Quenching of -Cyanophenylalanine Fluorescence by Various Anions.

To expand the spectroscopic utility of the non-natural amino acid -cyanophenylalanine (Phe), we examine the quenching efficiencies of a series of commonly encountered anions toward its fluorescence. We find that iodide exhibits an unusually large Stern-Volmer quenching constant, making it a convenient choice in Phe fluorescence quenching studies. Indeed, using the villin headpiece subdomain as a testbed we demonstrate that iodide quenching of Phe fluorescence offers a convenient means to reveal protein conformational heterogeneity. Furthermore, we show that the amino group of Phe strongly quenches its fluorescence, suggesting that Phe could be used as a local pH sensor.