Photophysics of a Fluorescent Non-natural Amino Acid: p-Cyanophenylalanine.

The non-natural amino acid p-cyanophenylalanine (Phe(CN)) has recently emerged as a useful fluorescent probe of proteins; however, its photophysical properties have not been systematically examined. Herein, we measure the fluorescence quantum yield and the fluorescence lifetime of Phe(CN) in a series of solvents. It is found that the fluorescence lifetime of Phe(CN) shows a linear dependence on the Kamlet-Taft parameter α of the protic solvents used, indicating that the solute-solvent hydrogen bonding interactions mediate the non-radiative decay rate. Thus, results of this study provide a basis for quantitative application of Phe(CN) fluorescence in protein conformational studies.