Femtosecond spectroscopy of excitation energy transfer and initial charge separation in the reaction center of the photosynthetic bacterium Rhodopseudomonas viridis

Controlling Symmetry Breaking Charge Transfer in BODIPY Pairs

ConspectusSymmetry breaking charge transfer (SBCT) is a process in which a pair of identical chromophores absorb a photon and use its energy to transfer an electron from one chromophore to the other, breaking the symmetry of the chromophore pair. This excited state phenomenon is observed in photosynthetic organisms where it enables efficient formation of separated charges that ultimately catalyze biosynthesis. SBCT has also been proposed as a means for developing photovoltaics and photocatalytic systems that operate with minimal energy loss. It is known that SBCT in both biological and artificial systems is in part made possible by the local environment in which it occurs, which can move to stabilize the asymmetric SBCT state. However, how environmental degrees of freedom act in concert with steric and structural constraints placed on a chromophore pair to dictate its ability to generate long-lived charge pairs via SBCT remain open topics of investigation.In this Account, we compare a broad series of dipyrrin dimers that are linked by distinct bridging groups to discern how the spatial separation and mutual orientation of linked chromophores and the structural flexibility of their linker each impact SBCT efficiency. Across this material set, we observe a general trend that SBCT is accelerated as the spatial separation between dimer chromophores decreases, consistent with the expectation that the electronic coupling between these units varies exponentially with their separation. However, one key observation is that the rate of charge recombination following SBCT was found to slow with decreasing interchromophore separation, rather than speed up. This stems from an enhancement of the dimer's structural rigidity due to increasing steric repulsion as the length of their linker shrinks. This rigidity further inhibits charge recombination in systems where symmetry has already enforced zero HOMO-LUMO overlap. Additionally, for the forward transfer, the active torsion is shown to increase LUMO-LUMO coupling, allowing for faster SBCT within bridging groups.By understanding trends for how rates of SBCT and charge recombination depend on a dimer's internal structure and its environment, we identify design guidelines for creating artificial systems for driving sustained light-induced charge separation. Such systems can find application in solar energy technologies and photocatalytic applications and can serve as a model for light-induced charge separation in biological systems.

Ultrafast structural changes within a photosynthetic reaction centre

Photosynthetic reaction centres harvest the energy content of sunlight by transporting electrons across an energy-transducing biological membrane. Here we use time-resolved serial femtosecond crystallography¹ using an X-ray free-electron laser² to observe light-induced structural changes in the photosynthetic reaction centre of Blastochloris viridis on a timescale of picoseconds. Structural perturbations first occur at the special pair of chlorophyll molecules of the photosynthetic reaction centre that are photo-oxidized by light. Electron transfer to the menaquinone acceptor on the opposite side of the membrane induces a movement of this cofactor together with lower amplitude protein rearrangements. These observations reveal how proteins use conformational dynamics to stabilize the charge-separation steps of electron-transfer reactions.

Visualizing a protein quake with time-resolved X-ray scattering at a free-electron laser

We describe a method to measure ultrafast protein structural changes using time-resolved wide-angle X-ray scattering at an X-ray free-electron laser. We demonstrated this approach using multiphoton excitation of the Blastochloris viridis photosynthetic reaction center, observing an ultrafast global conformational change that arises within picoseconds and precedes the propagation of heat through the protein. This provides direct structural evidence for a 'protein quake': the hypothesis that proteins rapidly dissipate energy through quake-like structural motions.

The three-dimensional structures of bacterial reaction centers

This review presents a broad overview of the research that enabled the structure determination of the bacterial reaction centers from Blastochloris viridis and Rhodobacter sphaeroides, with a focus on the contributions from Duysens, Clayton, and Feher. Early experiments performed in the laboratory of Duysens and others demonstrated the utility of spectroscopic techniques and the presence of photosynthetic complexes in both oxygenic and anoxygenic photosynthesis. The laboratories of Clayton and Feher led efforts to isolate and characterize the bacterial reaction centers. The availability of well-characterized preparations of pure and stable reaction centers allowed the crystallization and subsequent determination of the structures using X-ray diffraction. The three-dimensional structures of reaction centers revealed an overall arrangement of two symmetrical branches of cofactors surrounded by transmembrane helices from the L and M subunits, which also are related by the same twofold symmetry axis. The structure has served as a framework to address several issues concerning bacterial photosynthesis, including the directionality of electron transfer, the properties of the reaction center-cytochrome c 2 complex, and the coupling of proton and electron transfer. Together, these research efforts laid the foundation for ongoing efforts to address an outstanding question in oxygenic photosynthesis, namely the molecular mechanism of water oxidation.

