Fluorescent Amino Acid Undergoing Excited State Intramolecular Proton Transfer for Site-Specific Probing and Imaging of Peptide Interactions

Fluorescent amino acids bearing environment-sensitive fluorophores are highly valuable tools for site-selective probing of peptide/ligand interactions. Herein, we synthesized a fluorescent l-amino acid bearing the 4′-methoxy-3-hydroxyflavone fluorophore (M3HFaa) that shows dual emission, as a result of an excited state intramolecular proton transfer (ESIPT). The dual emission of M3HFaa was found to be substantially more sensitive to hydration as compared to previous analogues. By replacing the Ala30 and Trp37 residues of a HIV-1 nucleocapsid peptide, M3HFaa was observed to preserve the peptide structure and functions. Interaction of the labeled peptides with nucleic acids and lipid vesicles produced a strong switch in their dual emission, favoring the emission of the ESIPT product. This switch was associated with the appearance of long-lived fluorescence lifetimes for the ESIPT product, as a consequence of the rigid environment in the complexes that restricted the relative motions of the M3HFaa aromatic m...