The redox properties of the cytochromes of purified Complex III

Purified Complex III from beef heart contains two b cytochromes: a high-potential (E_{m 7.2} = +93 mV) cytochrome b-562 which can be enzymatically reduced, and a low-potential (E_{m 7.2} = −34 mV) cytochrome b-565 which is reduced only by dithionite. The two components each contribute approximately 50% to the total cytochrome b of Complex III. Cytochrome c₁ of Complex III titrates with a half-reduction potential of +232 mV.     

Thermodynamic properties of the photochemical reaction center of Heliobacterium chlorum

The thermodynamic and spectral properties of the photochemical reaction center components of Heliobacterium chlorum have been examined. The primary electron donor bacteriochlorophyll has E_m,7 = +225 mV, and the ‘primary acceptor’ E_m,10 = −510 mV. The former has an EPR signal in its oxidised form near G = 2.0025, ΔH = 0.95 mT, reminiscent of the properties of the primary donor in bacteria containing bacteriochlorophyll a. The ‘primary acceptor’ has properties similar to those of the iron-sulfur cluster acceptors of green sulfur bacteria. H. chlorum contains a c-type cytochrome (E_m,7 = +170 mV) that donates electrons to the photooxidised primary donor with . The reaction center of H. chlorum is thus very similar to that found in representative green sulfur bacteria, but the cellular architecture and photopigments of this group are quite distinct from those of H. chlorum.     

The respiratory chain of plant mitochondria. XV. Equilibration of cytochromes c549, b553, b557 and ubiquinone in mung bean mitochondria: Placement of cytochrome b557 and estimation of the midpoint potential of ubiquinone

1. 1. Cycles of oxidation followed by reduction at pH 7.2 have been induced in uncoupled anaerobic mung bean mitochondria treated with succinate and malonate by addition of oxygen-saturated medium. Under the conditions used, cytochromes b₅₅₇, b₅₅₃, c₅₄₉ (corresponding to c₁ in mammalian mitochondria) and ubiquinone are completely oxidized in the aerobic state, but become completely reduced in anaerobiosis.

2. 2. The time course of the transition from fully oxidized to fully reduced in anaerobiosis was measured for cytochromes c₅₄₉, b₅₅₇, and b₅₅₃. The intramitochondrial redox potential (IMP_h) was calculated as a function of time for each of the three cytochromes from the time course of the oxidized-to-reduced transition and the known midpoint potentials of the cytochromes at pH 7.2. The three curves so obtained are superimposable, showing that the three cytochromes are in redox equilibrium under these conditions during the oxidized-to-reduced transition.

3. 3. This result shows that the slow reduction of cytochrome b₅₅₇ under these conditions, heretofore considered anomalous, is merely a consequence of its more negative midpoint potential of +42 mV at pH 7.2, compared to +75 mV for cytochrome b₅₅₃ and +235 mV for cytochrome c₅₄₉. Cytochrome b₅₅₇ is placed on the low potential side of coupling site II and transfers electrons to cytochrome c₅₄₉ via the coupling site.

4. 4. The time course of the transition from fully oxidized to fully reduced was also measured for ubiquinone. Using the change in intramitochondrial potential IMP_h with time obtained from the three cytochromes, the change in redox state of ubiquinone with IMP_h was calculated. When replotted as IMP_h versus the logarithm of the ratio (fraction oxidized)/(fraction reduced), two redox components with n = 2 were found. The major component is ubiquinone with a midpoint potential E_m7.2 = + 70 mV. The minor component has a midpoint potential E_m7.2 = − 12 mV; its nature is unknown.

Photo-accumulation of the P+QB- radical pair state in purple bacterial reaction centres that lack the QA ubiquinone

Photo-excitation of membrane-bound Rhodobacter sphaeroides reaction centres containing the mutation Ala M260 to Trp (AM260W) resulted in the accumulation of a radical pair state involving the photo-oxidised primary electron donor (P). This state had a lifetime of hundreds of milliseconds and its formation was inhibited by stigmatellin. The absence of the Q_A ubiquinone in the AM260W reaction centre suggests that this long-lived radical pair state is P⁺Q_B⁻, although the exact reduction/protonation state of the Q_B quinone remains to be confirmed. The blockage of active branch (A-branch) electron transfer by the AM260W mutation implies that this P⁺Q_B⁻ state is formed by electron transfer along the so-called inactive branch (B-branch) of reaction centre cofactors. We discuss how further mutations may affect the yield of the P⁺Q_B⁻ state, including a double alanine mutation (EL212A/DL213A) that probably has a direct effect on the efficiency of the low yield electron transfer step from the anion of the B-branch bacteriopheophytin (H_B⁻) to the Q_B ubiquinone.     