Dynamical theory of primary processes of charge separation in the photosynthetic reaction center

A dynamical theory has been developed for primary separation of charges in the course of photosynthesis. The theory deals with both hopping and superexchange transfer mechanisms. Dynamics of electron transfer from dimeric bacteriochlorophyll to quinone has been calculated. The results obtained agree with experimental data and provide a unified explanation of both the hierarchy of the transfer time in the photosynthetic reaction center and the phenomenon of coherent oscillations accompanying the transfer process.

Hopping Maps for Photosynthetic Reaction Centers()

Photosynthetic reaction centers (PRCs) employ multiple-step tunneling (hopping) to separate electrons and holes that ultimately drive the chemistry required for metabolism. We recently developed hopping maps that can be used to interpret the rates and energetics of electron/hole hopping in three-site (donor-intermediate-acceptor) tunneling reactions, including those in PRCs. Here we analyze several key ET reactions in PRCs, including forward ET in the L-branch, and hopping that could involve thermodynamically uphill intermediates in the M-branch, which is ET-inactive in vivo. We also explore charge recombination reactions, which could involve hopping. Our hopping maps support the view that electron flow in PRCs involves strong electronic coupling between cofactors and reorganization energies that are among the lowest in biology (≤ 0.4 eV).

How Quantum Coherence Assists Photosynthetic Light Harvesting

This perspective examines how hundreds of pigment molecules in purple bacteria cooperate through quantum coherence to achieve remarkable light harvesting efficiency. Quantum coherent sharing of excitation, which modifies excited state energy levels and combines transition dipole moments, enables rapid transfer of excitation over large distances. Purple bacteria exploit the resulting excitation transfer to engage many antenna proteins in light harvesting, thereby increasing the rate of photon absorption and energy conversion. We highlight here how quantum coherence comes about and plays a key role in the photosynthetic apparatus of purple bacteria.

Kinetics and energetics of electron transfer in reaction centers of the photosynthetic bacterium Roseiflexus castenholzii

The kinetics and thermodynamics of the photochemical reactions of the purified reaction center (RC)-cytochrome (Cyt) complex from the chlorosome-lacking, filamentous anoxygenic phototroph, Roseiflexus castenholzii are presented. The RC consists of L- and M-polypeptides containing three bacteriochlorophyll (BChl), three bacteriopheophytin (BPh) and two quinones (Q(A) and Q(B)), and the Cyt is a tetraheme subunit. Two of the BChls form a dimer P that is the primary electron donor. At 285K, the lifetimes of the excited singlet state, P*, and the charge-separated state P(+)H(A)(-) (where H(A) is the photoactive BPh) were found to be 3.2±0.3 ps and 200±20 ps, respectively. Overall charge separation P*→→ P(+)Q(A)(-) occurred with ≥90% yield at 285K. At 77K, the P* lifetime was somewhat shorter and the P(+)H(A)(-) lifetime was essentially unchanged. Poteniometric titrations gave a P(865)/P(865)(+) midpoint potential of +390mV vs. SHE. For the tetraheme Cyt two distinct midpoint potentials of +85 and +265mV were measured, likely reflecting a pair of low-potential hemes and a pair of high-potential hemes, respectively. The time course of electron transfer from reduced Cyt to P(+) suggests an arrangement where the highest potential heme is not located immediately adjacent to P. Comparisons of these and other properties of isolated Roseiflexus castenholzii RCs to those from its close relative Chloroflexus aurantiacus and to RCs from the purple bacteria are made.

