Excited states of photosynthetic reaction centers at low recox potentials

Recombination dynamics in bacterial photosynthetic reaction centers

The time dependence of magnetic field effects on light absorption by triplet-state and radical ions in quinone-depleted reaction centers of Rhodopseudomonas sphaeroides strain R-26 has been investigated. Measurements on the time scale of the hyperfine interaction in the radical pair [(BChl)2+. ...BPh-.)] provided kinetic data characterizing the recombination process. The results have been interpreted in terms of a recently proposed model that assumes an intermediate electron acceptor (close site) between the bacteriochlorophyll "special pair" (BChl)2 and the bacteriopheophytin BPh (distant site). Recombination is assumed to proceed through this intermediate acceptor. The experiments led to effective recombination rates for the singlet and triplet channel: k(Seff) = 3.9 . 107 s-1 and k(Teff) = 7.4 . 10(8) s-1. These correspond to recombination rates ks = 1 . 10(1) s-1 and kT = 7.1 . 10(11) s-1 in the close configuration. The upper bound of the effective spin dephasing rate k2eff approximately equal to 1 . 10(9) s-1 is identical with the rate of the electron hopping between the distant site of zero spin exchange interaction and the close site of large interaction. Interpretation of data for the case of direct recombination yields the recombination rates, spin dephasing rate, and exchange interaction in a straightforward way.

Temperature dependence of electron transfer between bacteriopheophytin and ubiquinone in protonated and deuterated reaction centers of Rhodopseudomonas sphaeroides

The rate of the electron-transfer reaction between bacteriopheophytin and the first quinone in isolated reaction centers of Rhodopseudomonas sphaeroides has an unusual temperature dependence. The rate increases about threefold with decreasing temperature between 300 and 25 K, and decreases abruptly at temperatures below 25 K. Partial deuteration of the reaction centers alters the temperature dependence of the rate constant. Qualitative features of the temperature dependence can be understood in the context of a theory of nonadiabatic electron transfer (Sarai, 1980. Biochim. Biophys. Acta 589:71-83). We conclude that very low-energy (10-50 cm-1) processes, perhaps skeletal vibrations of the protein, are important to electron transfer. Higher-energy vibrations, possibly involving the pyrrolic N--H bonds of bacteriopheophytin, also are important in this process.

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.

The effect of temperature and transmembrane potentials on the rates of electron transfer between membrane-bound biological redox components

We have investigated rate data for the temperature and free energy dependence of the primary electron-transfer processes in bacterial photosynthesis. Rather than representing the whole electronic-nuclear coupling by a frequently applied discrete single-mode model, we have incorporated a continuum of modes characterized by a certain distribution function. In this way, we can illuminate the role of both a broad distribution of low-frequency modes representing the medium and a narrow distribution representing local nuclear modes. Furthermore, it emerges from the calculations that both sets are important in the overall scheme of primary photosynthetic electron-transfer processes. By means of this model and quantum-mechanical rate theory, we can reproduce a number of important features of the primary photosynthetic processes concerning in particular the temperature (tunnelling or thermally activated nuclear motion) and free energy dependence ('normal', 'activation-less', or 'inverted' regions) of the rate constants and estimate such parameters as nuclear-reorganization energy electron-exchange integrals and electron-transfer distances. We have finally considered some of the important factors which determine the potential drop across the membrane and estimated the extent to which variations in the potential drop affect the rate constants of the electron-transfer processes.

A possible new mechanism of temperature dependence of electron transfer in photosynthetic systems

A new theory for the electron transfer by the non-adiabatic process is formulated taking into account the origin shift and the frequency change of the vibration. The resultant formulas are quite similar to those of Jortner (Jortner, J. (1976) J. Chem. Phys. 64, 4860-4867) except that the free energy gap delta G is used instead of the energy gap delta E. By applying this theory to the photosynthetic electron transfer, the role of the remarkable temperature dependence of the electron transfer from cytochrome to P+ in Chromatium vinosum and the experimental data were reproduced very well using a small value of the coupling strength in contrast with the previous theory. This implies that proteins play a role to exclude many of the solvent molecules from the region of the electron transfer reaction between the donor and acceptor molecules. The negative activation process in the back electron transfer from QA- to P+, the very slow back electron transfer from I- to P+ and the solvent isotope effect on the cytochrome oxidation are also successfully explained by this new theory. It is shown that even a qualitative conclusion as to the molecular parameters obtained from the temperature dependence of the electron transfer is different between the present theory and that of Jortner.

