DAMAGE IN AEROBIC AND ANAEROBIC SYSTEMS 



33 



expression of the effectiveness with which oxygen can compete at 

 different intracelhilar concentrations for reaction with radicals or other 

 intermediates to give rise to loss of re])roductive integrity. 



Measnrements between and 30 C show no systematic variation of 

 K with temperature and enables us to assign an upper limit of 6 kcal/ 

 mole to the differences between the activation energies of the two re- 

 actions represented by equation (1). 



If the lifetime of the intermediate in question is longer than the rate 

 constant for diffusion into the cell (estimated at ~ 1 msec) it may be 

 possible to determine the lifetime directly by the use of our ])ulsed 

 source. Figure 4 shows the result of an experiment conducted by Dewey 



Fig. 



5 10 15 20 25 30 35 

 Dose (kilorad) 



4. Comparison between pulsed and low dose-rate irradiation of Serrntia tnnr- 



cescens. (Redrawai, by permission, from Dewey and Boag, 19(iO). 

 O, 2 /xsec. pulse; • , 1,000 rad/min for 0-5 per cent oxygen in the gas phase 



and Boag (1959, 1960) in which the effects of doses of radiation de- 

 livered at normal dose-rate, and in a single 2 sec pulse, were compared 

 with respect to loss of reproductive integrity. The bacteria were in sus- 

 pension in a medium maintained at an oxygen concentration of about 

 10 /xmoles/1 by bubbling. Under these conditions the bacteria displayed 

 almost their maximum sensitivity when irradiated at normal dose-rates, 

 but appeared progressively more and more radio-resistant with in- 

 creasing doses above 5 krad when this is delivered in a single pulse. 

 This was interpreted as due to the radiochemical utilization of all the 

 oxygen initially present in the cells by the reactions arising from the 



