Intra- and intermolecular factors affecting the excited states of aromatic amino acids

The Luminescence of the Aromatic Amino Acids

“…These measurements and those of Weber and Teale(70) for phenol and indole have been widely accepted and have been routinely used to compute absolute quantum yields of fluorescence for proteins and aromatic model compounds by the comparative method. More recently, these values have been corroborated by Bishai et al, (42) although no discussion of the associated experimental uncertainty was included.…”

“…There is some indication of a similar effect for phenylalanine. (42) Teale and Weber (6) found that the fluorescence excitation spectra of the aromatic amino acids over the 200-320 nm range closely corresponded to their respective absorption spectra. This indicates that the quantum yields are independent of the excitation wavelength over the same range.…”

“…However, the use of quinine as a standard may present problems, also. (72 -75) Furthermore, Bishai et al (42) used the same method as Chen (71) and arrived at different conclusions.…”

“…Thus Bishai et al (42) found that in a rigid glass (0.5 % glucose) at 77°K the total emission yield (fluorescence plus phosphorescence) from tyrosine, phenylalanine, and tryptophan approaches unity. In addition, the wavelength of maximum fluorescence for tryptophan is "blue-shifted" by about 25 nm or more from its value at room temperature.…”

“…The yield is reduced by protonation of the carboxylate group and greatly reduced by ionization of the phenolic hydroxyl. (42,69) In strong acid, hydrogen ion quenching becomes important. There is some indication of a similar effect for phenylalanine.…”