Exciton interaction among chlorophyll molecules in bacteriochlorophyll a proteins and bacteriochlorophyll a reaction center complexes from green bacteria

Absorption and CD spectra of bacteriochlorophyll a proteins and bacteriochlorophyll a reaction center complexes from two strains of Chlorobium limicola were recorded at 77 °K. Visual inspection showed that the Q_y-band of chlorophyll in either protein was split into at least five components. Analysis of the spectra in terms of asymmetric Gaussian component pairs by means of computer program GAMET showed that six components are necessary to fit the spectra from strain 2K. These six components are ascribed to an exciton interaction between the seven bacteriochlorophyll a molecules in each subunit. The clear difference between the exciton splitting in the two bacteriochlorophyll a proteins shows that the arrangement of the chlorophyll molecules in each subunit must be slightly different.

The spectra for the bacteriochlorophyll a reaction center complexes have a component at 834 nm (absorption) and 832 nm (CD) which does not appear in the spectra of the bacteriochlorophyll a proteins. The new component is ascribed to a reaction center complex which is combined with bacteriochlorophyll a proteins to form the bacteriochlorophyll a reaction center complex. The complete absorption (or CD) spectrum for a given bacteriochlorophyll a reaction center complex can be described to a first approximation in terms of the absorption (or CD) spectrum for the corresponding bacteriochlorophyll a protein plus the new component ascribed to the reaction center complex.     

A light-initiated,dark oxidation of bacteriochlorophyll from reaction center particles

When reaction center particles from Rhodopseudomonas spheroides strain R26 are illuminated and then extracted with methanol, about one-third of the extracted bacteriochlorophyll slowly becomes oxidized, The oxidation does not occur under anaerobic conditions or in the absence of the detergent lauryldimethylamine oxide. Alkaline conditions also prevent the reaction. A dark interval between illumination and extraction delays the onset of bacteriochlorophyll oxidation in a predictable way. These results are consistent with the hypothesis that illumination generates a reaction initiator which is fairly stable in methanol but decays with a half-life of about 4.5 min in reaction center particles after illumination ceases.     

Fluorescence emission spectra of cells and subcellular preparations of a green photosynthetic bacterium. Effects of dithionite on the intensity of the emission bands

Fluorescence emission spectra were measured of intact cells and subcellular preparations of the green photosynthetic bacterium Prosthecochloris aestuarii in the presence and in the absence of dithionite. A 3–5-fold increase in bacteriochlorophyll a fluorescence at 816 nm occurred upon addition of dithionite in a membrane vesicle preparation (Complex I), in a photochemically active pigment-protein complex and in a bacteriochlorophyll a protein complex free from reaction centers. The pigment-protein complex showed a relatively strong long-wave emission band (835 nm) of bacteriochlorophyll a, which was preferentially excited by light absorbed at 670 nm and was not stimulated by dithionite. With Complex I, which contains some bacteriochlorophyll c in addition to bacteriochlorophyll a, a 3–4-fold stimulation of bacteriochlorophyll c emission was also observed. Emission bands at shorter wavelengths, probably due to artefacts, were quenched by dithionite. With intact cells, the effect of dithionite was smaller, and consisted mainly of an increase of bacteriochlorophyll a emission.

The results indicate that the strong increase in the yield of bacteriochlorophyll emission that occurred upon generating reducing conditions is, at least mainly, due to a direct effect on the light-harvesting systems, and does not involve the reaction center as had been earlier postulated.     

High-order derivative spectroscopy of infrared absorption spectra of the reaction centers from Rhodobacter sphaeroides

The infrared absorption spectra of reduced and chemically oxidized reaction center preparations from the purple bacterium Rhodobacter sphaeroides were investigated by means of high-order derivative spectroscopy. The model Gaussian band with a maximum at 810 nm and a half-band of 15 nm found in the absorption spectrum of the reduced reaction center preparation is eliminated after the oxidation of photoactive bacteriochlorophyll dimer (P). This band was related to the absorption of the P(+)y excitonic band of P. On the basis of experimental results, it was concluded that the bleaching of the P(+)y absorption band at 810 nm in the oxidized reaction center preparations gives the main contribution to the blue shift of the 800 nm absorption band of Rb. sphaeroides reaction centers.     