The Photosynthetic Reaction Center from the Purple Bacterium Rhodopseudomonas viridis

The history and methods of membrane protein crystallization are described. The solution of the structure of the photosynthetic reaction center from the bacterium Rhodopseudomonas viridis is described, and the structure of this membrane protein complex is correlated with its function as a light-driven electron pump across the photosynthetic membrane. Conclusions about the structure of the photosystem II reaction center from plants are drawn, and aspects of membrane protein structure are discussed.

Gaussian free-energy dependence of electron-transfer rates in iridium complexes

The kinetics of photoinduced electron-transfer (ET) reactions have been measured in a series of synthetic donor-acceptor complexes. The electron donors are singlet or triplet excited iridium(I) dimers (Ir(2)), and the acceptors are N-alkylpyridinium groups covalently bound to phosphinite ligands on the Ir(2) core. Rate constants for excited-state ET range from 3.5 x 10(6) to 1.1 x 10(11) per second, and thermal back ET (pyridinium radical to Ir(2)(+)) rates vary from 2.0 x 10(10) to 6.7 x 10(7) per second. The variation of these rates with driving force is in remarkably good agreement with the Marcus theory prediction of a Gaussian free-energy dependence.

Picosecond kinetics of fluorescence and absorbance changes in photosystem II particles excited at low photon density

Oxygen-evolving photosystem II particles (from Synechococcus) with about 80 chlorophyll molecules per primary electron donor (P(680)) were used for a correlated study of picosecond kinetics of fluorescence and absorbance changes, detected by the single-photon-timing technique and by a pump-probe apparatus, respectively. Chlorophyll fluorescence decays were biexponential with lifetimes tau(1) = 80 +/- 20 ps and tau(2) = 520 +/- 120 ps in open reaction centers and tau(1) = 220 +/- 30 ps and tau(2) = 1.3 +/- 0.15 ns in closed reaction centers. The corresponding fluorescence yield ratio F(max)/F(o) was 3-4. Absorbance changes were monitored in the spectral range of 620-700 nm after excitation at 675 nm with 10-ps pulses sufficiently weak (<7 x 10(12) photons/cm(2) per pulse) to avoid singlet-singlet annihilation. With open reaction centers, the absorbance changes could be fit to the sum of three exponentials. The associated absorbance difference spectra were attributed to (i) exciton trapping and charge separation (tau = 100 +/- 20 ps), (ii) the electron-transfer step P(680) (+) I(-) Q(A) --> P(680) (+) I Q(A) (-) (where I is the primary electron acceptor and Q(A) is the first quinone acceptor) (tau = 510 +/- 50 ps), and (iii) the reduction of P(680) (+) by the intact donor side (tau > 10 ns). With closed reaction centers, the absorbance changes were biexponential with lifetimes tau(1) = 170-260 ps and tau(2) = 1.6-1.75 ns. The results are explained in terms of a kinetic model that assumes P(680) to constitute a shallow trap. The results show that Q(A) reduction in these photosystem II particles decreases both the apparent rate and the yield of the primary charge separation by a factor of 2-3 and increases the mean lifetime of excitons in the antenna by a factor of 3-4. Thus, we conclude that the long-lived, nanosecond chlorophyll fluorescence is not charge-recombination luminescence but rather emission from equilibrated excited states of antenna chlorophylls.

Determination of the primary charge separation rate in isolated photosystem II reaction centers with 500-fs time resolution

We have measured directly the rate of formation of the oxidized chlorophyll a electron donor (P680(+)) and the reduced electron acceptor pheophytin a(-) (Pheoa(-)) following excitation of isolated spinach photosystem II reaction centers at 4 degrees C. The reaction-center complex consists of D(1), D(2), and cytochrome b-559 proteins and was prepared by a procedure that stabilizes the protein complex. Transient absorption difference spectra were measured from 440 to 850 nm as a function of time with 500-fs resolution following 610-nm laser excitation. The formation of P680(+)-Pheoa(-) is indicated by the appearance of a band due to P680(+) at 820 nm and corresponding absorbance changes at 505 and 540 nm due to formation of Pheoa(-). The appearance of the 820-nm band is monoexponential with tau = 3.0 +/- 0.6 ps. The time constant for decay of (1*)P680, the lowest excited singlet state of P680, monitored at 650 nm, is tau = 2.6 +/- 0.6 ps and agrees with that of the appearance of P680(+) within experimental error. Treatment of the photosystem II reaction centers with sodium dithionite and methyl viologen followed by exposure to laser excitation, conditions known to result in accumulation of Pheoa(-), results in formation of a transient absorption spectrum due to (1*)P680. We find no evidence for an electron acceptor that precedes the formation of Pheoa(-).