Trapping, loss and annihilation of excitations in a photosynthetic system: II. Experiments with the purple bacteria Rhodospirillum rubrum and Rhodopseudomonas capsulata

In this paper a number of experiments with the purple bacteria Rhodospirillum rubrum and Rhodopseudomonas capsulata is described in which the total fluorescence yield and/or the total fraction of reaction centers closed after a picosecond laser pulse were measured as a function of the pulse intensity. The conditions were such that the reaction centers were either all in the open or all in the closed state before the pulse arrived. These experiments are analysed using the theoretical formalism discussed in the preceding paper (Den Hollander, W.T.F., Bakker J.G.C., and Van Grondelle, R., Biochim. Biophys. Acta 725, 492–507). From the experimental results the number of connected photosynthetic units, λ, the rate of energy transfer between neighboring antenna molecules, k_h, and the rate of trapping by an open reaction center, k^o_t, can be estimated. For R. rubrum it is found that λ = 14−17, k_h = (1−2)·10¹² s⁻¹ and k^o_t = (4−6)·10¹¹ s⁻¹, for Rps. capsulata λ ≈ 30, k_h ≈ 4·10¹¹ s⁻¹ and k^o_t ≈ 3·10¹¹ s⁻¹. The findings are discussed in terms of current models for the structure of the antenna and the kinetic properties of the decay processes occurring in these purple bacteria.     

Mitochondrial respiratory chain of Tetrahymena pyriformis The properties of submitochondrial particles and the soluble b and c type pigments

Submitochondrial particles isolated from Tetrahymena pyriformis contain essentially the same redox carriers as those present in parental mitochondria: at pH 7.2 and 22 °C there are two b-type pigments with half-reduction potentials of −0.04 and −0.17 V, a c-type cytochrome with a half reduction potential of 0.215 V, and a two-component cytochrome a₂ with E_m7.2 of 0.245 and 0.345 V.

EPR spectra of the aerobic submitochondrial particles in the absence of substrate show the presence of low spin ferric hemes with g values at 3.4 and 3.0, a high spin ferric heme with g = 6, and a g = 2.0 signal characteristic of oxidized copper. In the reduced submitochondrial particles signals of various iron-sulfur centers are observed.

Cytochrome c₅₅₃ is lost from mitochondria during preparation of the submitochondrial particles. The partially purified cytochrome c₅₅₃ is a negatively charged protein at neutral pH with an E_m7.2 of 0.25 V which binds to the cytochrome c-depleted Tetrahymena mitochondria in the amount of 0.5 nmol/mg protein with a K_D of 0.8 · 10⁻⁶ M. Reduced cytochrome c₅₅₃ serves as an efficient substrate in the reaction with its own oxidase. The EPR spectrum of the partially purified cytochrome c₅₅₃ shows the presence of a low spin ferric heme with the dominant resonance signal at g = 3.28.

A pigment with an absorption maximum at 560 nm can be solubilized from the Tetrahymena cells with butanol. This pigments has a molecular weight of approx. 18 000, and E_m7.2 of −0.17 V and exhibits a high spin ferric heme signal at g = 6.     

Glutathione half-cell reduction potential: A universal stress marker and modulator of programmed cell death?

The elucidation of factors that contribute to cell viability loss is presently compromised by the lack of a universal measure that quantifies “stress.” We have investigated mechanisms of viability loss in plant seeds to find a reliable marker of stress response. Oxidative damage has previously been correlated with degenerative processes and death, but how exactly this contributes to viability loss is unknown. We show in four species subjected to ageing or desiccation that seed viability decreased by 50% when the half-cell reduction potential of glutathione (E_GSSG/2GSH), a major cellular antioxidant and redox buffer, increased to −180 to −160 mV. We then conducted a metaanalysis of data representative of 13 plant and fungal orders to show that plant stress generally becomes lethal when E_GSSG/2GSH exceeds −160 mV. We put forward that this change in E_GSSG/2GSH is part of the signaling cascade that initiates programmed cell death (PCD), finally causing internucleosomal DNA fragmentation in the final, or execution phase, of PCD. E_GSSG/2GSH is therefore a universal marker of plant cell viability and allows us to predict whether a seed will live, germinate, and produce a new plant, or if it will die.     