Relations between pigments and proteins in the photosynthetic membranes of Rhodopseudomonas spheroides

We have isolated from Rhodopseudomonas spheroides a pigment-protein complex of apparent weight 9 kdaltons that bears more than 60% of the light harvesting bacteriochlorophyll. The isolation procedure involved exposure to 1% lauryl dimethyl amine oxide (LDAO). The purified 9-kdalton fraction showed the light harvesting bacteriochlorophyll components B800 and B850, plus carotenoids. The ratio of bacteriochlorophyll to protein was 17%. This protein is probably the same as the “band 15” protein of Fraker and Kaplan. It may exist in vivo as characteristic aggregates of higher molecular weight. LDAO added to Rps. spheroides chromatophores converted the bacteriochlorophyll component B870 to a form absorbing at 770 nm but had little effect on the “B800 + B850” system, causing only a reversible shift of the 850-nm band to 845 nm. Anti-reaction center serum, added to subcellular fractions from Rps. spheroides with 1% LDAO, precipitated reaction center chromoprotein unaccompanied by light harvesting bacteriocholorophyll. Other antisera precipitated light harvesting components and left the reaction center chromophores in solution. A major protein of apparent weight 45 kdaltons was found in relatively nonpigmented fractions from Rps. spheroides, associated with cell wall fragments. The 45-kdalton protein showed considerable interstrain variability, whereas the 9-kdalton and reaction center proteins appeared constant.     

Absorbance and fluorescence properties of the bacteriochlorophyll a reaction center complex and bacteriochlorophyll a protein in green bacteria

Absorbance, emission and excitation spectra were measured at both room and liquid-nitrogen temperatures for a photochemically active bacteriochlorophyll a reaction center complex and a bacteriochlorophyll a protein isolated from Chlorobium limicola and Chlorobium thiosulfatophilum. The low-temperature absorbance spectrum for the complex has a band centered at 833 nm, which is not seen in the spectrum of the bacteriochlorophyll a protein. We attribute this difference to a modification of the bacteriochlorophyll a protein in the active complex. The room-temperature fluorescence spectra for the bacteriochlorophyll a protein and the complex are similar, as are those measured at low temperatures. The 833-nm component of the low-temperature absorbance spectrum of the complex is relatively nonfluorescent.     

Spectral and photochemical properties of subchromatophore fractions derived from carotenoid-deficient Chromatium by Triton treatment

Triton treatment of chromatophores of carotenoid-deficient Chromatium followed by density-gradient centrifugation led to a separation into three subchromatophore fractions. Unlike the case with chromatophores of regular Chromatium, Triton releases about 1/3 of the total bulk bacteriochlorophyll into one fraction (designated G, for green) whose major absorption-band maximum is at 780 nm. One fraction (H, for heavy) absorbs at 805 and 885 nm, with an absorbance ratio A₈₈₅ nm/A₈₀₅ nm between 1.5 and 2; another fraction (L, for light) absorbs at 805 nm and has a shoulder at 825 nm. The absorption and fluorescence emission spectra of the three fractions at room temperature and 77°K indicate that the different bacteriochlorophyll forms are efficiently separated by Triton treatment.

The reaction center P890 is concentrated exclusively in the H-fraction, at a level of 5–7% of the bulk bacteriochlorophyll. The solubilized bacteriochlorophyll absorbing at 780 nm can be totally and irreversibly bleached by 5 mM ferricyanide. The other bacteriochlorophyll forms in the H- and L-fractions are also irreversibly bleached by ferricyanide to variable extents. P890 is the only component that can be re-reduced by ascorbate after ferricyanide oxidation. The P890 content estimated by reversible chemical bleaching agrees well with that obtained by reversible light bleaching. The different bacteriochlorophyll forms, with the exception of the 780-nm absorbing form, are relatively stable toward light bleaching. Again, only P890 is reversibly bleached by light.

Cytochromes-555 and -553 are distributed in both the H-and L-fractions, but not in the solubilized-bacteriochlorophyll G-fraction. However, only cytochromes in the H-fraction which contains all of the P890 can undergo coupled oxidation. Excitation with 20-nsec ruby-laser pulses shows that cytochrome-555 can be oxidized in 2–3 μsec by photooxidized P890, indicating that necessary conformation for rapid electron transport is retained in the subchromatophore particles.

The data on fractionation and redox reactions obtained here, together with direct kinetic measurements recently reported in the literature lend further support to the view that oxidation of these two cytochromes is mediated by the same reaction center, P890.     