Kinetic and Energetic Model for the Primary Processes in Photosystem II

A detailed model for the kinetics and energetics of the exciton trapping, charge separation, charge recombination, and charge stabilization processes in photosystem (PS) II is presented. The rate constants describing these processes in open and closed reaction centers (RC) are calculated on the basis of picosecond data (Schatz, G. H., H. Brock, and A. R. Holzwarth. 1987. Proc. Natl. Acad. Sci. USA. 84:8414-8418) obtained for oxygen-evolving PS II particles from Synechococcus sp. with approximately 80 chlorophylls/P(680). The analysis gives the following results. (a) The PS II reaction center donor chlorophyll P(680) constitutes a shallow trap, and charge separation is overall trap limited. (b) The rate constant of charge separation drops by a factor of approximately 6 when going from open (Q-oxidized) to closed (Q-reduced) reaction centers. Thus the redox state of Q controls the yield of radical pair formation and the exciton lifetime in the Chl antenna. (c) The intrinsic rate constant of charge separation in open PS II reaction centers is calculated to be approximately 2.7 ps(-1). (d) In particles with open RC the charge separation step is exergonic with a decrease in standard free energy of approximately 38 meV. (e) In particles with closed RC the radical pair formation is endergonic by approximately 12 meV. We conclude on the basis of these results that the long-lived (nanoseconds) fluorescence generally observed with closed PS II reaction centers is prompt fluorescence and that the amount of primary radical pair formation is decreased significantly upon closing of the RC.

Excitation and electron transfer in reaction centers from Rhodobacter sphaeroides probed and analyzed globally in the 1-nanosecond temporal window from 330 to 700 nm

Global analysis of a set of room temperature transient absorption spectra of Rhodobacter sphaeroides reaction centers, recorded in wide temporal and spectral ranges and triggered by femtosecond excitation of accessory bacteriochlorophylls at 800 nm, is presented. The data give a comprehensive review of all spectral dynamics features in the visible and near UV, from 330 to 700 nm, related to the primary events in the Rb. sphaeroides reaction center: excitation energy transfer from the accessory bacteriochlorophylls (B) to the primary donor (P), primary charge separation between the primary donor and primary acceptor (bacteriopheophytin, H), and electron transfer from the primary to the secondary electron acceptor (ubiquinone). In particular, engagement of the accessory bacteriochlorophyll in primary charge separation is shown as an intermediate electron acceptor, and the initial free energy gap of approximately 40 meV, between the states P(+)B(A)(-) and P(+)H(A)(-) is estimated. The size of this gap is shown to be constant for the whole 230 ps lifetime of the P(+)H(A)(-) state. The ultrafast spectral dynamics features recorded in the visible range are presented against a background of results from similar studies performed for the last two decades.

Spatial distribution of dielectric shielding in the interior of Pyrococcus furiosus rubredoxin as sampled in the subnanosecond timeframe by hydrogen exchange

Experimental pK values of ionizable sidechains provide the most direct test for models representing dielectric shielding within the interior of a protein. However, only the strongly shifted pK values are particularly useful for discriminating among models. NMR titration studies have usually found only one or two such shifted pK values in each protein, so that the fitting of the experimental data to a uniform internal dielectric (epsilon(int)) model is not well constrained. The observed variation among proteins for such epsilon(int) estimates may reflect nonuniformity of dielectric shielding within each protein interior or qualitative differences between individual proteins. The differential amide kinetic acidities for a series of metal-substituted rubredoxins are shown to be consistent with Poisson-Boltzmann predictions of dielectric shielding that is relatively uniform for all of the amides that are sensitive to the metal charge, a region which corresponds to roughly 1/3 of the internal volume. The effective epsilon(int) values near 6 that are found in this study are significantly lower than many such estimates derived from sidechain pK measurements. The differing timeframes in which dielectric relaxation can respond to the highly transient peptide anion as compared to the longer lived states of the charged sidechains offers an explanation for the lower apparent dielectric constant deduced from these measurements.