Blue and red shifts of bacteriochlorophyll absorption band around 880 nm in Rhodospirillum rubrum

The redox potential dependence of the light-induced absorption changes of bacteriochlorophyll in chromatophores and subchromatophore pigment-protein complexes from Rhodospirillum rubrum has been examined. The highest values of the absorption changes due to the bleaching of P-870 and the blue shift of P-800 in chromatophores and subchromatophore complexes are observed in the 360–410 mV redox potential range. At potentials below 300 mV (pH 7.0), the 880 nm band of bacteriochlorophyll shifts to shorter wavelengths in subchromatophore complexes and to longer wavelengths in chromatophores.

The data on redox titration show that the red and blue shifts of 880-nm bacteriochlorophyll band represent the action of a non-identified component (C₃₄₀) which has an oxidation-reduction midpoint potential close to 340 mV (n = 1) at pH 6.0–7.6. The E_m of this component varies by 60 mV/pH unit between pH 7.6 and 9.2.

The results suggest that the red shift is due to the transmembrane, and the blue shift to the local intramembrane electrical field. The generation of both the transmembrane and local electrical fields is apparently governed by redox transitions of the component C₃₄₀.     

Light- and chemically-induced oxidation-reduction reactions in chromatophore fractions of Rhodospirillum rubrum

Chromatophore fractions of Rhodospirillum rubrum were prepared by centrifugation of a ‘classical’ chromatophore preparation in a sucrose density gradient. Most of the experiments were carried out with a fraction separating out at about 23% sucrose. Although no light-induced reactions of cytochromes could be detected, the presence of two bound cytochromes, C422 and C428, in this fraction was indicated by oxidation-reduction reactions induced by KIO₄ and dithionite. Cytochrome C422 appeared to be the ‘high-potential’ cytochrome detected earlier in preparations from this organism. Titration of chemically-induced absorbance changes due to a reduction of oxidized cytochrome C428 has led to an estimated redox midpoint potential of E_m7.5 = −0.01 V for this component in a four-electron transport reaction.

Light-induced absorbance changes in the near-infrared spectral region measured in the light chromatophore fraction at different environmental redox potentials suggested a bacteriochlorophyll component, P', with spectral characteristics which were different from those of the high-potential bacteriochlorophyll component P890. The spectral shifts observed upon oxidation, either by light or by air were a bleaching in the 860–880-nm spectral region and an appearance of an absorption band at about 900 nm. The component could be oxidized and reduced reversibly and had an estimated midpoint potential of +0.06 V in a one-electron reaction.     

Quinones as hydrogen carriers for a late step in anaerobic heme biosynthesis in Escherichia coli

A late step in anaerobic heme synthesis, the oxidation of protoporphyrinogen with fumarate as electron acceptor, was studied in extracts and particles of Escherichia coli mutants deficient in quinones or cytochromes. Mutants specifically deficient in menaquinone did not couple protoporphyrinogen oxidation to fumarate reduction, whereas mutants containing menaquinone but deficient in either ubiquinone or cytochromes exhibited this activity. These findings indicate that this coupled reaction is dependent upon menaquinone as hydrogen carrier but independent of ubiquinone and cytochromes. Other characteristics of this coupled reaction were also studied. The activity was located exclusively in the membrane fraction of cell-free extracts. Coproporphyrinogen III could not replace protoporphyrinogen as substrate. Methylene blue, triphenyl tetrazolium and nitrate, but not nitrite, could replace fumarate as anaerobic hydrogen acceptor. These findings have implications for the mechanism and regulation of microbial heme and chlorophyll synthesis and for the physiology of cytochrome synthesis in anaerobic microorganisms.     

Spectral study of b cytochromes in yeast mitochondria and intact cells

Three types of b cytochromes are demonstrated in Candida utilis mitochondria. One of these b cytochromes has a symmetrical -band at 561.5 nm at room temperature. This b cytochrome is readily reduced either by anaerobiosis or by cyanide treatment in the presence of glycerol 1-phosphate or succinate both in coupled and uncoupled mitochondria. The second b cytochrome has a double -band at 565 nm and 558 nm. This b cytochrome is readily reduced either by anaerobiosis or by cyanide treatment in the presence of glycerol 1-phosphate or succinate in coupled mitochondria, but in uncoupled mitochondria it is slowly reduced after anaerobiosis and this reduction rate is enhanced by antimycin A addition. Thus the oxidation-reduction state of this cytochrome is energy dependent. The first cytochrome is spectroscopically identified as cytochrome b_K and the second as cytochrome b_T. The third b cytochrome has an -band around 563 nm (b₅₆₃) and is reduced slowly after anaerobiosis in uncoupled mitochondria but faster than the b_T. Further properties of this component are not known. Midpoint potentials of cytochromes b_T, b₅₆₃ and b_K are approximately −50 mV, +5 mV, and +65 mV, respectively.

