Proc. Nat. Acad. Sci. USA Vol. 70, No. 2, pp. 294-297, February 1973 Detection of a Free Radical in the Primary Reaction of Chloroplast Photosystem II (photosynthesis/electron transfer/light reactions/electron paramagnetic resonance) RICHARD) MALKIN* AND ALAN J. BEARDENt * Department of Cell Physiology and tDonner Laboratory, University of California, Berkeley, Calif. 94720 Communicated by Daniel I. Arnon, November 17, 1972 ABSTRACT Electron paramagnetic resonance (EPR) Although electron paramagnetic resonance (EPR) spec- spectroscopy has revealed a new free-radical signal pro- duced by illumination of spinach chloroplasts at 770K. troscopy has been used to study the primary photoact in This signal is observed only when an oxidant (ferricyanide) lhotosynthetic bacteria (15-18) and to study the primary is added to the chloroplast suspension in the dark before reaction of chloroplast Photosystem I (19-22), EPR signals illumination. The EPR signal is produced at 770K by specificially associated with the primary reactants of chloro- illumination with 645-nm monochromatic light capable of plast Photosystem II have not heretofore been detected. An activating Photosystem II but not with 715-nm illumina- tion capable of activating Photosystem I. Furthermore, EPR signal, now known as "Signal II" (23-25), has been since the signal shows a relative increase in chloroplast associated with Photosystem II, but the nature of the com- fragments enriched in Photosystem II but is absent in ponent responsible for this signal and its role in photosyn- chloroplast fragments enriched in Photosystem I, we con- thetic electron transport is not clear (24, 25). clude that this new EPR signal is associated with the pri- mary photoact of Photosystem II in chloroplasts. On the In this palper, we report an EPR spectroscopic examination basis of the measured EPR parameters (g = 2.0026 i 0.0002, of chloroplasts and chloroplast fragments at 770K for changes linewidth = 8 G), it is suggested that the signal may be attributable to the primary photoreaction of Photosystem associated with the reaction-center chlorophyll of Photo- II. We have detected a new free-radical EPR signal that is system II. different from other previously described chloroplast free- The mechanism of oxygen evolution is one of the major unre- radical signals and that may be associated with the reaction- solved problems inl chloroplast photosynthesis. Oxygen evolu- center chlorophyll of Photosystem II. tion has been found to )roceed more efficiently in short-wave- MATERIALS AND METHODS length light (X < 700 nm), associated with Photosystem II, Whole spinach chloroplasts and washed, broken chloroplasts than in long-wavelength light (X > 700 nm), associated with were prepared from greenhouse spinach as described (26, 27). Photosystem I (1-3), but the nature of the primary reactants Digitonin chloroplast fragments enriched in Photosystem II of Photosystem II and the subsequent dark reactions in- (D-10) and Photosystem I (D-144) were prepared by the volved in the process of oxygen evolution have not yet been procedure of Hauska et al. (28). Triton chloroplast fragments fully characterized. enriched in Photosystein II were prepared from whole chloro- Two different light-induced changes in absorbance appear plasts by the procedure of Malkin (26). Chlorophyll concen- to be related to the primary photochemical event of Photo- trations and the chlorophyll a:b ratio were measured by the system II. A light-induced absorbance change at 550 nm was method of Arnon (29). discovered by Knaff and Arnon (4) and was found to be due Oxidation-reduction potentials of chloroplast suspensions to the Photosystem II photoreduction of a chloroplast com- at 50C in the presence of 10 mM ferricyanide were measured )onent, designated C550. The photoreduction of C550 was with a Radiometer PK-149 combined platinum-calomel elec- found to proceed not only at physiological temperature but trode and a Corning digital pH meter (model 110). The oxida- also at 770K, a finding that indicated a possible relation with tion-reduction potentials are reported relative to the standard a l)rimary l)hotochemical event. On the basis of this work, hydrogen electrode. which was confirmed and extended in other laboratories Chloroplast samples were placed in standard X-band quartz (5-10), C550 was lprol)osed (4-8) as the primary electron EPR tubes (3 mm i.d.) and illuminated for 30 sec at 770K acceptor of Photosystem II. D6ring et al. (11, 12) observed a directly in the EPR cavity with the apparatus described (20). different change in light-induced absorbance at physiological Baird-Atomic interference filters (715 or 645 nm) of half-band teml)erature in chloroplast fragments enriched in Photosystem width of 10 nm were used for the monochromatic illumina- II. This absorbance change was very rapid and had a spectrum tions. The incident light intensity