39 



BARRON: Perhaps the addition of the iminazole ring to the pyrimidine 

 in guanine made the molecule more stable than thymine, which has only a pyri- 

 midine ring. 



ALLEN: Are these ionic yields all for air-saturated water? 



BARRON: Yes. Irradiation in nitrogen-saturated water diminished 

 the yield. 



I am going now to present to you some experiments we have performed 

 on respiratory pigments with iron-porphyrin as the prosthetic group. X irradia- 

 tion of ferricytochrome produced an increase in absorption spectrum in the re- 

 gion corresponding to the protein moiety, around Z800 ^ , and a decrease (^f the 

 Soret band corresponding to the porphyrin nucleus. The Soret band, 4050 ^, is 

 more easily destroyed by X radiation when cytochrome is dissolved in 0. 005 M 

 HCL than when it is dissolved in neutral or alkaline solutions. This effect is 

 due to the oxidizing action of the OH and O2H radicals; it is decreased by half 

 when irradiation is performed in the presence of nitrogen. The decrease of 

 light absorption in the Soret band can also be demonstrated on irradiation of 

 protoporphyrin. Here, as the Soret band is decreased, there is also a decrease 

 in the red fluorescence characteristic of porphyrins. 



SHERMAN: Did you observe this effect in the absence of water? 



BARRON: I was informed that experiments had been performed with 

 nucleic acids dissolved in carbon tetrachloride and that they were presented as 

 an indication that the effect produced by irradiation was not due to the OH and 

 O7H radicals. Similar experiments were made with protoporphyrin. On X ir- 

 radiation, the porphyrin became green and the Soret band was greatly reduced 

 as well as the fluorescence. The same effect was obtained when porphyrin was 

 dissolved in X irradiated carbon tetrachloride. Porphyrin was converted into a 

 biliverdin. 



Pure, dry carbon tetrachloride, free from impurities, is relatively 

 stable. When it is X irradiated, a free radical and atomic chlorine are formed 

 which on recombination give hexachloroethane and CI2: 



CCI4 irradiation ^ CClo-» CI ; 



CCI3+ CCI3 »• C2CI6 



CI + CI ► CI2 



The oxidizing action of chlorine is responsible for the attack on the 

 porphyrin molecule. It consists of ':he opening of the methane bridge and the ox- 

 idation of this group. In the presence of water, chlorine gives ClOH, which also 

 is a powerful reagent. 



KAMEN: Was carbon tetrachloride irradiated in air'' 



BARRON: It was irradiated soon after redistillation and in a closed 



vessel. 



ALLEN: If you add chlorine to your system, does it have the same ef- 

 fect as irradiated carbon tetrachloride "^ 



BARRON: Chlorine and hypochlorous acid produce the same effect as 



