Effect of pH on aqueous phenylalanine studied using a 265-nm pulsed light-emitting diode

Recently, we described the characteristics and application of a 265-nm AlGaN light-emitting diode (LED) operated at 1-MHz repetition rate, 1.2-ns pulse duration, 1.32-microW average power, 2.3-mW peak power, and approximately 12-nm bandwidth. The LED enables the fluorescence decay of weakly emitting phenylalanine to be measured routinely in the condensed phase, even in dilute solution. For a pH range of 1-11, we find evidence for a biexponential rather than a monoexponential decay, whereas at pH 13, only a monoexponential decay is present. These results provide direct evidence for the dominance of two phenylalanine rotamers in solution with a photophysics closer to the other two fluorescent amino acids, tyrosine and tryptophan, than has previously been reported. Although phenylalanine fluorescence is difficult to detect in most proteins because of its low quantum yield and resonance energy transfer from phenylalanine to tyrosine and tryptophan, the convenience of the 265-nm LED may well take protein photophysics in new directions, for example, by making use of this resonance energy transfer or by observing phenylalanine fluorescence directly in specific proteins where resonance energy transfer is inefficient.