esters displayed spectroscopic properties similar to their native fluorophores, with high quantum yields and fluorescence in the visible region (420-520 nm). Steady-state as well as time-resolved studies indicated that benzo-thiadiazole derivatives 22-25 are promising probes for studying protein dynamics or molecular interactions, especially in the cell, due to their emissions in the visible region, their high quantum yields and their relatively long lifetimes (14.7-18.3 ns). Moreover, the fluorescence of 7-hydroxycoumarin-substituted amino ester 27 was very sensitive to the solvent proticity and pH value. This compound appears to be a suitable pH-sensitive amino acid for probing the local pH in cell compartments.