The Absence of Radiation-Induced Disulfide Interchanges 



285 



by CoLLiNSON et al. (8)) did not give any increase in the content of non-ether 

 soluble chromophoric material as compared with a control containing HaO, 

 alone. Even at the end of forty-eight hours no increase was apparent, although 

 ribonuclease would have been destroyed completely at the end of thirty minutes 

 (8). The increase in optical density in the HgOg control is small but significant 

 (20 per cent of theoretical at the end of forty-eight hours), and probably 

 represents oxidation to ether insoluble materials such as dialkyl sulfoxides 

 and cysteic acids. 



Irradiation in Aqueous Solution 



When aqueous solutions of mixtures of cystine and its Z)/5-dinitrophenyl 

 derivative were irradiated with 1 X 10' r, the results obtained were equivocal 

 because of the influence of side reactions causing change in the chromophoric 

 moiety. 



Possibly this effect is akin to the well known photo-destruction of dinitro- 

 phenyl derivatives generally. That such a process is occurring follows from 

 the observations that the samples irradiated with 6 X 10*^ r of Co^" gamma 

 rays were less intensely colored than the non-irradiated controls; the optical 

 density at 350 m/^ was reduced to one third that of the controls. The bulk of the 

 350 mn absorbing material after irradiation was insoluble in ether. This 

 product was clearly not the interchange product, because it possessed the wrong 

 spectrum (high absorption at 250 mfx, with only a shoulder at 380 m//, whereas 

 the wavelength for the maximum absorption of the mono-dinitrophenyl cystine 

 under these conditions is 360 m//). In addition, the same material was produced 

 (in higher yield) in the control containing no cystine. 



An attempt was made to lower the dose to a point where the above side 

 reaction would not obscure the possible interchange reaction (see Table II). 



Table II. The Effect o/Co^" Gamma-ray Irradiation ofhis-DNP Cystine 



The complete system contains 1 x 10"^ M cystine; 1.25 x 10^* M 

 bis-DNP cystine; 1.2 x 10"- M versene buflFer, pH 7.9; where indi- 

 cated, 5 x 10"* M A^-ethyl maleimide (nemi) and 5 x 10"* M/7-chloro 

 mercuri-benzoate (pcmb). The samples were irradiated with 4 x 10* r 

 of Co*" gamma-rays in a 60-minute period. 



Optical density 



Before ether After ether Component with 

 extraction extraction Rp = 0.6 



After a dose of 4 X 10^ r no colored material could be detected with the Rp 

 of 0.6, which is the R^ of mono-dinitrophenyl cystine, although significant 



