BARRY COMMONER 



375 



narrow, uiisinu lured ESR signals at about this g value in a preparation 

 ol i)urificc[ ( lilorophyll. The signal is present in the dark, increases 

 on illumination, and decays very slowly when illumination ceases. 

 But there is no clear evidence that the ESR signal is due to a free 

 radical form ol chlorophyll. The number ol unpaired electrons ob- 

 served is less than 0.1% of the chlorophyll present, and it is possi- 

 ble that the signal arises from an impurity. Similar results have been 

 reported by Brody, Newell, and Castner (1). ESR studies of the 

 Krasnovsky reaction are not very helpful either. We have found that 

 on illumination of the Krasnovsky preparation (chlorophyll, aqueous 

 pyridine, and ascorbate) an intense ESR signal is observed — but it is 

 clearly due to an ascorbate free radical. A similar observation has 

 been reported by Krasnovsky himself (2) . There is therefore only the 

 most slender basis at this time for relating the signal at g = 2.002 to 

 chlorophyll. 



Recent experiments with deuterated Chlorella provide additional 

 data on the identities of the ESR signals observed in photosynthetic 

 systems, and offer a hope that complete identification will be achieved 

 before long. Fig. 10, which is based on studies made in collaboration 

 with Dr. Joseph J. Katz of the Argonne National Laboratory, shows 

 the ESR signal from Chlorella cultured in 99.9% DoO. As indicated 

 earlier, the hyperfine splitting due to H^ and deuterium are distinc- 



g= 2.005 



CHLORELLA 



D2O CULTURED 



■ 1 L I I ' 



HO gaussH 



Fig. 10. ESR signals, under identical instrument conditions from HoO-cultured 

 Chlorella and Chlorella cultured in 99.9% DoO. Modulation amplitude: 3 gauss. 