In intact cells, cytochrome b_T is reduced immediately after anaerobiosis or cyanide treatment, and rapidly oxidized when uncoupler is added. Addition of antimycin A instead of uncoupler to the anaerobic cells causes oxidation of mainly cytochrome b_T while addition of antimycin A to the aerobic cells results in a reduction of the cytochrome b_T.     

Properties of the reaction center of the thermophilic purple photosynthetic bacterium Chromatium tepidum

Reaction centers were purified from the thermophilic purple sulfur photosynthetic bacterium Chromatium tepidum. The reaction center consists of four polypeptides L, M, H and C, whose apparent molecular masses were determined to be 25, 30, 34 and 44 kDa, respectively, by polyacrylamide gel electrophoresis. The heaviest peptide corresponds to tightly bound cytochrome. The tightly bound cytochrome c contains two types of heme, high-potential c-556 and low-potential c-553. The low-potential heme is able to be photooxidized at 77 K. The reaction center exhibits laser-flash-induced absorption changes and circular dichroism spectra similar to those observed in other purple photosynthetic bacteria. Whole cells contain both ubiquinone and menaquinone. Reaction centers contain only a single active quinone; chemical analysis showed this to be menaquinone. Reaction center complexes without the tightly bound cytochrome were also prepared. The near-infrared pigment absorption bands are red-shifted in reaction centers with cytochrome compared to those without cytochrome.     

Nature of photochemical reactions in chromatophores of Chromatium D. II. Quantum yield of photooxidation of cytochromes in Chromatium chromatophores

Initial rates of the light-induced absorption decrease in Chromatium chromatophores due to the oxidation of cytochromes were measured under various experimental conditions. The initial rate in the presence of 10 mM potassium ferrocyanide and 50 μM potassium ferricyanide was about one-half to two-thirds of that in the presence of 30 mM ascorbate or in a medium with a redox potential (E_h) of − 78 mV.

Light-minus-dark difference spectrum indicated that, in the presence of 10 mM ferrocyanide and 50 μM ferricyanide, only cytochrome c-555 was photooxidized. In the presence of 30 mM ascorbate or at E_h values lower than about 0 mV, both cytochrome c-555 and cytochrome c-552 were photooxidized. The quantum yield of cytochrome c-555 photooxidation was calculated to be about 0.4.

The results obtained in the present study are compared with other investigators' and the possibility of the presence of two types of associations between the cytochromes and reaction-center bacteriochlorophyll is discussed.     

Coulometric and potentiometric evaluation of the redox components of cytochrome c oxidase in situ

A new coulometric-potentiometric titration cuvette is described which permits accurate measurements of oxidation-reduction components in membranous systems. This cuvette has been utilized to measure the properties of cytochrome c oxidase in intact membranes of pigeon breast muscle mitochondria. The reducing equivalents accepted and donated by the portion of the respiratory chain with half-reduction potentials greater than 200 mV are equal to those required for the known components (cytochrome a₃ and the high-potential copper plus cytochrome a, ‘visible copper’, cytochrome c₁, cytochrome c, and the Rieske iron-sulfur protein). Titrations in the presence of CO show that formation of the reduced cytochrome a₃-CO complex requires two reducing equivalents per cytochrome a₃ (coulometric titration). Potentiometric titrations indicate (Lindsay, J.G., Owen, C.S. and Wilson, D.F. (1975) Arch. Biochem. Biophys. 169, 492–505) that both cytochromes a₃ and the high-potential copper must be reduced in order to form the CO complex (n=2.0 with a CO concentration-dependent half-reduction potential, E_m). By contrast, titrations in the presence of azide show that the E_m value of the high-potential copper is unchanged by the presence of azide and thus azide binds with nearly equal affinity whether the copper is reduced or oxidized.     