The balance between primary forward and back reactions in bacterial photosynthesis

The temperature dependence of the bacteriochlorophyll fluorescence and reaction center triplet yield in while cells of Rhodopseudomonas sphaeroides strain 2.4.1 and of the magnetic field-induced fluorescence increase are calculated, taking into account rate constants of losses in the antenna system and of charge separation and recombination in the reaction center. Triplet and singlet yield after recombination in the reaction center are described by the radical pair mechanism. Good fits of the theoretically calculated temperature dependence with published experimental results could be obtained, assuming that ks, the rate constant for recombination of the charges on the primary donor P+ and the reduced intermediate acceptor I- to the lowest excited singlet state P*I of the reaction center bacteriochlorophyll, is temperature-dependent via the Boltzmann factor Kso exp(-delta E/kT), where delta E is the energy difference between P*I and P+I- and kso is the frequency factor. kg and/or kt, the rate constants for recombination to the singlet ground and triplet states, respectively, were assumed to be temperature-independent, or temperature-dependent via their exothermicity factors ki = CiT-1/2 exp(-Ei/kT) with i = g, t. Depending on the particular choice for the temperature dependence of kg and kt, best fits were obtained for delta E = 45-75 meV and recombination rate constants at 300 K of ks = 0.4-0.8 ns-1, kg = 0.08-0.12 ns-1, and kt = 0.3-0.5 ns-1. The model predicts a lifetime of the radical pair P+I- that is somewhat larger than that of delayed fluorescence; a magnetic field increases both.

Transient states in reaction centers containing reduced bacteriopheophytin

Photosynthetic reaction centers isolated from Rhodopseudomonas sphaeroides strain R-26 were excited with non-saturating 7-ps, 600-nm flashes under various conditions, and the resulting absorbance changes were measured. If the quinone electron acceptor (Q) is in the oxidized state, flash excitation generates a transient state (PF), in which an electron has moved from the primary electron donor (P, a dimer of bacteriochlorophylls) to an acceptor complex involving a special bacteriopheophytin (H) and another bacteriochlorophyll (B). PF decays in 200 ps as an electron moves from H to Q and the acceptor complex are reduced photochemically before the excitation, the flash generates a different transient state of P with a high quantum yield. This state decays with a lifetime of 340 ps. There is no indication of electron transfer from P to B under these conditions, but this does not rule out the possibility that B is an intermediate electron carrier between P and H. Measurements of the yield of fluorescence from P under various conditions show that the 340 ps state is not the fluorescent excited singlet state of P. The transient state could be a triplet state, a charge-transfer state of P, or another excited singlet state that is not fluorescent.

Reaction center triplet states in photosystem I and photosystem II

A photosystem I (PS I) particle has been prepared by lithium dodecyl sulfate digestion which lacks the acceptor X, and iron-sulfur centers B and A. Illumination of these particles at liquid helium temperature results in the appearance of a light-induced spin-polarized triplet signal observed by EPR. This signal is attributed to the triplet state of P-700, the primary donor, formed by recombination of the light induced radical pair P-700+ A1- (where A1 is the intermediate acceptor). Formation of the triplet does not occur if P-700 is oxidized or if A1 is reduced, prior to the illumination. A comparison of the P-700 triplet with that of P-680, the primary donor of Photosystem II, shows several differences. (1) The P-680 triplet is 1.5 mT (15 G) wider than the P-700 triplet. This is reflected by the zero-field splitting parameters, which indicate that P-700 is a slightly larger species than P-680. The zero-field splitting parameters do not indicate that either P-700 or P-680 are dimeric. (2) The P-700 triplet is induced by red and far-red light, while the P-680 triplet is induced only by red light. (3) The temperature dependences of the P-700 triplet and the P-680 triplet are different.

