Recently, nitrile-derivatized amino acids have been used as infrared probes of local environment. Here, we show that phenylalanine-CN (PheCN) also exhibits interesting fluorescent properties. For example, its fluorescence quantum yield is roughly 4 times larger than that of Phe and its fluorescence spectrum is sensitive to solvent and, therefore, allowing measurements of local environment through PheCN fluorescence. In addition, its emission spectrum overlaps significantly with the absorption spectrum of Trp. Thus, PheCN and Trp make a good fluorescence resonance energy transfer (FRET) pair. Using a series of synthetic peptides that contain a single Trp and a single PheCN residue at different positions, we verified that indeed Förster energy transfer takes place between these two amino acids. Moreover, the distance-dependent FRET signals allowed us to obtain structural information on the peptides.
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