35 



or can you produce more at will if you irradiate longer? Does it reach an equi- 

 librium value? 



BARRON: That is a difficult question to answer because, when the ex- 

 posure is increased to produce more dimer, protein damage is also increased 

 and precipitation takes place. Furthermore, the presence of other solutes also 

 has great influence. A dilute solution of serum albumin that precipitates with 

 75, 000 r remains optically clear if irradiated in the presence of salts, NaCL 

 (0. 1M_) or phosphate buffer (0. OlM). 



WORF: Would small concentrations of amino acids have the same pro- 

 tective effect? 



BARRON: Yes. When aqueous solutions of albumin were irradiated in 

 the presence of cysteine, there was no dimer formation, presumably because of 

 reaction of the free radicals with cysteine, the "protecting action" of Dale. 



MAZIA: Do you ever get gel formation in concentrated solutions? 



BARRON: We have never irradiated concentrated solutions. 



MAZIA: I ask this in connection with Dr. Pollard's suggestion con- 

 cerning the breakage of disulfide bonds. If these bonds were broken and then re- 

 forined in new positions, it would be probable that a certain number of intermole- 

 cular S-S bonds would be formed, and such a polymerization might lead to gel 

 formation. 



BARRON: This relation between -SH groups and gel formation re- 

 minds me of the experiments of Huggins (8) who found that in the thermal coagu- 

 lation of serum albumin, the nature of the coagulum was influenced by minute 

 amounts of -SH reagent. At pH values from 6.9 to 7.4., thermal coagulation 

 produced a soft, opaque gel. Previous addition of -SH reagents produced clear, 

 elastic gels. The clot produced in the presence of -SH reagents could hold 3 to 

 4 times as much water as the control opaque gel. 



MAZIA: Pollard proposed the opening of the S-S bonds and reformation 

 in other places. You would, under these conditions, expect gelation. 



BARRON: Few proteins have -S-S- bridges. 



CURTIS: You actually do get this. Nims, in our laboratory found an 

 increased tendency of fibrinogen solutions to clot following the massive exposure 

 to radiation. Although the average size of the molecule was greatly reduced ac- 

 cording to the sedimentation constants, the clotting capacity of the solution had 

 increased. 



BARRON: Polymerization resulting from irradiation has been demon- 

 strated, and it is conceivable that substances containing a number of -SH groups 

 in their side chains may, on oxidation of these groups, polymerize to form gels 

 and macromolecules. 



I will speak now about some experiments we have done with nucleic acid, 

 pyrimidines, and purines. Taking advantage of the intense absorption of light in 

 the ultraviolet, we tested the action of X radiation on adenosine triphosphate. There 

 was a decrease in light absorption proportional to the X-ray exposure. The same 

 phenomenon occurs with all these substances. Purines and pyrimidines can add 1 



