joaMage in aerobic and anaerobic systems 25 



related B. subtilis, inferred from their studies witli very slow electrons 

 that the radiobiological ly iin])ortant material comprises only part of 

 the spore, being surrounded by a shell of very insensitive material 

 230 A thick. 



The log survival curves observetl by Powers are, under all conditions 

 of irradiation, strictly linear over almost their entire length, but show 

 a small curvature towards the origin. The extrapolation number (as 

 well as the slope) varies with the conditions of the irradiation, but is 

 always less than 2. Loss of reproductive integrity in the spore has, 

 therefore, the general features that I mentioned earlier. 



Certain features of Powers' work which are relevant to the present 

 discussion may be summarized as follows : 



There are four recognizable classes of events which may lead to loss 

 of reproductive integrity. These are characterized by the chemical 

 reactivity, thermal stability and lifetime of intermediates in the reac- 

 tion chains. Two involve molecular oxygen and two do not. 



Classes of event leading to reaction chains in ivhich the participation of 



molecular oxygen is not essential 



Class A events lead to reaction chains which are equally toxic under 

 all conditions so far tested. 



Class B events lead to reaction chains which are equally toxic under 

 all conditions tested, except treatment with H2S. They react with 

 H2S, the simplest of the sulphydryl compounds, if present during 

 irradiation, to give a product which is non-toxic. The lifetime of these 

 species is such that reaction with H2S is no longer possible after the 

 end of an irradiation of a few minutes' duration. 



Classes of event leading to reaction chains in ivhich the participation of 

 molecular oxygen is essential 



Class C events generate intermediates that react readily with oxygen 

 to give products leading to loss of reproductive integrity. In the absence 

 of oxygen the intermediates in question have a comparatively long life, 

 at room temperature, and may be stabilized against subsequent reaction 

 with oxygen by thermal treatment, or exposure to NO or H2S after the 

 end of irradiation. The stabilization is less complete if nitric oxide is 

 present during irradiation (see p. 34). 



Class D events generate intermediates which only become toxic by 

 reaction with oxygen. They have, however, a much shorter lifetime 

 than the intermediates derived from Class C events, and are only in- 

 fluenced by oxygen and H2S if these gases are present during irradiation. 



The physical reality of intermediates having lifetimes and chemical 



