Effect of pH and excitation light wavelength on the activation energy of thermal quenching of tryptophan fluorescence

The activation energy of temperature quenching (E) of tryptophan fluorescence at pH 1–13 exhibited a maximum corresponding to the isoelectric point (pH_i ≈ 6) of tryptophan at all exciting light wavelengths. The values of E nonmonotonically changed with the excitation wavelength. This behavior was explained by photoinduced electron transfer from the higher singlet-excited S_n (n>1) states of tryptophan.