4-Chloro-l-kynurenine as fluorescent amino acid in natural peptides

4-Chloro- l -kynurenine (3-(4-chloroanthraniloyl)- l -alanine, l -4-ClKyn), an amino acid known as a prospective antidepressant, was recently for the first time found in nature in the lipopeptide antibiotic taromycin. Here, we report another instance of its identification in a natural product: 4-chloro- l -kynurenine was isolated from acidic hydrolysis of a new complex peptide antibiotic INA-5812. l -4-ClKyn is a fluorescent compound responsible for the fluorescence of the above antibiotic. Whereas fluorescence of 4-chlorokynurenine was not reported before, we synthesized the racemic compound and studied its emission in various solvents. Next, we prepared conjugates of dl -4-ClKyn with two suitable energy acceptors, BODIPY FL and 3-(phenylethynyl)perylene (PEPe), and studied fluorescence of the derivatives. 4-Chloro- dl -kynurenine emission is not detected in both conjugates, thus evidencing effective energy transfer. However, BODIPY FL emission in the conjugate is substantially reduced, probably due to collisional or photoinduced charge-transfer-mediated quenching. The intrinsic fluorescence of l -4-ClKyn amino acid in antibiotics paves the way for spectral studies of their mode of action.