The primary charge separation, cytochrome oxidation and triplet formation in preparations from the green photosynthetic bacterium Prosthecochloris aestuarii

Flash-induced absorbance changes were measured in intact cells and subcellular preparations of the green photosynthetic bacterium Prosthecochloris aestuarii. In Complex I, a membrane vesicle preparation, photooxidation of the primary electron donor, P-840, and of cytochrome c-553 was observed. Flash excitation of the photosystem pigment complex caused in addition the generation of a bacteriochlorophyll a triplet. Triplet formation was the only reaction observed after flash excitation in the reaction center pigment-protein complex. The triplet had a lifetime of 90 microseconds at 295 K and of 165 microseconds at 120 K. The amount of triplet formed in a flash increased upon cooling from 295 to 120 K from 0.2 and 0.5 per reaction center to 0.45 and nearly 1 per reaction center in the photosystem pigment and reaction center pigment-protein complex, respectively. Measurements of absorbance changes in the near infrared in the reaction center pigment-protein complex indicate that the triplet is formed in the reaction center and that the reaction center bacteriochlorophyll a triplet is that of P-840. Formation of a carotenoid triplet did not occur in our preparations. Illumination with continuous light at 295 K of the reaction center pigment-protein complex produced a stable charge separation (with oxidation of P-840 and cytochrome c-553) in each reaction center, but with a low efficiency. This low efficiency, and the high yield of triplet formation is probably due to damage of the electron transport chain at the acceptor side of the reaction center of the reaction center pigment-protein complex. The halftime for cytochrome c-553 oxidation in Complex I and the photosystem pigment complex was 90 microseconds at 295 K; below 220 K no cytochrome oxidation occurred. At 120 K P-840+ was rereduced with a halftime of 20 ms, presumably by a back reaction with a reduced acceptor.

Energy transfer between the carotenoid and the bacteriochlorophyll within the B-800-850 light-harvesting pigment-protein complex of Rhodopseudomonas sphaeroides

Energy transfer between carotenoid and bacteriochlorophyll has been studied in isolated B-800-850 antenna pigment-protein complexes from different strains of Rhodopseudomonas sphaeroides which contain different types of carotenoid. Singlet-singlet energy transfer from the carotenoid to the bacteriochlorophyll is efficient (75-100%) and is rather insensitive to carotenoid type, over the range of carotenoids tested. The yield of carotenoid triplets is low (2-15%) but this arises from a low yield of bacteriochlorophyll triplet formation rather than from an inefficient triplet-triplet exchange reaction. The rate of the triplet-triplet exchange reaction between the bacteriochlorophyll and the carotenoid is fast (Ktt greater than or equal to 1.4 . 10(8) S-1) and also relatively independent of the type of carotenoid present.

Subpicosecond and picosecond studies of electron transfer intermediates in Rhodopseudomonas sphaeroides reaction centers

The primary electron transfer processes in isolated reaction centers of Rhodopseudomonas sphaeroides have been investigated with subpicosecond and picosecond spectroscopic techniques. Spectra and kinetics of the absorbance changes following excitation with 0.7-ps 610-nm pulses, absorbed predominantly by bacteriochlorophyll (BChl), indicate that the radical pair state P+BPh-, in which an electron has been transferred from the BChl dimer (P) to a bacteriopheophytin (BPh), is formed with a time constant no greater than 4 ps. The initial absorbance changes also reveal an earlier state, which could be an excited singlet state, or a P+BChl- radical pair. The bleaching at 870 nm produced by 7 ps excitation at 530 nm (absorbed by BPh) or at 600 nm (absorbed predominantly by BChl) shows no resolvable delay with respect to standard compounds in solution, suggesting that the time for energy transfer from BPh to P is less than 7 ps. However, the bleaching in the BPh band at 545 nm following 7-ps 600-nm excitation, exhibits an 8- to 10-ps lag with respect to standard compounds. This finding is qualitatively similar to the 35-ps delay previously observed at 760 nm by Shuvalov at al. (Shuvalov, V.A., Klevanik, A.V., Sharkov, A.V., Matveetz, Y.A. and Kryukov, P.G. (1978) FEBS Lett. 91, 135-139) when 25-ps 880-nm excitation flashes were used. A delay in the bleaching approximately equal to the width of the excitation flash can be explained in terms of the opposing effects of bleaching due to the reduction of BPh, and absorbance increases due to short-lived excited states (probably of BChl) that turn over rapidly during the flash. The decay of the initial bleaching at 800 nm produced by 7-ps 530- or 600-nm excitation flashes shows a fast component with a 30-ps time constant, in addition to a slower component having the 200-ps kinetics expected for the decay of P+BPh-. the dependence on excitation intensity of the absorbance changes due to the 30-p]s component indicate that the quantum yield of the state responsible for this step is lower than that observed for the primary electron transfer reactions. This suggests that at least part of the transient bleaching at 800 nm is due to a secondary process, possibly caused by excitation with an excessive number of photons. If the 800-nm absorbing BChl (B) acts as an intermediate electron carrier in the primary photochemical reaction, electron transfer between B and the BPh must have a time constant no greater than 4 ps.

