R. TIRRELL 



The effect of irradiation may be stated as follows: (?) There is no shift in 

 the position of any band, (u) The strength of bands after irradiation is 

 approximately 90 per cent of that of bands for the non-irradiated solution. 

 This is of the same order as previously found when the Soret band only 

 was examined. 



(b) Ferriprotoporphyrin 



The irradiation of ferriprotoporphyrin produced no shift in the position 

 of the absorption bands of the spectrum {Table 2). It did, however, produce 

 a reduction in strength of the three bands, comparable with that found for 

 protoporphyrin. 



Table 2. Effect of irradiation on absorption bands of ferriprotoporphyrin 

 examined as the pyridine heniochrome 



DISCUSSION OF RESULTS 



The experiments on the effect of dose rate indicate a positive correlation 

 between the rate at which radiation is given and the rate at which the proto- 

 porphyrin is converted to the product tetrapyrrole. This was not unexpected, 

 for two reasons, the first being the protective effect of the plasma proteins. 

 There is evidence that serum albumin can react with OH radicals^. It has 

 also been found that, to some extent, added protein can protect haemocyanin 

 from the action of X-rays'^. Therefore, with the irradiation of protoporphyrin 

 in plasma, the number of free radicals available for reaction with the proto- 

 porphyrin would depend on the number reacting with the proteins. The 

 number of such available free radicals would probably increase with 

 increasing dose late. 



Secondly, the molecular structure of protoporphyrin suggests that some 

 of the energy absorbed by, say, the methene bridges, may be transferred to, 

 and taken up by, the aromatic pyrrole rings. A similar effect has been found 

 with the synthetic copolymer of isobutylene and styrene^, in which the 

 energy which is absorbed by the isobutylene part of the chain is transferred 

 to the styrene part of the chain, where it is dissipated by the aromatic ring 

 system, resulting in a comparatively small amount of chemical change. If 

 this is so, for the protoporphyrin molecule, the amount of energy available 

 for reaction with the methene bridges [i.e., left over after transfer to the 

 pyrrole rings) could be dependent on the amount of energy available to the 

 protoporphyrin molecule in unit time, i.e., dose rate. In this case, the effect 

 should be found when protoporphyrin is irradiated in alkaline solution. 

 This aspect will be investigated further. 



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