366 LIGHT AND LIFE 



lor decay. In well-washed chloroplasts, the onset of signal I became 

 more rapid (time constant about 2 seconds) while the rate of decay 

 was reduced (time constant about 26 seconds) . 



These observations suggested that signal II arises from a sub- 

 stance produced in association with the decay of the component that 

 causes signal I. Since signal II is also present in the dark, and be- 

 cause its hyperfine structure is characteristic of an organic free radi- 

 cal, it appeared probable that it represented a free radical intermediate 

 of a dark oxidation-reduction process. Thus in the more nearly 

 intact chloroplast, the rate of decay of component I is so fast as to 

 preclude the accumulation of a detectable steady-state concentration. 

 In this circumstance, the light-induced unpaired electrons appear 

 to be transferred to component II, which by virtue of its relatively 

 low decay rate, then tends to accumulate to a degree. 



The component which yields ESR signal II has a hyperfine struc- 

 ture which is characteristic of an organic free radical, and further 

 evidence of its structure will be cited below. The foregoing data 

 offer no further characterization of the component which is the source 

 of the signal I. Its possible relation to a free radical form of chloro- 

 phyll will be discussed below. 



More recently we have turned our attention toward living Chlorella 

 in order to study free radical processes in a functionally complete 

 photosynthetic system. The algae are harvested, washed, and either 

 packed into an ESR cell, or examined while flowing through the 

 cavity in a circulation system which includes a gas-exchanger. Typi- 

 cal ESR signals from living Chlorella under various intensities of 

 illumination are shown in Fig. 3. The ESR signals are quite similar 

 to those observed in isolated chloroplasts. In the dark, Chlorella ex- 

 hibits an ESR signal centered at g = 2.005, w'ith a half-width of 

 about 20 gauss, with 5 hyperfine peaks at 6 gauss intervals. On 

 illumination the intensity of this signal increases, but in addition (at 

 appropriate temperatures) a second signal is observed which en- 

 hances the absorption at the low-g side of the signal. By subtraction 

 of suitable signals it can be shown that the second signal is centered 

 'dt g =z about 2.002, has a half-width of about 9 gauss, and no ob- 

 servable hyperfine structure. 



Thus, the two ESR components I and II observed in spinach and 

 tobacco chloroplasts also occur in living Chlorella. That these com- 

 ponents are closely related to the photosynthetic processes in green 

 algae is shown by the results shown in Fig. 4. The upper curve 

 shows the ESR signals exhibited by the normal green strain of Euglena, 