Magnetic field affects the fluorescence yield in reaction center preparations from Rhodopseudomonas spaeroides R-26

Purified photochemical reaction centers from Rhodopseudomonas sphaeroides R-26 were reduced with Na2S2O4 so as to block their photochemical electron-transfer reaction. The magnetic field induced an increase in the emission yield. Our results support the hypothesis that under these conditions, charge recombination in the singlet radical pair composed of the oxidized primary donor and reduced primary acceptor predominantly generates the excited singlet state of the reaction cnter bacteriochlorophyll. The maximum relative fluorescence change and the value of the magnetic field at which half-saturation of the effect is achieved (B1/2) at room temperature are 5.5% and 75 G, respectively. For the whole cells of Rps. sphaeroides R-26 these parameters are 1.2% and 120G. The relative fluorescence change at 600 G, deltaF/F(600), and B1/2 are studied as functions of temperature. The temperature dependencies of deltaF/F(600) for reaction centers and whole cells of Rps. sphaeroides R-26 are qualitatively the same, with the maximum effect (8% for reaction centers) occurring at 230 K. However, the B1/2 curves for the two preparations are different.

Carotenoid triplet yields in normal and deuterated Rhodospirillum rubrum

Quantum yields of carotenoid triplet formation in Rhodospirillum rubrum wild type and fully deuterated cells and chromatophores were determined in weak laser flashes for excitation wavelength lambda i = 530 nm (mainly absorbed by the carotenoid spirilloxanthin) and for lambda i = 608 nm (mainly absorbed by bacteriochlorophyll) in the presence and absence of magnetic fields. All experiments were performed at room temperature and in the absence of oxygen. The quantum yield of reaction center bacteriochlorophyll oxidation in wild type preparations, in which all reaction centers are in state PIX, at lambda i = 608 nm is close to unity, whereas the quantum yield of antenna carotenoid triplet formation is low (about 5%); P is the primary electron donor, a bacteriochlorophyll dimer, I the primary acceptor, a bacteriopheophytin, and X the secondary acceptor, an iron-ubiquinone complex. In cells in which the reaction centers are in the state P+IX(-), the antenna carotenoid triplet yield is about 0.2. In contrast, at lambda i = 530 nm, the quantum yield of P+ formation is relatively low (0.3) and the yield of the antenna carotenoid triplet state in state PIX unusually high (0.3). At increasing light intensities of 530 nm only about 3 carotenoids per reaction center of the 15 carotenoids present are efficiently photoconverted into the triplet state, which indicates that there are two different pools of carotenoids, one with a low efficiency for transfer of electronic excitation to bacteriochlorophyll and a high yield for triplet formation, the other with a high transfer efficiency and a low triplet yield. The absorption difference spectrum of the antenna carotenoid triplet, excited by 608 or 530 nm light in the state P+IX(-) does not show the peak at 430 nm, that is present in the difference spectrum of the reaction center carotenoid triplet, mainly observed at lambda i = 608 nm with weak flashes. The yield of the reaction center carotenoid triplet, generated in chromatophores in the state PIX(-), is decreased by about 10% by a magnetic field of 0.6 T. In a magnetic field of 0.6 T the yield of the carotenoid triplet, formed by 530 nm excitation in chromatophores at ambient redox potential, is decreased by about 45%. The high quantum yield of formation and the pronounced magnetic field effect for the carotenoid triplet generated by direct excitation at 530 nm can be explained by assuming that this triplet is not formed by intersystem crossing, but by fission of the singlet excitation into two triplet excitations and subsequent annihilation (triplet pair mechanism), or by charge separation and subsequent recombination (radical pair mechanism). Fully deuterated bacteria give essentially the same triplet yields, both in the reaction center and in the antenna carotenoids and show the same magnetic field effects as non-deuterated samples. This indicates that hyperfine interactions do not play a major role in the dephasing of the spins in the radical pair P+I- nor in the formation of the antenna carotenoid triplet.

