The luminescence of trypsin in an ethylene glycol-water solution and in the dry state, as obtained during irradiation with x-rays and ultraviolet light in vacuum at 77°K, has been compared to that observed for L-tryptophan and L-tyrosine in a similar solution under the same conditions. The emission spectra of the x-ray-induced luminescence in the trypsin samples were similar to those excited by 270-nm UV light, except that in the former case the phosphorescence was greatly enhanced relative to the fluorescence. The spectra of trypsin, both in the dry state and in solution, resembled that of tryptophan, although in solution tyrosine appeared to give rise to about 10% of the phosphorescence for the two types of irradiation. For the dry trypsin no contribution from tyrosine could be detected. These results were supported by the decay pattern of the phosphorescence of the protein, which was largely similar for x-rays and UV light. It was concluded that the luminescence was due to the same excited levels with both types of irradiation. The enhanced phosphorescence-to-fluorescence ratios, observed with x-irradiation, prompted the proposition that, in addition to excitation directly from the ground state, ion-electron recombination gives rise to excitation of the radiative levels in this case. The relative importance of the two types of excitation was evaluated. Comparison of the emission yields for trypsin in solution and in the dry state with those of tryptophan and tyrosine in solution indicated that there was no significant contribution to the x-ray-induced luminescence of trypsin from energy transfer, either intermolecular or between solvent and solute.

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