Products of Excited State Molecules: Evidence for Hydrogen Atom Generation Within a Protein

Abstract

The UV photolysis of the aromatic amino acid, tryptophan (Trp), in the Ca2+ binding protein, cod parvalbumin, Type III, was studied using electron paramagnetic resonance (EPR) spectroscopy in the temperature range 4-80 K. For the Ca2+-bound protein, irradiation with UV light (250-400 nm) resulted in the generation of atomic hydrogen with a hyperfine splitting of 50.8 mT, whereas in the Ca2+-free form, where the Trp is exposed to solvent, the trapped atomic hydrogen was not in evidence. This is the first report of H atom generation in a protein, and it is suggested that the highly reactive free radical species may be involved in the many reactions that Trp is known to undergo.