Identification of the carotenoid present in the B-800-850 antenna complex from Rhodopseudomonas capsulata as that which responds electrochromically to transmembrane electric fields

Mild proteolysis of Rhodopseudomonas capsulata chromatophores results in a parallel loss of the 800 nm bacteriochlorophyll absorption band a blue shift in the carotenoid absorption bands associated with the B-800-850 light-harvesting complex. Both the light-induced and the salt-induced electrochromic carotenoid band shift disappear in parallel to the loss of the 800 nm bacteriochlorphyll absorption upon pronase treatment of chromatophores. During the time required for the loss of the 800 nm bacteriochlorophyll absorption and the loss of the electrochromic cartenoid band shift photochemistry is not inhibited and the ionic conductance of the membrane remains very low. We conclude that the carotenoid associated with the B-800-850 light-harvesting complex is the one that responds electrochromically to the transmembrane electric field. Analysis of the pigment content of Rps. capsulata chromatophores indicates that all of the carotenoid may be accounted for in the well defined pigment-protein complexes.

Possible role of protein in photosynthetic electron transfer

Photosynthetic electron transfer is discussed from a theoretical viewpoint. Theoretical description of electron transfer including the effect of low-frequency mode of protein is first discussed briefly. Then typical electron transfers in the primary photosynthesis are discussed as examples for the comparison between the theory and experiments. Attention is focussed on the fact that the photosynthetic system organizes a variety of electron transfers in a systematic manner to achieve a vary efficient energy conversion, and it is suggested that the protein environment plays an important role in controlling the rate of electron transfer and maximizing the efficiency of the primary reaction. We emphasize that not only the electronic but also vibrational interactions are important for the regulation of electron transfer. Some novel processes such as activationless and negative activation transfers are shown to be connected with the significance of vibrational factor.

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 delta Hpp = 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 4.10(8) s-1 is derived, corresponding to an exchange interaction of 3--5 G.

Spectroscopic and kinetic properties of the transient intermediate acceptor in reaction centers of Rhodopseudomonas sphaeroides

The photoreductive trapping of the transient, intermediate acceptor, I-, in purified reaction centers of Rhodopseudomonas sphaeroides R-26 was investigated for different external conditions. The optical spectrum of I- was found to be similar to that reported for other systems by Shuvalov and Klimov ((1976) Biochim. Biophys. Acta 400, 587--599) and Tiede et al. (P.M. Tiede, R.C. Prince, G.H. Reed and P.L. Dutton (1976) FEBS Lett. 65, 301--304). The optical changes of I- showed characteristics of both bacteriopheophytin (e.g. bleaching at 762, 542 nm and red shift at 400 nm) and bacteriochlorophyll (bleaching at 802 and 590 nm). Two types of EPR signals of I- were observed: one was a narrow singlet at g = 2.0035, deltaH = 13.5 G, the other a doublet with a splitting of 60 G centered around g = 2.00, which was only seen after short illumination times in reaction centers reconstituted with menaquinone. The optical and EPR kinetics of I- on illumination in the presence of reduced cytochrome c and dithionite strongly support the following three-step scheme in which the doublet EPR signal is due to the unstable state DI-Q-Fe2+ and the singlet EPR signal is due to DI-Q2-Fe2+. : formula: (see text), where D is the primary donor (BChl)2+. The above model was supported by the following observations: (1) During the first illumination, sigmoidal kinetics of the formation of I- was observed. This is a direct consequence of the three-sequential reactions. (2) During the second and subsequent illuminations first-order (exponential) kinetics were observed for the formation of I-. This is due to the dark decay, k4, to the state DIQ2-Fe2+ formed after the first illumination. (3) Removal of the quinone resulted in first-order kinetics. In this case, only the first step, k1, is operative. (4) The observation of the doublet signal in reaction centers containing menaquinone but not ubiquinone is explained by the longer lifetime of the doublet species I-(Q-Fe2%) in reaction centers containing menaquinone. The value of tau2 was determined from kinetic measurements to be 0.01 s for ubiquinone and 4 s for menaquinone (T = 20 degrees C). The temperature and pH dependence of the dark electron transfer reaction I-(Q-Fe2+) yields I(Q2-Fe2+) was studied in detail. The activation energy for this process was found to be 0.42 eV for reaction centers containing ubiquinone and 0.67 eV for reaction centers with menaquinone. The activation energy and the doublet splitting were used to calculate the rate of electron transfer from I- to MQ-Fe2+ using Hopfield's theory for thermally activated electron tunneling. The calculated rate agrees well with the experimentally determined rate which provides support for electron tunneling as the mechanism for electron transfer in this reaction. Using the EPR doublet splitting and the activation energy for electron transfer, the tunneling matrix element was calculated to be 10(-3) eV. From this value the distance between I- and MQ- was estimated to be 7.5--10 A.

