Excitation trapping and primary charge stabilization in Rhodopseudomonas viridis cells, measured electrically with picosecond resolution

Lel A. Drachev and the Direct Electrometric Method

In the bioenergetics studies, the direct electrometric method played an important role. This method is based on measuring the electrical potential difference (Δψ) between two compartments of the experimental cell generated by some membrane proteins. These proteins are incorporated into closed lipid-protein membrane vesicles associated with an artificial lipid membrane that separates the compartments. The very existence of such proteins able to generate Δψ was one of the consequences of Peter Mitchell's chemiosmotic concept. The discovery and investigation of their functioning contributed to the recognition of this concept and, eventually the well-deserved awarding of the Nobel Prize to P. Mitchell. Lel A. Drachev (1926-2022) was one of the main authors of the direct electrometrical method. With his participation, key studies were carried out on the electrogenesis of photosynthetic and respiratory membrane proteins, including bacteriorhodopsin, visual rhodopsin, photosynthetic bacterial reaction centers, cytochrome oxidase and others.

Transmembrane charge transfer in photosynthetic reaction centers: some similarities and distinctions

This mini review presents a general comparison of structural and functional peculiarities of three types of photosynthetic reaction centers (RCs)--photosystem (PS) II, RC from purple bacteria (bRC) and PS I. The nature and mechanisms of the primary electron transfer reactions, as well as specific features of the charge transfer reactions at the donor and acceptor sides of RCs are considered. Comparison of photosynthetic RCs shows general similarity between the core central parts of all three types, between the acceptor sides of bRC and PS II, and between the donor sides of bRC and PS I. In the latter case, the similarity covers thermodynamic, kinetic and dielectric properties, which determine the resemblance of mechanisms of electrogenic reduction of the photooxidized primary donors. Significant distinctions between the donor and acceptor sides of PS I and PS II are also discussed. The results recently obtained in our laboratory indicate in favor of the following sequence of the primary and secondary electron transfer reactions: in PS II (bRC): Р(680)(Р(870)) → Chl(D1)(В(А)) → Phe(bPhe) → Q(A); and in PS I: Р(700) → А(0А)/A(0B) → Q(A)/Q(B).

Electrogenic reactions and dielectric properties of photosystem II

This review is focused on the mechanism of photovoltage generation involving the photosystem II turnover. This large integral membrane enzyme catalyzes the light-driven oxidation of water and reduction of plastoquinone. The data discussed in this work show that there are four main electrogenic steps in native complexes: (i) light-induced charge separation between special pair chlorophylls P(680) and primary quinone acceptor Q(A); (ii) P(680)(+) reduction by the redox-active tyrosine Y(Z) of polypeptide D1; (iii) oxidation of Mn cluster by Y(Z)(ox) followed by proton release, and (iv) protonation of double reduced secondary quinone acceptor Q(B). The electrogenicity related to (i) proton-coupled electron transfer between Q(A)(-) and preoxidized non-heme iron (Fe(3+)) in native and (ii) electron transfer between protein-water boundary and Y(Z)(ox) in the presence of redox-dye(s) in Mn-depleted samples, respectively, were also considered. Evaluation of the dielectric properties using the electrometric data and the polarity profiles of reaction center from purple bacteria Blastochloris viridis and photosystem II are presented. The knowledge of the profile of dielectric permittivity along the photosynthetic reaction center is important for understanding of the mechanism of electron transfer between redox cofactors.

Dielectric and photoelectric properties of photosynthetic reaction centers

A brief review of studies of dielectric and photoelectric properties of photosynthetic reaction centers of purple bacteria as well as photosystem I and photosystem II of cyanobacteria and higher plants is given. A simple kinetic model of the primary processes of electron transfer in photosynthesis is used to discuss possible mechanisms of correlation between rate constant of charge transfer reaction, free energy of electron transition, and effective dielectric constant in the locus of corresponding carriers.

