Electron Spin Resonance in the Study of Radiation Damage 255 



only pattern, despite the fact that the cystine or cysteine content is only a few 

 percent. The fact that the same pattern, but one very dilTerent from any so 

 far obtained from non-sulfur compounds, is observed for many sulfur-containing 

 proteins and for cysteine, cystine, and glutathione convinces us that the odd 

 electron giving these resonances is essentially localized on sulfur. Whether it 

 is on a single sulfur or is shared by two sulfurs of the S — S link, as originally 

 suggested, remains a question to be answered by later work. That the odd 

 electron is localized mainly on one or two atoms is borne out by the large amount 

 of residual spin orbit coupling evidenced by the anisotropy in the observed g 

 factor, as already explained. 



Because cysteine with only — SH sulfur gives the same type of resonance 

 as cystine with — SS — sulfur, it is uncertain whether the electron wliich 

 gives rise to the characteristic resonance of Figs. 4 and 9 is on a single S or is 

 shared between two sulfurs to form a 'three-electron bond'. When the plus 

 charge accompanying the odd character arrives at the S of the — SH of cysteine, 

 it would probably 'shock' off either the naked proton to leave the neutral free 

 radical RS-, or the H atom to leave RS+, where R represents the part of the 

 cysteine exclusive of the SH group. In the latter case, the H atom would 

 escape through the lattice or react with something. (We have been unable to 

 detect the free hydrogen radical at room temperature in any irradiated substances.) 

 We do not know at tliis time which if either of these two events occurs. Inter- 

 estingly, RS+ is not a free radical, and no resonance would be detected for this 

 case until further events had transpired. At room temperature, however, the 

 molecules may flop about sufficiently to allow the RS+ to react with the — SH 

 of a neighbor and to release another H and form the same charged disulfide 

 radical which has been postulated for irradiated cystine. The common patterns 

 of cystine and cysteine might be thus explained. I should say, however, that 

 the two patterns although alike are not identical: the resonance pattern of 

 cysteine has a slightly greater over-all width than that of cystine, a variation 

 which we believe arises from the difference in dielectric medium. If the radicals 

 were diff'erent — if one were RS- and the other were R • (SS)'" • R — a much 

 greater diff'erence would be expected. 



If the resonance in irradiated cysteine arises from RS- mentioned above, 

 the resonance of cystine must arise from the same radical, which would result 

 first from ionization of the cystine molecule and later from rupture of the 

 S — S bond to leave RS- and RS+. There is no evident mechanism by which 

 the positive charge could disrupt the S — S bond other than the initial 'shock' 

 of the sudden arrival of the charge. Such 'shock' effects can be anticipated from 

 the Franck-Condon principle (28). They would hardly be expected to break 

 the S — S link, because its potential curve would be lowered and its bond length 

 shortened by the removal of an anti-bonding electron. The positive charge 

 would have no preference for either sulfur; and, if the S — S bond holds, the 

 odd electron would be shared equally by both sulfurs to form an additional 

 half-bond. The Franck-Rabinowitch caging principle (28) would also tend 

 to prevent the breaking of the S — S link by the 'shock' efi'ect. The two S atoms 

 are in a sense caged and hindered from flying apart by the large inert R groups 

 to which they are attached. Any 'shock' energy would probably be dissipated 

 as vibrational energy throughout the whole dimeric molecule. Such a charged 