The involvement of iron and ubiquinone in electron transfer reactions mediated by reaction centers from photosynthetic bacteria

Reaction centers from Rhodopseudomonas sphaeroides strain R-26 were prepared with varying Fe and ubiquinone (Q) contents. The photooxidation of P-870 to P-870+ was found to occur with the same quantum yield in Fe-depleted reaction centers as in control samples. The kinetics of electron transfer from the initial electron acceptor (I) to Q also were unchanged upon Fe removal. We conclude that Fe has no measurable role in the primary photochemical reaction. The extent of secondary reaction from the first quinone acceptor (QA) to the second quinone acceptor (QB) was monitored by the decay kinetics of P-870+ after excitation of reaction centers with single flashes in the absence of electron donors, and by the amount of P-870 photooxidation that occurred on the second flash in the presence of electron donors. In reaction centers with nearly one iron and between 1 and 2 ubiquinones per reaction center, the amount of secondary electron transfer is proportional to the ubiquinone content above one per reaction center. In reaction centers treated with LiClO4 and o-phenanthroline to remove Fe, the amount of secondary reaction is decreased and is proportional to Fe content. Fe seems to be required for the secondary reaction. In reaction centers depleted of Fe by treatment with SDS and EDTA, the correlation between Fe content and secondary activity is not as good as that found using LiClO4. This is probably due in part to a loss of primary photochemical activity in samples treated with SDS; but the correlation is still not perfect after correction for this effect. The nature of the back reaction between P-870+ and Q-B was investigated using stopped flow techniques. Reaction centers in the P-870+ Q-B state decay with a 1-s half-time in both the presence and absence of o-phenanthroline, an inhibitor of electron transfer between Q-B and QB. This indicates that the back reaction between P-870+ and Q-A is direct, rather than proceeding via thermal repopulation of Q-A. The P-870+ Q-B state is calculated to lie at least 100 mV in free energy below the P-870+ Q-A state.

Electron acceptors associated with P-700 in Triton solubilized photosystem I particles from spinach chloroplasts

Flash-induced absorption changes of Triton-solubilized Photosystem I particles from spinach were studied under reducing and/or illumination conditions that serve to alter the state of bound electron acceptors. By monitoring the decay of P-700 following each of a train of flashes, we found that P-430 or components resembling it can hold 2 equivalents of electrons transferred upon successive illuminations. This requires the presence of a good electron donor, reduced phenazine methosulfate or neutral red, otherwise the back reaction of P-700+ with P-430 occurs in about 30 ms. If the two P-430 sites, designated Centers A and B, are first reduced by preilluminating flashes or chemically by dithionite under anaerobic conditions, then subsequent laser flashes generate a 250 microseconds back reaction of P-700+, which we associate with a more primary electron acceptor A2. In turn, when A2 is reduced by background (continuous) illumination in presence of neutral red and under strongly reducing conditions, laser flashes then produce a much faster (3 microseconds) back reaction at wavelengths characteristic of P-700. We associate this with another more primary electron acceptor, A1, which functions very close to P-700. The organization of these components probably corresponds to the sequence P-700-A1-A2-P-430[AB]. The relation of the optical components to acceptor species detected by EPR, by electron-spin polarization or in terms of peptide components of Photosystem I is discussed. Preliminary experiments with broken chloroplasts suggest that an analogous situation occurs there, as well.

