Electron Spin Resonance in the Study of Radiation Damage 253 



There are far too many radicals already observed in irradiated amino acids 

 and peptides to discuss them here. I should like to mention one more, 

 however. The pattern of Fig. 7 for acetyl valine consists mainly of a set of 

 nine syimTietrical doublets spread over 200 gauss. There is another resonance 

 near the center of the group which I ignore for the present discussion. Seemingly, 

 the nine doublets must arise from eight equally-coupling protons and a ninth 

 with coupling only about half as much as each of the eight at room temperature, 

 and only about a fourth as much at liquid air temperature. This pattern 

 requires an almost unimaginable radical. The odd electron must spread 

 two-fifths of its total density in \s orbitals of the eight equivalent hydrogens. 

 This indicates a radical with a high concentration of hydrogens. It is 

 difficult to design a radical with eight equally coupling hydrogens, especially 

 with a ninth coupling differently. The (CH3)3C radical would have nine 

 equally coupling hydrogens wliich would be expected to give a hyperfine 

 spread of the order of 200 gauss. If we should assume that one of the hydrogens 

 in (CH3)3C is replaced by a group RH with only one coupling hydrogen (such 

 as OH) and one which does not noticeably disturb the couphng of the other 

 two, we would have a radical which might account for the acetyl valine pattern 

 of nine doublets. 



IV. RADIATION DAMAGE IN PROTEINS 



In contrast to the varied hyperfine patterns found for the resonances of 

 the x-irradiated amino acids and simple peptides, we have found mainly (but 

 not exclusively) two patterns either singly or in combination for numerous 

 proteins. One of these patterns consists of a simple doublet arising from 

 interaction of the odd electron spin Vv'ith a single proton spin. The other 

 pattern is a field-dependent one like that of powdered or polycrystalline cystine, 

 cysteine, or glutathione. Fig. 8 illustrates the first type; Fig. 9, the second; 

 and Fig. 10 is a combination of the two patterns. 



In our first papers on electron resonances in irradiated proteins (4), we 

 suggested that the doublet pattern in the proteins might arise from an odd 

 electron localized mainly on an oxygen joined by a hydrogen bridge as indicated 

 in Structure II. 



Model I represents a structural section of the unirradiated /?-keratin protein. 

 The doublet structure, we thought, might arise from dipole-dipole interaction 

 of the electron spin with the proton of the hydrogen bridge. Partly to test this 

 hypothesis, H. W. Shields and the author (25) have made observations on 

 strands of irradiated silk directed along the applied magnetic field, and also on 

 strands directed perpendicular to it. It is known from infrared and x-ray 

 studies (26) that hydrogen bridges in silk he approximately in a plane perpen- 

 dicular to the direction of the silk strands. If we assume, for simplicity, that 

 the odd electron density is symmetrically localized on the oxygen, the of 

 equation (18) would measure the angle of the O — H axis with the magnetic 

 field. Hence, when the silk strands are along the apphed field, 6 equals 90° 

 for all hydrogen bridges, and the doublet splitting is the same for all radicals 

 of the silk. Under these conditions one would expect a clear resolution of the 

 doublet. When the silk strands are perpendicular to the applied field, the 



