292 Discussion 



The result, I think, is that one should probably not speak about traditional valence bonds 

 in traditional directions in a molecular species which has been ionized by ions or radiation. 

 The notion of the classical valence bond is peculiar to a closed shell system, that is, an electroni- 

 cally closed shell with a filled lower energy band and with a large energy gap between the highest 

 filled state and the lowest empty state. I think that it might not be possible to attribute the 

 electrons whose spins Professor Gordy detects to a particular site in the original primitive 

 molecule, because these bonds may have been completely rearranged. Also it may not be 

 possible to attribute Professor Platzman's type of damage to a particular section of the 

 molecule, if this sort of thing, cyclopropyl or tropylium ion, is very common. There may 

 be a large section of the molecule which is doing a merry-go-round of interchangeable carbon 

 atoms before the system settles down. 



Platzman : I should like to record my skepticism as to the ubiquitous and decisive role of 

 disulfide-bond cleavage in radiobiology. This is not to question the participation of such 

 breakage in denaturation by ionizing radiation or any other agent : since disulfide bonds make 

 an important contribution to the structural stability of many proteins they must certainly be 

 involved in structural breakdown. Dr. Augenstine has, indeed, attempted here to describe the 

 relationship between the contribution of secondary bonds and that of disulfide bonds. However, 

 the argument — frequently heard in recent years — that the great sensitivity of a protein molecule 

 to ionizing radiation can be understood in terms of migration of an electron vacancy produced 

 almost anywhere in the molecule to a disulfide bond, with resultant cleavage of that bond and 

 unfolding of the molecule, is open to most serious objections. In the first place, such long- 

 range migration is unsupported by independent evidence and, as indicated in my paper, is 

 likewise unsupported by physical principles. The fact that electron holes are observed to move 

 freely in certain nonmetallic crystals is of doubtful relevance because of the different dielectric 

 properties of such crystals, and because of their periodic structure. Moreover, even though 

 Professor Gordy's proof of the existence of free valences at sulfur atoms is impressive (although 

 the precise number of such radicals in relation to the radiation dose is still uncertain), a simple 

 causal connection between formation of the radicals and inactivation of the protein has yet to 

 be established, and, indeed, may not exist at all. It is quite possible that they are a secondary 

 factor in denaturation, however conspicuous they may be in the paramagnetic-resonance 

 spectrum. Furthermore, the logical link between disulfide-bond cleavage and electron- 

 vacancy migration is also unproven. A simpler and plausible mechanism for a strong 

 sulfur-atom signal is given by the action of subexcitation electrons : that these can attack the 

 disulfide bonds effectively even though the latter are present in low concentration is strongly 

 suggested by the small dissociation energy of such bonds and also by the marked red displace- 

 ment of the absorption spectrum of cystine in relation to the spectra of most other amino acids. 



Gordy : I certainly think that Professor Platzman's suggestion is worthy of consideration. 

 I try to keep the sulphur-sulphur bonds in my brain open when discussing these complex 

 systems. 