Flash-induced changes in the in vivo bacteriochlorophyll fluorescence yield at low temperatures and low redox potentials in carotenoid-containing strains of photosynthetic bacteria

The changes in the in vivo bacteriochlorophyll fluorescence induced by a Xenon flash at low temperatures (77--200 K) with the "primary" acceptor X chemically prereduced have been examined in whole cells of several species of photosynthetic bacteria which contain carotenoids absorbing in the visible part of the absorption spectrum. Two groups of species with different behaviour could be distinguished. In both cases a flash-induced rise of the fluorescence yield was observed with X prereduced at 77 k; as the temperature was increased the ratio of the maximum fluorescence (FM) and the basal fluorescence (F0) decreased and the kinetics of the decay of the high fluorescent state, as observed during the tail of the flash, apparently accelerated. Of the species examined the flash-induced changes in fluorescence-yield kinetics appeared to occur at higher temperatures in the members of one group (Chromatium vinosum, Rhodopseudomonas gelatinosa and Rhodopseudomonas palustris) than in the members of the other (Rhodopseudomonas palustris) than in the members of the other (Rhodopseudomonas sphaeroides and Rhodospirillum rubrum). These effects are interpreted in terms of the light-induced generation of triplet states within the reaction centre. It is suggested that the species-dependent differences may reflect differences in the molecular organisation of the reaction centre. It was found that in all species the reaction centre carotenoid triplet does not act as a fluorescence quencher under these conditions.

Electron spin resonance in zero magnetic field of the reaction center triplet of photosynthetic bacteria

The decay rates kx, ky, kz of the individual spin levels of the light-induced triplet state have been accurately measured by the zero-field resonance technique under conditions of very low light intensity and a microwave sweep rate of 2.5 MHz/microseconds, which is in excess of that commonly used in optical detection magnetic resonance experiments. The rates ku found correspond well with those previously determined under somewhat different conditions (Hoff, A.J. (1976) Biochim. Biophys. Acta 440, 765--771) and with those inferred from the decay at 4.2 degrees K of the triplet-triplet absorption after picosecond excitation (Parson, W.W. and Monger, T.G. (1977) Brookhaven Symp. Biology 28, 195--212). Thus there seems no reason to doubt that PR corresponds to the triplet state detected by ESR. In a recent publication Clarke and Connors (Clarke, R.H. and Conners, R.E. (1976) Chem. Phys. Lett. 42, 69--72) published values of the rates ku which differ substantially from ours and which lead to a mean lifetime in excess of that of PR. We show that erroneous rates are obtained when the microwave sweep rate is not made fast relative to the decay of the individual spin levels. Zero-field splitting parameters for a member of photosynthetic bacteria have been measured with an accuracy of better than 0.4% for D and 1% for E. The enhanced precision as compared to conventional ESR allows one to discriminate between species of one family. Deuteration reduces the ku values by a factor of about 2, with little spin selectivity. This effect is much larger than previously observed for chlorophyll a. The present results explain the decrease in fluorescence intensity observed on microwave saturation in zero-field optical detection magnetic resonance experiments, and they also show that the simple exciton model is inadequate to derive the geometry of the reaction center dimer from the observed zerofield splitting and decay rates.

