HARRY COMMONER 365 



siin|)le ami lacking in hyi)erfine slriuUirc. However, the signal was 

 poorly resolved (ihe spectrometer sensitivity being too low), and pre- 

 cluded any effective cjuantitative data, or conclusions regarding the 

 true shape ol the signal. Similar residts were later obtained by Calvin 

 et al. in 1957, who showed also that the light response was exhibited 

 to a certain degree at licjuid nitrogen temperatures and that signals 

 resembling those of chloroplasts could be observed in samples of 

 Rliodospirillum rubrum (10) . 



In the meantime Professor Townsend managed to provide us with 

 a further increase in sensitivity, and we were finally in a position to 

 obtain highly resolved signals from photosynthetic systems at 30- 

 to 90-second scanning speeds. 



^Vhen the chloroplast system was reinvestigated with the new 

 spectrometer, it was discovered that what had previously been ob- 

 served in illuminated chloroplasts as a simple ESR signal was in 

 reality due to t^vo components (5) . Chloroplasts observed in the 

 dark under conditions of high resolution revealed for the first time 

 an ESR signal with distinctive hyperfine splitting. This signal was 

 centered at g =z 2.005, had a half-width of about 19 gauss, and 5 

 hyperfine peaks evenly spaced at 6 gauss intervals. When chloroplasts 

 prepared by ordinary methods were illuminated the signal became 

 more intense, but its shape changed, and it exhibited a somewhat 

 lower g value. When the light and dark signals were integrated and 

 subtracted from each other, it became apparent that on illumination 

 a new ESR signal was superimposed on the signal observed in the 

 dark. The signal due to light was centered at g = 2.002, had a 

 half-width of about 9 gauss, and exhibited no hyperfine structure. 

 These results are shown in Fig. 2. In what follows, the ESR signal 

 at g 1= 2.002, and the component which gives rise to it will be desig- 

 nated as I; while II will refer to the signal centered at g = 2.005, and 

 the free radical to which it is due. 



Further investigations (5) suggested that the two ESR signals arose 

 from components that probably represented sequential steps in a 

 light-induced electron transfer process. It was observed that signal 



I appeared only in chloroplasts that were thoroughly washed or 

 dialyzed. When chloroplasts prepared with a minimum of washing 

 were illuminated, they exhibited only an increased intensity of the 

 signal (II) , which was also present in the dark. Kinetic studies 

 showed that signal I was associated with a relatively rapid rate of 

 onset and decay (5- to 10-second half-time), while the onset of signal 



II followed a time constant of 40 seconds for onset and 100 seconds 



