The low-temperature luminescence of aromatic amino acids in peptides and proteins

The aromatic amino acids, tryptophan and tyrosine exhibited fluorescence and phosphorescence at 77°K; phenylalanine was found to be non-luminescent. The luminescence elf peptides and proteins containing tryptophan was dominated by that amino acid. In general, tyrosine emission was only observed in the absence of tryptophan. This effect is explained in terms of energy transfer from the excited singlet state of tyrosine to tryptophan. The luminescence characteristics of tryptophan and tyrosine were sensitive to numerous environmental factors. In polar media and in the vicinity of a tyrosinate anion, the relative phosphorescence quantum yield of tryptophan was enhanced. The reverse occurred in apolar media. In addition, the excited states of tryptophan and tyrosine were perturbed by a heavy atom effect. [Continues.]