Escape probability and trapping mechanism in purple bacteria: revisited

Despite intensive research for decades, the trapping mechanism in the core complex of purple bacteria is still under discussion. In this article, it is attempted to derive a conceptionally simple model that is consistent with all basic experimental observations and that allows definite conclusions on the trapping mechanism. Some experimental data reported in the literature are conflicting or incomplete. Therefore we repeated two already published experiments like the time-resolved fluorescence decay in LH1-only purple bacteria Rhodospirillum rubrum and Rhodopseudomonas viridis chromatophores with open and closed (Q(A)(-)) reaction centers. Furthermore, we measured fluorescence excitation spectra for both species under the two redox-conditions. These data, all measured at room temperature, were analyzed by a target analysis based on a three-state model (antenna, primary donor, and radical pair). All states were allowed to react reversibly and their decay channels were taken into consideration. This leads to seven rate constants to be determined. It turns out that a unique set of numerical values of these rate constants can be found, when further experimental constraints are met simultaneously, i.e. the ratio of the fluorescence yields in the open and closed (Q(A)(-)) states F(m)/F(o) approximately 2 and the P(+)H(-)-recombination kinetics of 3-6 ns. The model allows to define and to quantify escape probabilities and the transfer equilibrium. We conclude that trapping in LH1-only purple bacteria is largely transfer-to-the-trap-limited. Furthermore, the model predicts properties of the reaction center (RC) in its native LH1-environment. Within the framework of our model, the predicted P(+)H(-)-recombination kinetics are nearly indistinguishable for a hypothetically isolated RC and an antenna-RC complex, which is in contrast to published experimental data for physically isolated RCs. Therefore RC preparations may display modified kinetic properties.

Light gradients in spherical photosynthetic vesicles

Light-gradient photovoltage measurements were performed on EDTA-treated thylakoids and on osmotically swollen thylakoids (blebs), both of spherical symmetry but of different sizes. In the case of EDTA vesicles, a negative polarity (due to the normal light gradient) was observed in the blue range of the absorption spectrum, and a positive polarity, corresponding to an inverse light gradient, was observed at lambda = 530 and lambda = 682 nm. The sign of the photovoltage polarity measured in large blebs (swollen thylakoids) is the same as that obtained for whole chloroplasts, although differences in the amplitudes are observed. An approach based on the use of polar coordinates was adapted for a theoretical description of these membrane systems of spherical symmetry. The light intensity distribution and the photovoltage in such systems were calculated. Fits to the photovoltage amplitudes, measured as a function of light wavelength, made it possible to derive the values of the dielectric constant of the protein, epsilons = 3, and the refractive index of the photosynthetic membrane for light propagating perpendicular and parallel to the membrane surface, nt = 1.42 and nn = 1.60, respectively. The latter two values determine the birefringence of the biological membrane, Deltan = nn - nt = 0.18.

Evidence that serine L223 is involved in the proton transfer pathway to QB in the photosynthetic reaction center of Rhodopseudomonas viridis

In the reaction center of purple photosynthetic bacteria, the reducing equivalents produced by primary charge separation are exported via an ubiquinone molecule working as a two-electron shuttle. This loosely-bound quinone, called QB, accepts in successive flashes two electrons from the tightly bound primary quinone acceptor QA, along with two protons from the external medium. The surrounding protein plays an important role in stabilizing the semiquinone anion and in providing a pathway for protons from the cytoplasmic phase to QB. Herbicides of the triazine type compete with QB for the binding pocket and their binding is controlled by nearby amino acid residues. We have studied the kinetics of the first and second electron transfer from QA to QB in two herbicide-resistant mutants from Rhodopseudomonas viridis, T1 (ArgL217-->His,Ser L223-->Ala) and MAV5 (Arg L217-->His, Val L220-->Leu), in order to determine whether these residues are involved in proton transfer to the reduced QB. The main effect of the mutant T1 was a drastic (600-fold at pH 7) decrease in the rate of the second electron transfer to QB compared to the wild type. In contrast, the rate of the second electron transfer in the mutant MAV5 was decreased only slightly (10-fold) in the pH range from 7 to 11. We attribute the inhibition of the second electron transfer in the Ser L223-->Ala mutation to an essential role of Ser L223 in the donation of the first proton to the reduced QB.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-wavelength absorbing antenna pigments and heterogeneous absorption bands concentrate excitons and increase absorption cross section

The light-harvesting apparatus of photosynthetic organisms is highly optimized with respect to efficient collection of excitation energy from photons of different wavelengths and with respect to a high quantum yield of the primary photochemistry. In many cases the primary donor is not an energetic trap as it absorbs hypsochromically compared to the most red-shifted antenna pigment present (long-wavelength antenna). The possible reasons for this as well as for the spectral heterogeneity which is generally found in antenna systems is examined on a theoretical basis using the approach of thermal equilibration of the excitation energy. The calculations show that long-wavelength antenna pigments and heterogeneous absorption bands lead to a concentration of excitons and an increased effective absorption cross section. The theoretically predicted trapping times agree remarkably well with experimental data from several organisms. It is shown that the kinetics of the energy transfer from a long-wavelength antenna pigment to a hypsochromically absorbing primary donor does not represent a major kinetic limitation. The development of long-wavelength antenna and spectrally heterogeneous absorption bands means an evolutionary advantage based on the chromatic adaptation of photosynthetic organelles to spectrally filtered light caused by self-absorption.

