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Since the original detection of electron spin resonance (ESR) signals from free radicals formed by ionizing irradiation in the amino acids and proteins,' numerous ESR studies of these and other powdered peptides have been made.2-'0 The free radicals that have been identified in these studies have been formed for the most part by the breakage of a CaH bond to leave the electron spin concenltrated mainly on Ca, or by breakage of a bond to a sulfur atom to leave the spin density concentrated on S. In this work we have obtained proof of the formation of secondary radicals by hydrogen-addition reactions on the aromatic ringed groups of phenylalanine, tyrosine, and tryptophan. We have produced the H-addition radicals by subjecting powdered samples to thermal hydrogen atoms produced in a gaseous discharge or by exposing them to irradiation and have observed their ESR spectra at a frequency of 9300 Mc/ sec. The H-bombardment method was used earlier to produce H-addition radicals in the nucleic acid bases."-'3 Snipes and Schmidt'0 bombarded a number of amino acids with thermal H atoms but reported radicals formed by H abstraction only. L-Phenylalanine and Poly-L-P-henylalanine.-Figure 1 shows the ESR spectra obtained for ILphienylalanine and poly-L-pheniylalanine after exposure to thermal hydrogen atoms at 3000K and at 770K, respectively. In both patterns there is a strong signal in the central region like that reported earlier' 8 for samples that had been subjected to ionizing irradiation. In addition, there are quartet patterns on either side of the central region that are part of the spectra of radicals formed by H-addition on a meta-carbon of the ring. The radical can be described approximately as a hybrid of the following three valence-bond states:

Hydrogen-addition radicals formed in the aromatic rings of amino acids, polyamino acids, and proteins.