Generation of electric current by chromatophores of Rhodospirillum rubrum and reconstitution of electrogenic function in subchromatophore pigment-protein complexes

Lipoprotein complexes, containing (1) bacteriochlorophyll reaction centers, (2) bacteriochlorophyll light-harvesting antenna or (3) both reaction centers and antenna, have been isolated from chromatophores of non-sulphur purple bacteria Rhodospirillum rubrum by detergent treatments. The method of reconstituting the proteoliposomes containing these complexes is described. Being associated with planar azolectin membrane, proteoliposomes as well as intact chromatophores were found to generate a light-dependent transmembrane electric potential difference measured by Ag/AgCl electrodes and voltmeter. The direction of the electric field in proteoliposomes can be regulated by the addition of antenna complexes to the reconstitution mixture. The reaction center complex proteoliposomes generate an electric field of a direction opposite to that in chromatophores, whereas proteoliposomes containing reaction center complexes and a sufficient amount of antenna complexes produce a potential difference as in chromatophores. ATP and inorganic pyrophosphate, besides light, were shown to be usable as energy sources for electric generation in chromatophores associated with planar membrane.     

Photoactivation and conformational gating for manganese binding and oxidation in bacterial reaction centers

The influence of illumination history of native bacterial reaction centers (BRCs) on the ability of binding and photo-induced oxidation of manganous ions was investigated in the pH range between 8.0 and 9.4. Binding of manganous ions to a buried site required 6 to 11-fold longer incubation periods, depending on the pH, in dark-adapted BRCs than in BRCs that were previously illuminated prior to manganese binding. The intrinsic electron transfer from the bound manganese ion to the photo-oxidized primary electron donor was found to be limited by a 2 to 5-fold slower precursor conformational step in the dark-adapted samples for the same pH range. The conformational gating could be eliminated by photoactivation, namely if the BRCs were illuminated prior to binding. Unlike in Photosystem II, photoactivation in BRCs did not involve cluster assembly. Photoactivation with manganese already bound was only possible at elevated detergent concentration. In addition, also exclusively in dark-adapted BRCs, a marked breaking point in the Arrhenius-plot was discovered around 15 °C at pH 9.4 indicating a change in the reaction mechanism, most likely caused by the change of orientation of the 2-acetyl group of the inactive bacteriochlorophyll monomer located near the manganese binding site.     

Absorption changes of carotenoids and bacteriochlorophyll in energized chromatophores of Rhodospirillum rubrum

The energization of Rhodospirillum rubrum chromatophores by the light, ATP, PP_i, by dark electron transfer via energy-coupling sites of the redox chain, by the combination of KC1 and valinomycin causes absorption changes of carotenoids and bacteriochlorophyll. These changes due to the absorption-band shifts of the pigments are sensitive to the uncoupler p-trifluoromethoxycarbonyl cyanide phenylhydrazone (FCCP) but not to the combination of KC1 and nigericin, which abolishes fluorescence changes of atebrin. Dithionite and ferricyanide depress the light-induced absorption changes of bacteriochlorophyll but have no inhibitory effect on the PP_i-induced changes. Analysis of bacteriochlorophyll absorption changes in the infra-red region shows that the photooxidation of bacteriochlorophyll reaction centers with the negative peak in the region of 890 nm is accompanied by red and blue shifts of bacteriochlorophyll absorption bands. These shifts are due to a transmembrane electrochemical gradient of H⁺ and a local electric field arising as a result of oxidation of the reaction centers. It appears that the superposition of the (1) red shift which is characterized by negative and positive peaks at 865 and 895 nm, respectively, and (2) photobleaching of bacteriochlorophyll reaction centers in the region of 890 nm cause overall absorption changes with the negative peak at 865 nm.     

