HARRY COMMONER 363 



24 gauss mod. 



...3 gauss mod. 



10 gauss 



Fig. 1. ESR signals from a sample of living Chlorella under two different instru- 

 ment conditions (24 gauss modulation amplitude and 3 gauss modulation ampli- 

 tude) to show the effect of high resolution conditions (3 gauss modulation) in re- 

 ducing instrument sensitivity. See legend to Fig. 2 for other details. 



This account suggests the crucial relation between the characteris- 

 tics of the ESR spectrometer, and the level of information which it 

 yields regarding biochemical and biological systems. With sufficient 

 sensitivity the spectrometer can detect free radicals at very low con- 

 centrations in functional systems. High sensitivity also permits the 

 resolution of separable components, and an estimate, from the in- 

 tensities of the separate signals, of their concentrations. If such sensi- 

 tivity can be achieved at rapid scanning speeds, it also becomes possi- 

 ble to obtain quantitative kinetic data regarding the free radical 

 concentration in a functioning system. Such data lend themselves 

 to analysis with respect to reaction mechanism. But in their absence, 

 the spectrometer tells us little more than that some component which 

 contains one or more unpaired electrons is present. 



ESR Studies of Photosynthesis 



Our first observations of ESR signals in frozen-dried preparations 

 of biological material, which began in 1952, included investigations 

 of green leaves. These relatively crude observations showed that the 

 concentration of unpaired electrons in etiolated leaves is rather low 

 but increases markedly after they are exposed to light and become 

 green (7) . It was also foimd that the ESR signal in a green leaf 

 frozen-dried after exposure to light is significantly greater than the 

 signal yielded by preparations made previous to illumination (4) . 



Following the completion of a spectrometer of improved sensitivity 

 in 1956, it was possible to study aqueous suspensions of chloroplasts 

 for the first time. An ESR signal was observed which became con- 

 siderably intensified on illumination (6) . The signal was apparently 



