Photochemical Modifications of the Tryptophan Residues of Wheat‐Germ Agglutinin in the Presence of Trichloroethanol

Trichloroethanol is an efficient quencher of indole fluorescence of model compounds and proteins [Eftink, M. R. and Ghiron, C. A. (1976) J. Phys. Chem. 80 , 486–493]. At low quencher concentrations, the quenching follows the classical Stern‐Volmer law. Bimolecular rate constants calculated from measured quenching constants and lifetimes are equal to 6 × 10 9 M −1 s −1 and 1.2 × 10 9 M −1 s −1 for N ‐acetyltryptophanamide and wheat germ agglutinin, respectively. Upon ultraviolet irradiation in the presence of trichloroethanol, transformation of fluorescent trypophan occurs, leading to a fluorescent photoproduct. This can be easily used as a method for the quantitative determination of fluorescent tryptophan residues in proteins. In good agreement with previous results, two fluorescent tryptophan residues per polypeptide chain are found in wheat germ agglutinin. Concomitantly with the photochemical reactions, the hemagglutinating protein activity and its affinity constant towards chitin oligomers are reduced. A probable location of tryptophan residues in the binding sites of wheat germ agglutinin is proposed.