Energy transfer and bacteriochlorophyll fluorescence in purple bacteria at low temperature

Emission spectra of bacteriochlorophyll a fluorescence and absorption spectra of various purple bacteria were measured at temperatures between 295 and 4 K. For Rhodospirillum rubrum the relative yield of photochemistry was measured in the same temperature region. In agreement with earlier results, sharpening and shifts of absorption bands were observed upon cooling to 77 K. Below 77 K further sharpening occurred. In all species an absorption band was observed at 751-757 nm. The position of this band and its amplitude relative to the concentration of reaction centers indicate that this band is due to reaction center bacteriopheophytin. The main infrared absorption band of Rhodopseudomonas sphaeroides strain R26 is resolved in two bands at low temperature, which may suggest that there are two pigment-protein complexes in this species. Emission bands, like the absorption bands, shifted and sharpened upon cooling. The fluorescence yield remained constant or even decreased in some species between room temperature and 120 K, but showed an increased below 120 K. This increase was most pronounced in species, such as R. rubrum, which showed single banded emission spectra. In Chromatium vinosum three (835, 893 and 934 nm) and in Rps. sphaeroides two (888 and 909 nm) emission bands were observed at low temperature. The temperature dependence of the amplitudes of the short wavelength bands indicated the absence of a thermal equilibrium for the excitation energy distribution in C. vinosum and Rps. sphaeroides. In all species the increased in the yield was larger when all reaction centers were photochemically active than when the reaction centers were closed. In R. rubrum the increase in the fluorescence yield was accompanied by a decrease of the quantum yield of charge separation upon excitation of the antenna but not of the reaction center chlorophyll. Calculation of the F?rster resonance integral at various temperatures indicated that the increase in fluorescence yield and the decrease in the yield of photochemistry may be due to a decrease in the rate of energy transfer between antenna bacteriochlorophyll molecules. The energy transfer from carotenoids to bacteriochlorophyll was independent of the temperature in all species examined. The results are discussed in terms of existing models for energy transfer in the antenna pigment system.     

Cytochrome b oxidation and reduction reactions in the ubiquinone-cytochrome b/c2 oxidoreductase from Rhodopseudomonas sphaeroides

1. The kinetics of cytochrome b reduction and oxidation in the ubiquinone-cytochrome b/c2 oxidoreductase of chromatophores from Rhodopseudomonas sphaeroides Ga have been measured both in the presence and absence of antimycin, after subtraction of contributions due to absorption changes from cytochrome c2, the oxidized bacteriochlorophyll dimer of the reaction center, and a red shift of the antenna bacteriochlorophyll. 2. A small red shift of the antenna bacteriochlorophyll band centered at 589 nm has been identified and found to be kinetically similar to the carotenoid bandshift. 3. Antimycin inhibits the oxidation of ferrocytochrome b under all conditions; it also stimulates the amount of single flash activated cytochrome b reductions 3- to 4-fold under certain if not all conditions. 4. A maximum of approximately 0.6 cytochrome b-560 (Em(7) = 50 mV, n = 1, previously cytochrome b50) hemes per reaction center are reduced following activating flashes. This ratio suggests that there is one cytochrome b-560 heme functional per ubiquinone-cytochrome b/c2 oxidoreductase. 5. Under the experimental conditions used here, only cytochrome b-560 is observed functional in cyclic electron transfer. 6. We describe the existence of three distinct states of reduction of the ubiquinone-cytochrome b/c2 oxidoreductase which can be established before activation, and result in markedly different reaction sequences involving cytochrome b after the flash activation. Poising such that the special ubiquinone (Qz) is reduced and cytochrome b-560 is oxidized yields the conditions for optimal flash activated electron transfer rates through the ubiquinone-cytochrome b/c2 oxidoreductase. However when the ambient redox state is lowered to reduce cytochrome b-560 or raised to oxidize Qz, single turnover flash induced electron transfer through the ubiquinone-cytochrome b/c2 oxidoreductase appears impeded; the points of the impediment are tentatively identified with the electron transfer step from the reduced secondary quinone (QII) of the reaction center to ferricytochrome b-560 and from the ferrocytochrome b-560 to oxidized Qz, respectively.     

Analysis by exciton theory of the optical properties of the Chloroflexus aurantiacus reaction center

