Thermoluminescence from gamma-irradiated biochemicals. Investigation of emission spectra

Crystalline powders of the aromatic amino acids and the enzyme trypsin were exposed at 77°K to Co-60 γ rays, and the spectral distribution (between 3000 and 6500 A) of the thermoluminescence emitted upon subsequent heating was determined using band-pass filters. At a temperature corresponding to a particular peak in a plot of emission intensity vs T, a large fraction of the emission (except from phenylalanine) consisted of one characteristic spectral component. In general, the proportion of emission at the longer wavelengths increased with increasing temperature. The thermal-activation energies of thermoluminescence, estimated from plots of lnI_T vs 1/T (about 0.2 ev for the amino acids and 0.1 ev for trypsin), are lower than similar determinations at comparable temperatures for alkali halide crystals. The γ-ray induced thermoluminescence from amino acid crystals is composed of longer wavelengths than the uv-excited fluorescence (at room temperature) or long-lived phosphorescence (at 77°K). The possible limitations within which lattice imperfections might account for these effects are discussed briefly. The spectral results for trypsin provide additional evidence that, in accord with previous predictions, at least some of the electronic rearrangements arising from γ-ray interaction become preferentially localized in only a restricted number of configurations. The metastable species which persist at 77°K likely depend upon the native conformation of the protein, and the available evidence suggests that the decay of an excited triplet state of tryptophan accounts for the majority of the trypsin emission. Even so, the activation energies and spectra indicate that the initial trapping and untrapping does not occur at the aromatic amino acid residues. Accordingly, mechanisms are considered whereby electronic rearrangements at other sites in trypsin could lead to the excitation of an aromatic residue followed by phosphorescence from a tryptophan residue.