290 L. G. AUGENSTINE 



energies suggests that the target volume should decrease as the temperature 

 at which protein is irradiated is decreased. Consistent with this is the fact 

 that the x-irradiation cross-section of phage Tl has been found to be a linear 

 function of the irradiating temperature (19); also the inactivation of trypsin 

 ultraviolet-irradiated at 300°K is about three times as great as at 90°K (10). 

 The target volume should also decrease as the quantum of energy absorbed is 

 decreased: the inactivation cross-section of bovine serum albumin bombarded 

 with very low energy electrons was found to increase with increasing ^ energy 

 and a measurable cross-section was obtained with particle energies as low as 

 10 ev (20). 



The recent resuUs and interpretations of Yalov/ (21) are particularly per- 

 tinent to the hypothesis discussed here. Her irradiations of insuhn, serum 

 albumin and cystine indicated that disulfide bonds are reduced both under 

 conditions where direct and indirect effects should predominate. However, she 

 proposed that the splitting occurred between the C and S (leaving an S — S — C 

 configuration) rather than between the sulfurs. Although the data cited pre- 

 viously appear to indicate a splitting of the S — S bond, most of those same 

 data would be compatible with a reduction of the C — S bond instead. To 

 select between the two possibilities may be difficult, since the energy required 

 to spht a given bond in a compound such as cystine may be quite different 

 than that required in a protein; as Lumry and Eyring (6) point out, various 

 of the intramolecular bond angles of proteins may be distorted in order to 

 effect structural compromises which minimize free energy. However, Yalow (21) 

 has pointed out that the production of a C — S — S- radical is probably more 

 consistent with Gordy's findings than the other alternative. 



The failure of Koch (22) to detect radiation-induced disulfide inter- 

 changes either in solution or the dry state does not disprove the hypothesis 

 proposed here. The dosages they used (up to 3 X 10' r) are much larger than 

 those required by other workers (3 X 10^ r) to liberate sulfur groups (21, 23) 

 from similar compounds. These results indicate that although the splitting of 

 disulfide bonds may well be critically involved in protein inactivation, seven 

 per cent or less* of the liberated — SH groups undergo interchange. 



REFERENCES 



1. L. Augenstine: Structural interpretations of denaturation data. In: Information Theory 

 in Biology, ed. by H. Quastler, 119-124, University of Illinois Press, Urbana (1953). 



2. L. Augenstine and R.Ray: Trypsin monolayers at the air-water interface. III. Structural 

 postulates on inactivation. /. Phys. Chem. 61, 1385-1388 (1957). 



3. L. Augenstine: Trypsin monolayers at the water-air interface. Ph.D. thesis, University 

 of Illinois (1956). 



4. E. C. Pollard: The direct effect of radiation on proteins, viruses, and other large mole- 

 cules. In: Biochemical Aspects of Basic Mechanisms in Radiobiology, ed. by Harvey M. 

 Patt, Nat. Res. Council Washington Publ. no. 367, 1-29 (1954). 



* The \le dose, D*, for a spherical molecule of mol. wt. 240 and density 1.35 is 2 x 10* r. 

 A dose of 3 X 10' r could potentially rupture 1.5 per cent of the S— S bonds (from dNjN = 

 dNjD* = (3 X 107)/(2 X 109)). If one interchange per 1000 disulfides could have been 

 detected (22) but none was found, then seven per cent (1/15) or less of the 'hit' molecules were 

 eventually involved in disulfide interchanges. 



