376 LIGHT AND LIFE 



live, and one can predict that an ESR signal affected by protons will 

 become very much narrowed if deuterons are substituted. Fig. 10 

 shows that the D.O-cultured algae yield an ESR signal which resem- 

 bles that of ordinary Chi or el In— except that it is very much com- 

 pressed. Signal II, which in normal algae has a half-width of 19 

 gauss, exhibits a half-width of about 5.5 gauss in the DgO material. 

 The corresponding half-widths for signal I are 9 gauss and about 3 

 gauss respectively. 



In the case of component II the deuterium effect is so marked as 

 to indicate that the unpaired electron is affected exclusively by as- 

 sociation with hydrogen atoms. Once this conclusion is reached, it 

 follows that the 5-line hyperfine structure observed in the ordinary 

 signal from component II is due to protons. This being the case, it 

 can be concluded, on the basis of rules developed from studies of 

 known free radicals (see Weissman, 12 and 13), that this component 

 is an organic free radical in which the unpaired electron is associated 

 with a resonance system composed of four chemically equivalent car- 

 bons, to each of which a proton is attached. We are now studying 

 the effects of adding known substances to the DoO-cultured algae, and 

 expect, in this way, to make a positive identification of this free radical. 

 The D.O data also show that the unpaired electron of component I is 

 associated with hydrogen atoms. 



The association of both components with protons precludes the 

 possibility that the unpaired electrons that give rise to the two ESR 

 signals are conduction electrons in a semiconductor. Thus, the isotope 

 data indicate that no part of the ESR signal resulting from illumina- 

 tion can be attributed to conduction electrons, and provide a useful 

 partial description of the free radical that is responsible for one of 

 the observed signals. 



In presenting this sununary of our ESR studies of photosynthetic 

 systems, an effort has been made to describe some of the experimental 

 data, the principles on which the measurements are based, and the 

 limitations that these principles place on possible interpretations of 

 the results. It is evident that the physical principles which govern 

 ESR measurements and the relationships between the observed sig- 

 nals and molecular structure are essential as guides in these investi- 

 gations. However, our studies also show that biological principles 

 are ecjually important experimental guides. A frequent comment 

 about the earliest efforts to apply the ESR technique to biological 

 investigations was that the presence of numerous types of free radi- 

 cals in living cells would preclude precise data relevant to a specific 



