RELATIONSHIP BETWEEN LIGHT INDUCED EPR SIGNAL 

 AND PIGMENT P700 



Helmut Beinert and Bessel Kok 



In two previous communications (1,2) we have reported ob- 

 servations on the narrow (--^8 gauss) EPR signal induced by 

 light in photosynthetic materials. (This signal is also 

 known as signal I (Ref. 3) or signal II (Ref. 4) and rapidly 

 decaying or "R" signal (Ref. $)• Ti^e first paper (l) de- 

 scribed properties of the signal observed in a preparation of 

 the red alga strain TX 27 that was largely deprived of phyco- 

 bilin and enriched in "P700" by partial extraction of the 

 chlorophyll using 72% acetone (6). The signal appeared on 

 illumination at room or liquid nitrogen temperature or after 

 chemical oxidation by means of ferricyanide . Double integra- 

 tion of the derivative signal indicated a concentration of 

 spins nearly identical to the concentration of P700 as deter- 

 mined by difference spectroscopy assuming a molar extinction 

 of 10~5 M~ xcm~ and complete bleaching in the photoact. A 

 second extraction of the algal preparation, now with S0% in- 

 stead of 72% acetone, removed the EPR signal as well as the 

 difference spectroscopic signal at 700 mu. We concluded that 

 these results could be most readily explained if the photo- 

 oxidized form of P700 was responsible for the narrow, fast 

 decaying EPR signal in photosynthesis. 



The second paper (2) reported observations with whole 

 cells of the blue-green alga Anacystis at room temperature. 

 The relative strength of the narrow EPR signal was measured 

 as a function of intensity of either one or both of two wave- 

 lengths: 630 mu absorbed by phycocyanin and quite effective 

 in provoking photosynthesis and 710 mu absorbed by (long 

 wave) chlorophyll and rather ineffective in provoking photo- 

 synthesis. The EPR signal showed the same behavior as the 

 oxidized form of P700 which was observed earlier spectro- 

 scopically {?): Long wave light proved much more effective 

 in provoking the EPR signal than short wavelengths, or a 

 combination of the two lights. These data therefore also 

 indicated the possible identity of the EPR signal with oxi- 

 dized P7OO, presumably the oxidized moiety generated in the 

 long wave photoreaction of photosynthesis. Since its reduc- 

 tion requires the reduced moiety generated in the short wave 

 photoact, P700 tends to accumulate in the absence of the 

 short wave light. 



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