Activation by reduction of the resting form of cytochrome c oxidase: Tests of different models and evidence for the involvement of CuB

(1) The reaction of the resting form of oxidised cytochrome c oxidase from ox heart with dithionite has been studied in the presence and absence of cyanide. In both cases, cytochrome a reduction in 0.1 M phosphate (pH 7) occurs at a rate of 8.2 · 10⁴ M⁻¹ · s⁻¹. In the absence of cyanide, ferrocytochrome a₃ appears at a rate (k_obs) of 0.016 s⁻¹. Ferricytochrome a₃ maintains its 418 nm Soret maximum until reduced. The rate of a₃ reduction is independent of dithionite concentration over a range 0.9 mM–131 mM. In the presence or cyanide, visible and EPR spectral changes indicate the formation of a ferric a₃/cyanide complex occurs at the same rate as a₃ reduction in the absence of cyanide. A g = 3.6 signal appears at the same time as the decay of a g = 6 signal. No EPR signals which could be attributed to copper in any significant amounts could be detected after dithionite addition, either in the presence or absence of cyanide. (2) Addition of dithionite to cytochrome oxidase at various times following induction of turnover with ascorbate/TMPD, results in a biphasic reduction of cytochrome a₃ with an increasing proportion of the fast phase of reduction occurring after longer turnover times. At the same time, the predominant steady state species of ferri-cytochrome a₃ shifts from high to low spin and the steady-state level of reduction of cytochrome a drops indicating a shift in population of the enzyme molecules to a species with fast turnover. In the final activated form, oxygen is not required for fast internal electron transfer to cytochrome a₃. In addition, oxygen does not induce further electron uptake in samples of resting cytochrome oxidase reduced under anaerobic conditions in the presence of cyanide. Both findings are contrary to predictions of certain O-loop types of mechanism for proton translocation. (3) A measurement of electron entry into the resting form of cytochrome oxidase in the presence of cyanide, using TMPD or cytochrome c under anaerobic conditions, shows that three electrons per oxidase enter below a redox potential of around +200 mV. An initial fast entry of two electrons is followed by a slow (k_obs ≈ 0.02 s) entry of a third electron. Above +200 mV, the number of electrons taken up in the initial fast phase drops as a redox center (presumably Cu_A) titrates with an apparent mid-point potential of +240 mV. The slow phase of reduction remains at the more positive redox values. (4) The results are interpreted in terms of an initial fast reduction of cytochrome a (and Cu_A at redox values more negative than +240 mV) followed by a slow reduction of Cu_B. Cu_B reduction is proposed to spin-uncouple cytochrome a₃ to form a cyanide sensitive center, and trigger a conformational change to an activated form of the enzyme with faster intramolecular electron transfer.     

Folded conformation of an immunostimulating tetrapeptide rigin: high temperature molecular dynamics simulation study

Employing high temperature quenched molecular dynamics (QMD) simulations the conformational energy space of an immunostimulating tetrapeptide rigin: H-Gly³⁴¹-Gln-Pro-Arg³⁴⁴-OH, is explored. Using distance dependent dielectric (=r_ij) 31 different low energy starting structures with identical sequence were computed for their conformational preferences. According to the hypothesis of O'Connors et al. [J. Med. Chem. 35 (1992), 2870], 83 low-energy conformers resulted from unrestrained molecular dynamics (MD) simulations, could be classified into two energy minimized families: A and B, comprised of 64 (Pro C^γ-endo orientation) and 19 (Pro C^γ-exo orientation) structures, respectively. An examination of these families revealed the existence of a remarkably similar folded backbone conformation: torsion angles being φ_i+1 ≈−65°, ψ_i+1 ≈−65°, φ_i+2 ≈−65°, ψ_i+2 ≈−60°, characterizing a distorted type III β-turn structure across the central Gln-Pro segment. The folded conformation of rigin is devoid of a classical 1 ← 4 intra-molecular hydrogen bond nevertheless, the conformation is stabilized by an effective ‘salt-bridge’, i.e., Gly H₃N⁺… COO⁻ Arg interaction. Surprisingly, in both the families the unusual folded side-chain dispositions of the Gln residue favor the formation of a unique intra-residue ‘main-chain to side-chain’ H-bond, i.e., N–H…N^ε interaction, encompassing a seven-membered ring motif. The conformational attributes may be valuable in de novo construction of structure-based drug candidates having sufficient stimulating activity.     

