Electron Spin Resonance in the Study of Radiation Damage 257 



the proteins gave such shnple patterns with the same few features, described 

 above, repeating so often either singly or together. We were forced to conclude 

 that the electron hole or vacancy created by an ionizing quantum or particle 

 at any given locality in the protein can move through the polypeptide chain 

 until it reaches one of a few traps or sinks where it becomes lodged. One such 

 low-energy trap we believe is sulfur. Both — SH and — S — S — groups are 

 effective traps. Possibly the unsaturated rings of certain side chains are an 

 important trap. 



Furthemiore, we must postulate that there are effective traps for the electrons 

 knocked away in the ionization process since these do not always seem to be 

 able to return readily to fill the hole. Because they have not given recognizable 

 resonances, we do not speculate on the negative traps. For most of them, the 

 resonances may be too broad for detection. 



V. PROTECTIVE MECHANISMS 



I am sure that there are many who have suspected that some proteins when 

 ionized can hold together and conduct the electron hole to certain side-chain 

 groups such as the sulfur link. I think that I have heard Professor E. C. 

 Pollard, of Yale, and members of his group express such views. However, 

 from my brief and sketchy acquaintance with the hterature in tliis field I surmise 

 that this question has been a highly debatable one. In the microwave resonances 

 we have a new and perhaps more direct type of evidence in favor of the migra- 

 tion of the electron holes to certain side-chain groups. 



Now that there is new evidence for effective resistance to the breaking 

 of the polypeptide backbone of the proteins by ionization, it is interesting to 

 speculate on the reasons why this is true. If one of the electrons of a locahzed, 

 covalent bond between two atoms were suddenly removed, the two atoms 

 might — according to Franck-Condon principle — become dissociated wliile 

 trying to adjust to the new and shallower potential curve with the longer 

 equilibrium distance commensurate with the 'one-electron bond'. It might be 

 supposed that the Franck-Rabinowitch caging would help to prevent any two 

 atoms of a protein chain faced with such an emergency from becoming dis- 

 sociated. However, the evidence which we have obtained for the migration of 

 the electron vacancy to a sink in the side chains indicates that a particular bond 

 of the polypeptide chain does not have to face the Franck-Condon catastrophe 

 because the bonds are not in a strict sense localized. If we imagine that charge 

 density equivalent to a single electron is removed completely from the localized 

 region of two adjacent atoms along the main chain, we must, at the same time 

 imagine that tliis charge density is restored quickly, before the atoms have time 

 to move significantly apart, by the flow of electronic charge from a side chain 

 group such as the S— S hnk. It might be better to think of the ionization as 

 taking place only at these sites where the electron vacancy is detected. A 

 molecular chain or polymer which can conduct a hole out to a non-essential 

 side-chain sink or to a point where a simple recapture of an electron restores 

 the status quo has, in effect, a built-in, remarkably effective method of self- 

 protection from radiation damage. Such polymers have a high survival value 

 in a world where ionizing radiations are ever present. 