on the sample was about with at 435 and 682 nm. These workers suggested this peaks 5 X 104 ergs cm-2 - sec-1. EPR spectra were recorded at 9.22 change was due to a form of chlorophyll a that functioned as GHz at 770K. Details of the EPR methods are the same as the reaction-center chlorophyll of Photosystem II. Later given in previous publications (19, 22, 27). measurements of the photoinduced absorbance change at 682 nm showed that the reaction occurre(1 at 770K (13). The RESULTS AND DISCUSSION possible relation of this absorbance change and C550 to the As shown in Fig. 1A, an EPR free-radical signal, identical to primary l)hotoact of Photosystem II has been discussed (14). "Signal II" (24, 25), is present in washed, broken chloroplasts 294 Downloaded by guest on September 25, 2021 Proc. Nat. Acad. Sci. USA 70 (1973) Primary Reaction of Chloroplast Photosystem II 295 at 770K in the dark. The addition of 10 mM ferricyanide to the chloroplasts at 50C in the dark followed by EPR examina- tion at 770K (Fig. 1B) results in the appearance of a second narrow free-radical signal centered at g = 2.002 and having a linewidth of about 8 G. This signal, referred to as "Signal I" (24, 25), has been shown to be due to the oxidized form of P700, the reaction-center chlorophyll of Photosystem I (24, 25, 30, 31). The concentration of ferricyanide added to these samples was sufficient to raise the ambient oxidation-reduc- tion potential of the suspension to about +540 mV. If the chloroplast sample is illuminated at 770K in the presence of ferricyanide with far-red light (715 nm), which activates pri- marily Photosystem I, there is no increase in the observed EPR signal (Fig. 1C). The intensity of far-red light used for the illumination was sufficient to completely photooxidize P700 at 770K when ferricyanide was omitted from the reac- FIG. 2. EPR difference spectra of chloroplast free-radical sig- tion mixture. This finding indicates that P700 was completely nals. (A) The difference spectrum in the dark between the sample oxidized chemically before illumination. If, however, the same treated with ferricyanide and the sample with no addition. (B) sample is subsequently illuminated with red light (645 nm), The difference spectrum of the sample illuminated with 645-nm which activates Photosystem II, a large increase in the EPR light (in the presence of ferricyanide) at 77°K and the sample signal is now observed (Fig. 1D). Since this light-induced illuminated with 715-nm light at 77°K (in the presence of ferri- change occurs at an oxidation-reduction potential where cyanide). Both difference spectra have been multiplied by a factor P700 is already fully oxidized, is produced by monochromatic of two. EPR spectra were recorded at 77°K as described in Fig. 1. illumination that activates Photosystem II, and is not pro- duced by illumination that activates Photosystem I, it is although broken chloroplasts were mainly used in these clear that the additional EPR signal must be due to a photo- studies to facilitate the interaction of ferricyanide with mem- reaction of a Photosystem II component. brane-bound components. The observation that the EPR signal appears when the Primary photoreactions in photosynthetic bacteria are re- chloroplast sample is illuminated at low temperatures (770K versible on the cessation of illumination even at temperatures or lower), at which chemical reactions would be strongly in- as low as 1.7°K (32, 15, 17). In contrast, EPR signals from hibited, is consistent with an association of the component chloroplasts or chloroplast fragments are irreversible up to responsible for the signal with a primary photochemical event. temperatures of at least 77°K (17, 19, 20). The light-induced The light-induced EPR signal observed in the presence of change observed in the presence of ferricyanide is similar to ferricyanide was also detected in whole spinach chloroplasts, other light-induced chloroplast EPR signals in this respect. Fig. 2 shows EPR difference spectra obtained from the data with washed, broken chloroplasts. (The signals have been / \ \Fe~~~~~~~F(11l),CN, 715 hv Fe(III)CN, 645 hi/ FIG. 1. Low-temperature, light-induced free-radical signal in chloroplasts in the presence of ferricyanide. The reaction mixture contained 50 mM Tricine (pH 7.8), 20 mM NaCl, washed, broken FIG. 3. Low-temperature, light-induced EPR free-radical chloroplasts (0.3 mg of chlorophyll per ml) and, where present, signal in Photosystem II chloroplast fragments in the presence 10 mM potassium ferricyanide. (A) No additions, dark; (B) plus of ferricyanide. The reaction mixture contained 50 mM Tricine ferricyanide, dark; (C) plus ferricyanide, illuminated at 77°K with (pH 7.8), 20 mM NaCl, 10 mM potassium ferricyanide, and D-10 715-nm light; (D) plus ferricyanide, illuminated at 77°K with chloroplast fragments (0.3 mg of chlorophyll per ml). (A) Plus 645-nm light.