Dynamics of energy transfer and trapping in the light-harvesting antenna of Rhodopseudomonas viridis

By low intensity picosecond absorption spectroscopy it is shown that the exciton lifetime in the light-harvesting antenna of Rhodopseudomonas (Rps.) viridis membranes with photochemically active reaction centers at room temperature is 60 +/- 10 ps. This lifetime reflects the overall trapping rate of the excitation energy by the reaction center. With photochemically inactive reaction centers, in the presence of P+, the exciton lifetime increases to 150 +/- 15 ps. Prereducing the secondary electron acceptor QA does not prevent primary charge separation, but slows it down from 60 to 90 +/- 10 ps. Picosecond kinetics measured at 77 K with inactive reaction centers indicates that the light-harvesting antenna is spectrally homogeneous. Picosecond absorption anisotropy measurements show that energy transfer between identical Bchlb molecules occurs on the subpicosecond time scale. Using these experimental results as input to a random-walk model, results in strict requirements for the antenna-RC coupling. The model analysis prescribes fast trapping (approximately 1 ps) and an approximately 0.5 escape probability from the reaction center, which requires a more tightly coupled RC and antenna, as compared with the Bchla-containing bacteria Rhodospirillum (R.) rubrum and Rhodobacter (Rb.) sphaeroides.

Chemiosmotic systems in bioenergetics: H(+)-cycles and Na(+)-cycles

The development of membrane bioenergetic studies during the last 25 years has clearly demonstrated the validity of the Mitchellian chemiosmotic H+ cycle concept. The circulation of H+ ions was shown to couple respiration-dependent or light-dependent energy-releasing reactions to ATP formation and performance of other types of membrane-linked work in mitochondria, chloroplasts, some bacteria, tonoplasts, secretory granules and plant and fungal outer cell membranes. A concrete version of the direct chemiosmotic mechanism, in which H+ potential formation is a simple consequence of the chemistry of the energy-releasing reaction, is already proved for the photosynthetic reaction centre complexes. Recent progress in the studies on chemiosmotic systems has made it possible to extend the coupling-ion principle to an ion other than H+. It was found that, in certain bacteria, as well as in the outer membrane of the animal cell, Na+ effectively substitutes for H+ as the coupling ion (the chemiosmotic Na+ cycle). A precedent is set when the Na+ cycle appears to be the only mechanism of energy production in the bacterial cell. In the more typical case, however, the H+ and Na+ cycles coexist in one and the same membrane (bacteria) or in two different membranes of one and the same cell (animals). The sets of delta mu H+ and delta mu Na+ generators as well as delta mu H+ and delta mu Na+ consumers found in different types of biomembranes, are listed and discussed.

Several forms of chromaffin granule cytochrome b-561 revealed by EPR spectroscopy

Low-temperature EPR spectra of chromaffin granule membranes from bovine adrenal medulla reveal 3 different signals of the ferric cytochrome b-561. A typical gZ signal of a low-spin cytochrome observed at g approximately 3 is comprised of a high-potential component with gZ = 3.14 and a low-potential one with gZ = 3.11, the low-potential signal showing significantly faster relaxation. In addition, a highly temperature-sensitive heme signal at g = 3.7 is observed which is fully retained in the preparation of granule membranes with b-561 reduced by 50% but disappears upon full reduction of the cytochrome by ascorbate. The signal is strikingly similar to that of the mitochondrial low-potential cytochrome b heme (bL or b-566). The presence of several forms of b-561 in chromaffin granule membranes may provide a structural basis for the transmembrane electron transfer believe to be catalyzed by this hemoprotein.

Photoelectric study on the kinetics of trapping and charge stabilization in oriented PS II membranes

Excitation energy trapping and charge separation in Photosystem II were studied by kinetic analysis of the fast photovoltage detected in membrane fragments from peas with picosecond excitation. With the primary quinone acceptor oxidized the photovoltage displayed a biphasic rise with apparent time constants of 100-300 ps and 550±50 ps. The first phase was dependent on the excitation energy whereas the second phase was not. We attribute these two phases to trapping (formation of P-680(+) Phe(-)) and charge stabilization (formation of P-680(+) QA (-)), respectively. A reversibility of the trapping process was demonstrated by the effect of the fluorescence quencher DNB and of artificial quinone acceptors on the apparent rate constants and amplitudes. With the primary quinone acceptor reduced a transient photoelectric signal was observed and attributed to the formation and decay of the primary radical pair. The maximum concentration of the radical pair formed with reduced QA was about 30% of that measured with oxidized QA. The recombination time was 0.8-1.2 ns.The competition between trapping and annihilation was estimated by comparison of the photovoltage induced by short (30 ps) and long (12 ns) flashes. These data and the energy dependence of the kinetics were analyzed by a reversible reaction scheme which takes into account singlet-singlet annihilation and progressive closure of reaction centers by bimolecular interaction between excitons and the trap. To put on firmer grounds the evaluation of the molecular rate constants and the relative electrogenicity of the primary reactions in PS II, fluorescence decay data of our preparation were also included in the analysis. Evidence is given that the rates of radical pair formation and charge stabilization are influenced by the membrane potential. The implications of the results for the quantum yield are discussed.

