Molecular Excitations in Radiation Damage as Studied by the Radiation-Induced Luminescence of Proteins

Abstract

This article reviews a study of the radiation-induced luminescence of tryptophan, tyrosine, and trypsin, with the intention of elucidating the possible importance of molecular excitations for the radiation damage of proteins. In all these substances, the spectrum of the X-ray induced emission, as obtained at 77°K, was similar to that excited by uv light, except that a considerable enhancement of the phosphorescence to fluorescence ratio occurred with X-irradiation. This could be accounted for by assuming that about 90 per cent of the emission resulted from electron-ion recombination, subsequently giving rise to excitation of the radiative levels. The G-value for the emission of light was 5 for both tryptophan and tyrosine, leading to the conclusion that at least 40 per cent of the total number of ionizations resulted in recombination and subsequent emission of light. The emission from trypsin appeared to originate almost exclusively from the tryptophan residues. The emission yield (G = 0.17) indicated that energy transfer did not contribute significantly to the emission of this protein. A possible relationship between the temperature dependence of the luminescence of trypsin and that of its radiation sensitivity is discussed.

Key Words:

• Energy transfer
• fluorescence
• phosphorescence
• protein
• tryptophan
• tyrosine
• X-ray induced luminescence