ABSTRACT
According to the Teale spectral classification, 1 proteins are divided into three classes: class A, proteins containing tyrosine residues, but not containing tryptophan residues; class B, proteins containing both tyrosine and tryptophan residues; and class C, proteins containing only phenylalanine residues. As a rule, proteins of class A display luminescence spectra which coincide with that of free tyrosine. Spectra of class B proteins are mainly due to the emission of tryptophan residues. The tyrosine component is revealed in this case only by means of a special analysis of the spectra. The small contribution of tyrosine emission in the fluorescence spectra of class B proteins is explained by the relatively low extinction coefficient of tyrosine, its low fluorescence quantum yield value, and the existence in some cases of excitation energy transfer from the tyrosine to tryptophan residues. Analogously, the absence of the phenylalanine contribution to the spectra of class A proteins is explained by the low extinction coefficient of phenylalanine and the existence of excitation energy transfer from phenylalanine to tyrosine chromophores. The fluorescence quantum yield for phenylalanine in proteins may be essentially higher than that of the free amino acid.