Investigations of the polarity of the photo-induced electrical signal of chloroplast suspensions

This paper reports data and considerations relevant to the question of what determines the polarity of the voltages induced between electrodes in a suspension of chloroplasts when irradiated with a flash of light from a laser or flash-lamp. We found positive polarity (electrode nearest the light source positive) with excitation by ns pulses at 694, 539 and 530 nm wavelength. This and the earlier finding (Meszéna et al. (1988) Studia Biophysica 126:77-86), confirmed in this work, of negative polarity at 420 nm confirm, in part, the action spectrum reported by Gräber and Trissl (1981 FEBS Let 123:95-99) using 50 μs flashes. Gräber and Trissl also showed that swelling the chloroplasts can reverse the polarity.Negative polarity is expected on the basis of a simple light-gradient in the sample together with what is known about photosynthetic charge movements. The cause of positive polarities has eluded explanation. Duration of flash was suspected. We tried a random series of short flashes averaging about 10 μs apart and found that all simply duplicated the first flash. If there is any effect of light following the first flash it must occur in less than about 10 μs.We suggest that the polarity is determined by a complicated interference pattern of the light in the chloroplast which can focus it onto different parts, front or back, depending upon the wavelength of the light and the structure of the chloroplast.

Electrogenic steps in the redox reactions catalyzed by photosynthetic reaction-centre complex from Rhodopseudomonas viridis

Electrogenic and redox events in the reaction-centre complexes from Rhodopseudomonas viridis have been studied. In contrast to the previous points of view it is shown that all the four hemes of the tightly bound cytochrome c have different Em values (-60, +20, +310 and +380 mV). The first three hemes reveal alpha absorption maxima at 554 nm, 552 nm and 556 nm respectively. The 380-mV heme displays a split alpha band with a maximum at 559 nm and a shoulder at 552 nm. Such a splitting is due to non-degenerated Qx and Qy transitions in the iron-porphyrin ring as demonstrated by magnetic circular dichroism spectra. Fast kinetic measurements show that, at redox potentials when only high-potential hemes c-559 and c-556 are reduced, heme c-559 appears to be the electron donor to P-960+ (tau = 0.32 microsecond) whereas heme c-556 serves to rereduce c-559 (tau = 2.5 microsecond). Upon reduction of the third heme (c-552), the P-960+ reduction rate increases twofold (tau = 0.17 microsecond) and all photoinduced redox events within the cytochrome appear to be complete in less than 1 microsecond after the flash. The following sequence of the redox centers is tentatively suggested: c-554, c-556, c-552, c-559, P-960. To study electrogenesis, the reaction-centre complexes from Rps. viridis were incorporated into asolectin liposomes, and fast kinetics of laser flash-induced electric potential difference has been measured in proteoliposomes adsorbed on a phospholipid-impregnated film. The electrical difference induced by a single 15-ns flash was found to be as high as 100 mV. The photoelectric response has been found to involve four electrogenic stages associated with (I) QA reduction by P-960; (II) reduction of P-960+ by heme c-559; (III) reduction of c-559 by c-556 and (IV) protonation of Q2-B. The relative contributions of stages I, II, III and IV are found to be equal to 70%, 15%, 5% and 10%, respectively, of the overall electrogenic process. At the same time, the first three respective distances along the axis normal to the membrane plane covered by electrons, calculated from X-ray data of Deisenhofer et al. [J. Mol. Biol. 180, 385-398 (1984)], are 22%, 18.5% and 26%. This indicates that the efficiency of electrogenic phases depends first of all upon the value of the dielectric constant of the respective membrane regions rather than upon the distance between the redox groups involved.(ABSTRACT TRUNCATED AT 400 WORDS)

Primary photochemistry of reaction centers from the photosynthetic purple bacteria

Photosynthetic organisms transform the energy of sunlight into chemical potential in a specialized membrane-bound pigment-protein complex called the reaction center. Following light activation, the reaction center produces a charge-separated state consisting of an oxidized electron donor molecule and a reduced electron acceptor molecule. This primary photochemical process, which occurs via a series of rapid electron transfer steps, is complete within a nanosecond of photon absorption. Recent structural data on reaction centers of photosynthetic bacteria, combined with results from a large variety of photochemical measurements have expanded our understanding of how efficient charge separation occurs in the reaction center, and have changed many of the outstanding questions.