102 E. L. POWERS 



radical forms the toxic RO2* radical. The radical that is not effectively 

 available for annealment apparently is not available for secondary 

 reaction after formation of an RO2' radical. It is trapped with respect 

 to annealment, and, though available to O2, as shown by the ESR 

 result, it remains effectively trapped after reaction with O2. Both are 

 available to NO, and are converted into non-radical RON species, but 

 smce only one (Ra*) is biologically important, the removal of Rb' by 

 NO, an effect that results in an ESR spectrum different from that 

 caused by annealment, does not cause reduction of damage beyond 

 that observed after annealment. 



A choice between these two general possibiUties is not required 

 by the evidence at hand today. 



THE SHORT-LIVED, OXYGEN-DEPENDENT SPECIES (CLASS II) 



Another class of radiation -induced chemical change can be infei'red 

 from the data previously examined. We have noted that O2 when 

 present at the time of irradiation increases radiation sensitivity by 30 

 per cent. We have also seen that the presence of nitric oxide at the 

 time of irradiation decreases the protective capacity of the gas by 20 

 per cent, as compared with nitric oxide given after irradiation; or. in 

 other words, nitric oxide at the time of radiation increases radiation 

 effect by 20 per cent, as compared with nitric oxide given after irradia- 

 tion. It can be allowed that oxygen and nitric oxide accomplish the 

 same thing in increasing radiation sensitivity. If oxygen and nitric 

 oxide are in some way preserving energy within the cell for damage 

 that is absorbed primarily by other molecules, then the lifetimes of 

 these "excited" molecules must be very short. We are considering at 

 the present time one possibility, namely, the action of nitric oxide and 

 oxygen in catalyzing certain degradations of excited species (Powers 

 et al., 1960a). One of these could be the quenching of fluorescence, 

 i.e. the non-radiative transformation of an excited species to some other 

 form (e.g. an excited singlet to a triplet). In this case energy that in the 

 absence of the two gases escapes from the cell in the form of a photon 

 is i^reserved within the cell and becomes damaging when nitric oxide 

 or oxygen is present. 



THE SHORT-LIVED, OXYGEN-INDEPENDENT SPECIES (CLASS lb) 



One other class of radiation-induced chemical change can be postu- 

 lated on the basis of the results of irradiating the spores in the presence 

 of hydrogen sulpliide. As noted earlier, radiation sensitivity appears 

 to be below that observed when hydrogen sulphide treatment is given 

 after irradiation (Powers and Kaleta, 1960). We conclude from this 



