Properties of partially purified photosynthetic reaction centers from Scenedesmus mutant 6E and Anabaena variabilis grown in the presence of diphenylamine

Sub-microsecond chlorophyll a delayed fluorescence from photosystem I. Magnetic field-induced increase of the emission yield

(1) In photosystem I (PS I) particles in the presence of dithionite and intense background illumination at 290 K, an external magnetic field (0-0.22 T) induced an increase, delta F, of the low chlorophyll a emission yield, F (delta F/F approximately or equal to 1-1.5%). Half the effect was obtained at about 35-60 mT and saturation occurred for magnetic fields higher than about 0.15 T. In the absence of dithionite, no field-induced increase was observed. Cooling to 77 K decreased delta F at 685 nm, but not at 735 nm, to zero. Measuring the emission spectra of F and delta F, using continuous excitation light, at 82, 167 and 278 K indicated that the spectra of F and delta F have about the same maximum at about 730, 725 and 700 nm, respectively. However, the spectra of delta F show more long-wavelength emission than the corresponding spectra of F. (2) Only in the presence of dithionite and with (or after) background illumination, was a luminescence (delayed fluorescence) component observed at 735 nm, ater a 15 ns laser flash (530 nm), that decayed in about 0.1 microseconds at room temperature and in approx. 0.2 microseconds at 77 K. A magnetic field of 0.22 T caused an appreciable increase in luminescence intensity after 250 ns, probably mainly caused by an increase in decay time. The emission spectra of the magnetic field-induced increase of luminescence, delta L, at 82, 167 and 278 K coincided within experimental error with those of delta F mentioned above. The temperature dependence of delta F and delta L was found to be nearly the same, both at 685 and at 735 nm. (3) Analogously to the proposal concerning the 0.15 microseconds luminescence in photosystem II (Sonneveld, A., Duysens, L.N.M. and Moerdijk, A. (1980) Proc. Natl. Acad. Sci. U.S.A. 77, 5889-5893), we propose that recombination of the oxidized primary donor P-700+ and the reduced acceptor A-, probably A-1, of PS I causes the observed fast luminescence. The effect of an external magnetic field on this emission may be explained by the radical pair mechanism. The field-induced increase of the 0.1-0.2 microseconds luminescence seems to be at least in large part responsible for the observed increase of the total (prompt + delayed) emission measured during continuous illumination in the presence of a magnetic field.

Fluorescence emission spectra of chloroplasts and subchloroplast preparations at low temperature

A study was made of the chlorophyll fluorescence spectra between 100 and 4.2 K of chloroplasts of various species of higher plants (wild strains and chlorophyll b mutants) and of subchloroplast particles enriched in Photosystem I or II. The chloroplast spectra showed the well known emission bands at about 685, 695 and 715--740 nm; the System I and II particles showed bands at about 675, 695 and 720 nm and near 685 nm, respectively. The effect of temperature lowering was similar for chloroplasts and subchloroplast particles; for the long wave bands an increase in intensity occurred mainly between 100 and 50 K, whereas the bands near 685 nm showed a considerable increase in the region of 50--4.2 K. In addition to this we observed an emission band near 680 nm in chloroplasts, the amplitude of which was less dependent on temperature. The band was missing in barley mutant no. 2, which lacks the light-harvesting chlorophyll a/b-protein complex. At 4.7 K the spectra of the variable fluorescence (Fv) consisted mainly of the emission bands near 685 and 695 nm, and showed only little far-red emission and no contribution of the band at 680 nm. From these and other data it is concluded that the emission at 680 nm is due to the light-harvesting complex, and that the bands at 685 and 695 nm are emitted by the System II pigment-protein complex. At 4.2 K, energy transfer from System II to the light-harvesting complex is blocked, but not from the light-harvesting to the System I and System II complexes. The fluorescence yield of the chlorophyll species emitting at 685 nm appears to be directly modulated by the trapping state of the reaction center.

Role of beta-carotene in the reaction centres of photosystems I and II of spinach chloroplasts prepared in non-polar solvents

