HYDROGEN-ATOMS ARE PRODUCED WHEN TRYPTOPHAN WITHIN A PROTEIN IS IRRADIATED WITH ULTRAVIOLET-LIGHT

The UV photolysis of the aromatic amino acid, tryptophan (Trp), in the Ca2+-binding protein, cod parvalbumin, type III, was studied using el ectron paramagnetic resonance (EPR) spectroscopy in the temperature ra nge 4-80 K, For the Ca2+-bound protein, irradiation with UV light (250 -400 nm) resulted in the generation of atomic hydrogen with a hyperfin e splitting of 50.9 mT,,whereas in the Ca2+;free form, where the Trp i s exposed to solvent, the trapped atomic hydrogen was not in evidence, In the same spectra, the radical signal in the g = 2.00 region could he detected, The line shape of the Ca2+-bound form is similar to the E PR line shape obtained for Trp in micellar systems, In contrast, the E PR line shape for the Ca2+-free form is essentially featureless up to 80 K, The EPR spectra of the photoproducts of Trp and the nature of th e photoreactions are therefore sensitive to the environment of Trp wit hin the protein.