Cytochrome b and photosynthetic sulfur bacteria

Chromatophores isolated from the purple sulfur bacterium Chromatium and the green sulfur bacterium Chlorobium exhibit absorbance changes in the cytochrome -band region consistent with the presence of a b-type cytochrome. Cytochrome content determined by reduced minus oxidized difference spectra and by heme analysis suggests that each bacterium contains one cytochrome b per molecule of photochemically active bacteriochlorophyll (reaction-center bacteriochlorophyll).

The b-type cytochrome in Chromatium has an -band maximum at 560 nm and a midpoint oxidation-reduction potential of −5 mV at pH 8.0. The b-type cytochrome in Chlorobium has an -band maximum at 564 nm and an apparent midpoint oxidation-reduction potential near −90 mV.

Chromatophores isolated from both Chromatium and Chlorobium cells catalyze a photoreduction of cytochrome b that is enhanced in the presence of antimycin A. Antimycin A and 2-n-heptyl-4-hydroxyquinoline-N-oxide inhibit endogenous (but not phenazine methosulfate-mediated) cyclic photophosphorylation in Chromatium chromatophores and non-cyclic electron flow from Na₂S to NADP in Chlorobium chromatophores. These observations suggest that b-type cytochromes may function in electron transport reactions in photosynthetic sulfur bacteria.     

The effect of o-phenanthroline on the midpoint potential of the primary electron acceptor of Photosystem II

The primary electron acceptor of Photosystem II has a midpoint oxidation-reduction potential of +95 mV at pH 7.0 in Photosystem II chloroplast fragments prepared by digitonin treatment. The midpoint potential of the acceptor has a pH dependence of −60 mV/pH unit. At concentrations that inhibit oxygen evolution, o-phenanthroline shifts the midpoint potential of the primary acceptor by +70 mV. The shifted potential retains the same dependence on pH. The effect of o-phenanthroline suggests that it interacts directly with the primary electron acceptor of Photosystem II in a manner similar to that reported previously for the primary electron acceptor in purple photosynthetic bacteria.     

Chloroplast cytochrome b6: Molecular composition as a lipoprotein

Disc electrophoretically homogeneous spinach-chloroplast cytochrome b₆ was found to be a lipoprotein whose redox potential was essentially unchanged during isolation. These results further support the hypothesis of Triton X-100/4 M urea, pH 8, as a useful extracting medium for membrane lipoproteins.

Cytochrome b₆ was found to have a heme equivalent dry weight of 1 mol of heme per 60 000 g. Of this, 20 000 g was lipid-extractable. The molecular weight was 60 000 with a partial specific volume of 0.84 ml/g. The protein portion of the molecule (40 000) consisted of 1 polypeptide chain of 20 000 daltons, 1 of 9600 daltons and 2 of 6600 daltons. A simple lipid composition (relative to the original membrane) was found consisting of 7 mol of chlorophyll a and 6 mol of cardiolipin per mol of cytochrome; these two lipids thus account for about 75–80% of the lipid content. An unidentified minor neutral lipid and minor polar lipid were also detected. At pH 7.0 in the presence of 0.5% Triton X–100, E′₀ was −0.080 V, and in the absence of Triton X–100, E′₀ was −0.120 V. At pH 8 in 0.5% Triton X–100, E′₀ was −0.084 V, thus indicating that the redox potential is independent of pH in the region 7–8. The redox reaction proceeded via a one-electron-transfer.     

Transfer of light-induced electron-spin polarization from the intermediary acceptor to the prereduced primary acceptor in the reaction center of photosynthetic bacteria

In reaction centers and chromatophores of photosynthetic bacteria strong light-induced emissive ESR signals have been found, not only after a flash, but also under continuous illumination. The signal, with g = 2.0048 and ΔH_pp = 7.6 G, is only present under reducing conditions in material in which the primary acceptor, ubiquinone, U and its associated high-spin ferrous ion are magnetically uncoupled. Its amplitude under continuous illumination is strongly dependent on light intensity and on microwave power.

The emissive signal is attributed to the prereduced primary acceptor, U⁻, which becomes polarized through transfer of spin polarization by a magnetic exchange interaction with the photoreduced, spin polarized intermediary acceptor, I⁻. A kinetic model is presented which explains the observed dependence of emissivity on light intensity and microwave power. Applying this analysis to the light saturation data, a value of the exchange rate between I⁻ and U⁻ of 4 · 10⁸ s⁻¹ is derived, corresponding to an exchange interaction of 3–5 G.