Bacteriochlorophyll fluorescence of purple bacteria at low redox potentials. The relationship between reaction center triplet yield and the emission yield

This work describes fluorescence yield measurements in suspensions of strains of Rhodospirillum rubrum and Rhodopseudomonas sphaeroides in which the iron . quinone complex (X) was chemically reduced (state [PIX-]; P is the reaction center bacteriochlorophyll dimer, I is the long wavelength bacteriopheophytin), and compares these with the fluorescence observed when all the traps are open (state [PIX]) and with the fluorescence observed when all the traps are closed (state [P+IX]). At 77 K the amplitude and the shape of the fluorescence emission spectrum in [PIX-] are identical to those observed in [PIX]. This is a strong indication that all the extra fluorescence observed at room temperature in [PIX-] is, in fact, caused by an efficient back reaction [P+I-X-] leads to [P*IX-]. Using an equation similar to the original Vredenberg-Duysens relationship (Vredenburg, W.J. and Duysens, L.N.M. (1963) Nature 197, 355-357) but now assuming that a single reaction center has a probability pt of trapping an excitation and (1--pt) of re-emitting it to the surroundings, we are able to calculate pt as a function of the temperature by measuring the fluorescence in [PIX], [PIX-] and [P+IX] as a function of the temperature. The calculated pt values agree reasonably well with triplet yields measured in isolated reaction centers. Finally, we have measured the reaction center triplet yield (PTR) in intact systems and we have shown that the sum of the triplet yield and the remaining loss processes (PL) in the antenna bacteriochlorophyll including the bacteriochlorophyll dimer (such as fluorescence, internal conversion or direct triplet formation) is approximately constant; if we assume that at 77 K the only process which occurs in the reaction center is the formation of a reaction center triplet, than PTR + PL=1. The energy barrier between [P*IX-] and [P+I-X-] was estimated to be 0.11--0.15 eV for a set of preparations.

Electron transfer and spin exchange contributing to the magnetic field dependence of the primary photochemical reaction of bacterial photosynthesis

The yield phiT of triplet products "PR" generated in reaction centers of Rhodopseudomonas sphaeroides in which the "primary" acceptor is reduced had been found to depend on external magnetic fields. The magnetic field dependence varies, however, between different reaction center preparations. By means of a theoretical description of the primary electron transfer processes and hyperfine coupling-induced electron spin motion the factors influencing the magnetic field behaviour of the triplet products are studied. The following quantities characteristic of the primary electron transfer in photosynthesis have a strong effect on phiT: (1) the rate constants of reversible electron transfer between the initially excited singlet state of the reaction center and an intermediate radical ion pair state; (2) the rate constants of irreversible electron transfer of the radical pair to the ground and excited triplet state of the reaction center; (3) the electron exchange interactions between the radical pair and the "primary" acceptor. From the observed magnetic field dependence of phiT estimates for these quantities are obtained. A temperature dependence of the magnetic field behaviour of phiT and a magnetic field effect on the fluorescence quantum yield of the reaction center are predicted.

Formation and development of photosynthetic units in repigmenting Rhodopseudomonas sphaeroides wild type and "Phofil" mutant strain

The formation of the photosynthetic apparatus in the wild type Rhodopseudomonas sphaeroides and in the "Phofil" mutant was investigated by analyzing absorption and fluorescence parameters and the kinetics of fluorescence induction. Repigmentation was induced by transfer of bleached cells to semi-aerobic culture conditions. 1. In the "Phofil" mutant, functional photosynthetic units appear at pigment cellular contents which depend on the physiological state of the inoculum. The unadapted mutant can reach the functional units stage at a cellular pigment level 20 times lower than that of the wild type, although the size and composition of the units are identical in both strains. This rules out the hypothesis of photosynthetic units forming by random integration of pigments into the membrane. Moreover, the fact that units are separate at this stage (as shown by the exponential shape of fluorescence induction kinetics) suggests that the units' formation proceeds from discrete predetermined membrane sites. 2. In repigmentng wild type cells, the multistep assembly of bacteriochlorophyll complexes appears correlated to their organization in photosynthetic units as follows: (i) During a first stage, B-875 is mostly synthesized, while the efficiency of transfer increases, suggesting that the pigments are initially interpersed at comparatively large average distances from each other in the bleached membrane. (ii) After 1.5 h of repigmentation, the transfer and trapping efficiencies reach a maximum. The units (26 B-875 + 11 B-850 per center) are still separate, as shown by exponential fluorescence kinetics. (iii) The increase in the units' size then essentially proceeds through B-850 incorporation. Energy transfer between units occurs at a comparatively late stage, i.e., 5 h repigmentation.