Spinach chloroplasts have been prepared nonaqueously using non-polar solvents (n-hexane, CCl4, n-hepatane) and the beta-carotene content extracted in a controlled manner. This procedure is reproducible and does not result in large structural or spectral changes of the chloroplasts. The organisation of the chlorophyll-proteins is unaltered, as fragmentation with digitonin results in the appearance of the same fractions as found previously for aqueously-prepared chloroplasts, including the pink zone containing cytochromes f and b6 in the ratio 1 : 2. The chloroplasts possess both Photosystem I activity (P-700 photo-bleaching, and NADP+ photoreduction) and Photosystem II activity (parabenzoquinone reduction with Mn2+ as electron donor, and chlorophyll fluorescence induction). Use of moderate intensity red illumination has allowed a study of the role of beta-carotene in photochemistry separate from its roles in energy transfer and photoprotection. Removal of the fraction of beta-carotene closely associated with the Photosystem I reaction centre caused the rate of NADP+ photoreduction to fall to a low, but significantly non-zero level. Thus, in the complete absence of beta-carotene, photochemistry can still be observed, however the specific association of beta-carotene with the reaction centre is required for maximal rates. We propose that beta-carotene bound at the reaction centre decreases the rate of transfer of excitation energy away from the reaction centre, and increases the rate of photochemistry. It is possible that this occurs via formation of an exciplex between ground state beta-carotene and chlorophyll in the first excited state.

Isolation and characterization of photosystem I and II membrane particles from the blue-green alga, Synechococcus cedrorum

Fractions enriched in either Photosystem I or Photosystem II activity have been isolated from the blue-green alga, Synechococcus cedrorum after digitonin treatment. Sedimentation of this homogenate on a 10--30% sucrose gradient yielded three green bands: the upper band was enriched in Photosystem II, the lowest band was enriched in Photosystem I, while the middle band contained both activities. Large quantities of both particles were isolated by zonal centrifugation, and the material was then further purified by chromatography on DEAE-cellulose. The resulting Photosystem II particles carried out light-induced electron transport from semicarbizide to ferricyanide of over 2000 mumol/mg Chlorophyll per h (which was sensitive to 3-(3,4-dichlorophenyl)-1, 1-dimethylurea), and was nearly devoid of Photosystem I activity. This particle contains beta-carotene, very little phycocyanin, has a chlorophyll absorption maximum at 675 nm, and a liquid N2 fluorescence maximum at 685 nm. The purest Photosystem II particles have a chlorophyll to cytochrome b-559 ratio of 50 : 1. The Photosystem I particle is highly enriched in P-700, with a chlorophyll to P-700 ratio of 40 : 1. The physical structure of the two Photosystem particles has also been studied by gel electrophoresis and electron microscopy. These results indicate that the size and protein composition of the two particles are distinctly different.

Further purification of "Triton subchloroplast fraction I" (TSF-I particles). Isolation of a cytochrome-free high-P-700 particle and a complex containing cytochromes f and b6, plastocyanin and iron-sulfur protein(s)

The "Triton Subchloroplast Fraction I" or "TSF-I particles" can be further fractionated into a cytochrome fraction and a P-700-containing fraction essentially free of cytochromes. The cytochrome complex contains cytochromes f and b6 in approx. equimolar amounts, and, in addition, also plastocyanin and one iron-sulfur protein, all in the bound state. Bound plastocyanin was characterized by EPR spectroscopy. The EPR spectrum of the bound iron-sulfur protein resembles that previously detected in Phostosystem I particles under highly reducing conditions at lower than -560 mV. The redox potential of P-700 in the cytochrome-free high-P-700 particles was measured to be +468 mV; those of cytochromes f and b6 are +345 and -140 mV, respectively. Among the four components present in the complex, only cytochrome f can be coupled to a Photosystem I particle and undergoes photooxidation. This coupled photooxidation is totoally inhibited by KCN and only partially inhibited by HgCl2. The similarity of the complex containing cytochromes f and b6, plastocyanin, and an iron-sulfur protein to complexes III and IV of the mitochondrial respiratory redox chain and a possible involvement of the complex in cyclic photophosphorylation are noted and discussed.

Enrichment of photosystem I reaction center chlorophyll from spinach chloroplasts

The reaction center chlorophyll of Photosystem I in spinach chloroplasts was highly enriched. Preparations having 5-9 chlorophylls per 1 P700 were obtained by treating the Photosystem I particles prepared by digitonin treatment of chloroplasts with wet diethyl ether. All P700 present in the extracted particles was found to be photoactive, undergoind oxidation upon illumination.

Physiology and cytological chemistry blue-green algae

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A new photosynthetic pigment, "P430": its possible role as the priary electron acceptor of photosystem I

The technique of flash kinetic spectrophotometry was used to demonstrate a broad absorption band around 430 nm, which was kinetically different from P700, in several photosystem-I particles from spinach and blue-green algae. The component represented by this absorption band, designated as "P430", was bleached as fast as P700. Its recovery in the dark was accelerated by ferredoxin and by various artificial electron acceptors with redox potentials as low as -521 mV. The recovery kinetics have been demonstrated to be identical to those of a concomitant reduction of several of the artificial electron acceptors used. Tentatively, "P430" has been proposed as a possible primary electron acceptor of photosystem I.