Electron spin resonance of irradiated single crystals of L-phenylalanine-HCl.

Single crystals of L-phenylalanine-HCl have been observed with electron spin resonance after irradiation with a cobalt 60 gamma-ray source. The predominant signals observed are from long-lived benzyl radicals. The results indicate that one form of radiation damage to proteins containing this amino acid is breakage of the carbon-carbon bond to release the benzyl radical from the polypeptide chain. Hyperfine structure due to proton couplings of the two methylene hydrogens and of the hydrogens on the ring of the benzyl radical is observed and analyzed to give the electron spin density on the various carbons. The spin density on the methylene carbon is found to be 0.71; that on carbon C((2)), C((4)), or C((6)) is 0.17. The sum of the spin densities on C((1)), C((3)), and C((5)) is -0.22.