The optical properties of the reaction center of the filamentous green bacterium Chloroflexus aurantiacus, that contains three bacteriochlorophyll (BChl) a and three bacteriopheophytin (BPh) a molecules, were analyzed in the near-infrared region with the aid of exciton theory. The coordinates obtained from the X-ray analysis of the reaction center of Rhodopseudomonas viridis (Deisenhofer, J., Epp, O., Miki, K., Huber, R. and Michel, H. (1984) J. Mol. Biol. 180, 385–398) were used for the geometry of the reaction center of C. aurantiacus, with the replacement of one of the ‘accessory’ BChl molecules by BPh. The results were found to be in good agreement with experimental low-temperature absorption spectra, linear and circular dichroism and fluorescence polarization spectra and lead to the following conclusions. The allowed, low-energy exciton transition of the primary electron donor (P-865) is located at 887 nm and carries the dipole strength of approx. two BChl a monomers; the high-energy exciton transition, around 790 nm, is mixed with wave functions of other pigments, which explains its relatively small angle with respect to the 887 nm transition. The optical transition of the accessory BChl a molecule near 812 nm has some contribution of the BChls that constitute P-865. This can account for the experimentally observed reorientation and shift of this transition upon oxidation of P-865. Two of the BPh molecules are located on the same (probably the M) polypeptide subunit and show a clear splitting of absorption bands (11 nm) due to exciton coupling; the single BPh on the opposite branch shows hardly any exciton shift. Similar calculations for reaction centers of purple bacteria that contain four BChl a and two BPh a molecules resulted in a very low dipole strength for the high-energy transition of the primary donor due to antisymmetric mixing with both accessory BChl a wave functions and gave very little splitting of the absorption bands of BPh a. Our results indicate that the arrangement of the chromophores in reaction centers of C. aurantiacus is very similar to that in purple bacteria. The functional L-chains of the reaction centers of purple and filamentous green bacteria consist of pigments of the same type in a probably very similar arrangement.     

Effect of formate on Mössbauer parameters of the non-heme iron of PS II particles of cyanobacteria

Mössbauer spectra were measured for PSII particles having an active water-splitting system. The particles were isolated from the thennophilic cyanobacterium Synechococcus elongatus enriched in⁵⁷Fe. The Mössbauer resonance absorption spectrum is a superposition of 3 doublets with the following quadrupole splitting and chemical shift: 1, δ = 0.40, Δ = 0.85; II, δ = 1.35,Δ =2.35; III, δ = 0.25, Δ = 1.65. The δ and Δ values of doublets I, II, III are characteristic of proteins with iron-sulphur center, non-heme iron of the reaction center of higher plants and of the oxidized cytochrome 6–559. Treatment with sodium formate to remove bicarbonate affects only the doublet of non-heme iron, causing its quadrupole splitting to reduce to 1.75 and the chemical shift to reduce to 0.90. After washing out the formate, the Mossbauer spectrum of non-heme iron is restored. The data suggest that bicarbonate is a ligand for the non-heme iron of the reaction center of cyanobacteria.     

Isolation and characterization of bacteriochlorophyll-protein complexes from an aerobic bacterium,Pseudomonas radiora

Bacteriochlorophyll(Bchl)-protein complexes were isolated from a strictly aerobic and facultative methylotrophic bacterium Pseudomonas radiora strain MD-1. They were identified as the reaction center (RC)-B870 and the B870 complexes on the basis of their absorption spectra, light induced spectral changes and polypeptide compositions. The RC-B870 complex of this bacterium showed similar properties to those of typical purple photosynthetic bacteria, and contained c-type cytochrome which was oxidized upon illumination.Abbreviations Bchl bacteriochlorophyll - RC reaction center - SDS sodium dodecylsulfate - PAGE polyacrylamide gel electrophoresis     

Prompt and delayed fluorescence in pigment-protein complexes of a green photosynthetic bacterium

Excitation spectra were measured at 4 K of bacteriochlorophyll a fluorescence in reaction center containing pigment-protein complexes obtained from the green photosynthetic bacterium Prosthecochloris aestuarii. Excitation spectra for the longest-wave emission (838 nm) showed bands of bacteriochlorophyll a, carotenoid, and of a pigment with absorption bands at 670, 438 and possibly near 420 nm, which is probably identical to an unidentified porphyrin described in the preceding paper (Swarthoff, T., Kramer, H.J.M. and Amesz, J. (1982) Biochim. Biophys. Acta 681, 354–358). At room temperature the longest-wave emission is stimulated by a magnetic field, which indicates that at least part of the emission is delayed fluorescence brought about by a reversal of the primary charge separation. Below about 150 K no stimulation was observed. The excitation spectra for short-wave emission (828 nm) were very similar to the absorption spectrum of the isolated antenna bacteriochlorophyll a-protein complex, and showed bands of bacteriochlorophyll a only. This indicates that two forms of the antenna protein exist that are spectroscopically similar: a soluble form that is released by treatment with guanidine hydrochloride and a bound form that remains attached to the reaction center complex. The bands of the antenna complexes were weak in the excitation spectra of the 838 nm fluorescence, which indicates that the efficiency of energy transfer to the reaction center complex is low.     

