RELATIVE CONTRIBUTIONS OF TRYPTOPHAN and TYROSINE TO THE PHOSPHORESCENCE EMISSION OF HUMAN SERUM ALBUMIN AT LOW TEMPERATURES

Abstract— Phosphorescence emission and excitation spectra, as well as decay profiles of human serum albumin, were investigated in the wavelength regions of the tryptophan and tyrosine absorption and emission spectra in potassium phosphate buffer at 77 K. Emission and excitation spectra were found to be linear superpositions of the contributions of the tryptophan and tyrosine residues. It is suggested, therefore, that there is no significant tyrosine to tryptophan energy transfer in this protein at low temperature. The phosphorescence decay is, in general, multiexponential with lifetime components of 5.95, 2.7, and 1.2 s. The longest lifetime is characteristic of tryptophan, whereas the two short components are attributed to two types of tyrosine residues located in different environments within the protein. The latter is confirmed by a detailed analysis of the phosphorescence decay profiles determined at different emission wavelengths, and utilizing different wavelengths of excitation favoring either the tryptophan or tyrosine residues.