Excitation of Tryptophan in Solution during Irradiation with X-Rays and UV Light between 77°K and 300°K

The temperature dependence of the yields of the x-ray-induced fluorescence and phosphorescence of tryptophan in ethylene glycol-water solution, between 77°K and 300°K, was found to be quite different from that observed upon excitation with non-ionizing UV light. Thus, while the quantum yields of the fluorescence and phosphorescence, induced by 270-nm UV light, were independent of the temperature up to about 220°K and 140°K, respectively, the yields of the x-ray-induced emissions decreased by more than a factor of two within the same ranges of temperature. For both types of irradiation, the phosphorescence-to-fluorescence ratio remained constant up to 140°K, being 3.6 for x-rays and 0.46 for UV light. However, the marked drop in this ratio observed, in both cases, around 160°K was much more pronounced for x-irradiation. The temperature dependence of the phosphorescence decay time was identical for the two types of irradiation. The difference between the results obtained with x-rays and UV light was attributed primarily to ion recombination. Thus, it was calculated that about 85% of the x-ray-induced luminescence at 77°K was due to this process, the remainder resulting from excitation directly from the ground state. The significance of ion recombination decreased markedly above 140°K and became negligible above 220°K. It was calculated that for x-irradiation, below 140°K, about 75% of the excitation by ion recombination was to the triplet manifold, while the corresponding percentage for excitation directly from the ground state was limited upwards to 35%.