Spectroscopic Determinants in the Reaction Center of Rhodopseudomonas Viridis

Assignments are proposed for the long wavelength absorption bands observed in the reaction center of Rhodopseudomonas viridis. The assignments are based on a theoretical treatment in which quantum mechanical calculations are first carried out on the individual chromophores of the reaction center. The energies and wave functions that are obtained are then introduced into an exciton-type perturbation treatment in which extensive configuration interaction is carried out between the excited states of the four bacteriochlorophylls and two bacteriopheophytins of the reaction center. Calculated values for absorption maxima, transition moments, linear dichroism, and rotational strength are compared with experiments in an attempt to distinguish among different assignments. The calculations alone do not lead to unambiguous assignments; indeed it is difficult to account for the reaction center spectra without introducing assumptions as to the effects of the protein on the energy levels of the individual molecules. Even if these effects are treated as free parameters, the experimental spectra still provide useful constraints that restrict the models that are possible. The major result of this work is that the weak 850-nm absorption band is due, primarily, to the higher energy exciton state of the bacteriochlorophyll special pair. Accounting for the 960-nm absorption band of the low energy exciton state of the special pair requires either that a large spectroscopic effect of the protein be introduced, or possibly, that charge transfer states play a major spectroscopic role. The difference in spectra seen in the formation of oxidized or triplet state reaction centers can be understood in terms of a combination of electrochromic effects and modified exciton interactions.     

A kinetic completion of the cyclic photosynthetic electron pathway of Rhodopseudomonas sphaeroides: Cytochrome b-cytochrome c2 oxidation-reduction

In Rhodopseudomonas sphaeroides, following a single-turnover flash of light, cytochrome c₂ is oxidized by reaction center bacteriochlorophyll, and a cytochrome b is reduced by the primary electron acceptor, probably via ubiquinone. In this report we show that, in the uncoupled state, the rate of re-oxidation of the cytochrome b is identical to the rate of reduction of the cytochrome c₂, a kinetic completion of the cyclic photosynthetic electron transport system.     

The FMO Protein

In this article I review the history of research on the Fenna-Matthews-Olson (FMO) protein with emphasis on my contributions. The FMO protein, which transfers energy from the chlorosome to the reaction center in green sulfur bacteria, was discovered in 1962 and shown to contain bacteriochlorophyll a. From the absorption and circular dichroism spectra, it was clear that there was an exciton interaction between the bacteriochlorophyll molecules. Low temperature spectra indicated a seven-fold exciton splitting of the Q(y) band. The FMO protein was crystallized in 1964, and the X-ray structure determined in 1979 by B.W. Matthews, R.E. Fenna, M.C. Bolognesi, M.F. Schmidt and J.M. Olson. The structure showed that the protein consisted of three subunits, each containing seven bacteriochlorophyll molecules. The optical spectra were satisfactorily simulated in 1997. In living cells the FMO protein is located between the chlorosome and the reaction centers with the C3 symmetry axis perpendicular to the membrane. The FMO protein may be related to PscA in the reaction center.     

Light-induced changes of absorbance and electron spin resonance in small Photosystem II particles

Photosystem II reaction center components have been studied in small system II particles prepared with digitonin. Upon illumination the reduction of the primary acceptor was indicated by absorbance changes due to the reduction of a plastoquinone to the semiquinone anion and by a small blue shift of absorption bands near 545 nm (C550) and 685 nm. The semiquinone to chlorophyll ratio was between 1/20 and 1/70 in various preparations. The terminal electron donor in this reaction did not cause large absorbance changes but its oxidized form was revealed by a hitherto unknown electron spin resonance (ESR) signal, which had some properties of the well-known signal II but a linewidth and g-value much nearer to those of signal I. Upon darkening absorbance and ESR changes decayed together in a cyclic or back reaction which was stimulated by 3-(3,4 dichlorophenyl)-1,1-dimethylurea. The donor could be oxidized by ferricyanide in the dark.

Illumination in the presence of ferricyanide induced absorbance and ESR changes, rapidly reversed upon darkening, which may be ascribed to the oxidation of a chlorophyll a dimer, possibly the primary electron donor of photosystem II. In addition an ESR signal with 15 to 20 gauss linewidth and a slower dark decay was observed, which may have been caused by a secondary donor.