The Photosynthetic Reaction Centre from the Purple Bacterium Rhodopseudomonasviridis

We first describe the history and methods of membrane protein crystallization, and show how the structure of the photosynthetic reaction centre from the purple bacterium Rhodopseudomonas viridis was solved. The structure of this membrane protein complex is correlated with its function as a light-driven electron pump across the photosynthetic membrane. Finally we draw conclusions on the structure of the photosystem II reaction centre from plants and discuss the aspects of membrane protein structure.

The First Picoseconds in Bacterial Photosynthesis?Ultrafast Electron Transfer for the Efficient Conversion of Light Energy

In this Minireview, we describe the function of the bacterial reaction centre (RC) as the central photosynthetic energy-conversion unit by ultrafast spectroscopy combined with structural analysis, site-directed mutagenesis, pigment exchange and theoretical modelling. We show that primary energy conversion is a stepwise process in which an electron is transferred via neighbouring chromophores of the RC. A well-defined chromophore arrangement in a rigid protein matrix, combined with optimised energetics of the different electron carriers, allows a highly efficient charge-separation process. The individual molecular reactions at room temperature are well described by conventional electron-transfer theory.

Light-induced electron transfer in porphyrin–calixarene conjugates

The fluorescence from a set of porphyrin-calixarene complexes is quenched upon addition of benzo-1,4-quinone (BQ) in fluid solution. In N,N-dimethylformamide solution, fluorescence quenching involves both static and dynamic interactions but there are no obvious differences between porphyrins with or without the appended calixarene. Under such conditions, the static quenching behaviour is attributed to pi-complexation between the reactants and it is concluded that the calixarene cavity does not bind BQ. An additional static component is apparent in dichloromethane solution. This latter effect involves partial fluorescence quenching, for which the intramolecular rate constant can be obtained by time-resolved fluorescence spectroscopy. The derived rate constants depend on molecular structure in a manner consistent with fluorescence quenching being due to electron transfer. In all cases, however, the dominant quenching step involves diffusional contact between the porphyrin nucleus and a non-bound molecule of BQ.

Energy transfer in photosystem I of cyanobacteria Synechococcus elongatus: model study with structure-based semi-empirical Hamiltonian and experimental spectral density

We model the energy transfer and trapping kinetics in PSI. Rather than simply applying Förster theory, we develop a new approach to self-consistently describe energy transfer in a complex with heterogeneous couplings. Experimentally determined spectral densities are employed to calculate the energy transfer rates. The absorption spectrum and fluorescence decay time components of the complex at room temperature were reasonably reproduced. The roles of the special chlorophylls (red, linker, and reaction center, respectively) molecules are discussed. A formally exact expression for the trapping time is derived in terms of the intrinsic trapping time, mean first passage time to trap, and detrapping time. The energy transfer mechanism is discussed and the slowest steps of the arrival at the primary electron donor are found to contain two dominant steps: transfer-to-reaction-center, and transfer-to-trap-from-reaction-center. The intrinsic charge transfer time is estimated to be 0.8 approximately 1.7 ps. The optimality with respect to the trapping time of the calculated transition energies and the orientation of Chls is discussed.

Kinetics, energetics, and electronic coupling of the primary electron transfer reactions in mutated reaction centers of Blastochloris viridis

Femtosecond spectroscopy in combination with site-directed mutagenesis has been used to study the dynamics of primary electron transfer in native and 12 mutated reaction centers of Blastochloris (B) (formerly called Rhodopseudomonas) viridis. The decay times of the first excited state P* vary at room temperature between of 0.6 and 50 ps, and at low temperatures between 0.25 and 90 ps. These changes in time constants are discussed within the scope of nonadiabatic electron transfer theory using different models: 1) If the mutation is assumed to predominantly influence the energetics of the primary electron transfer intermediates, the analysis of the room temperature data for the first electron transfer step to the intermediate P(+)B(A)(-) yields a reorganization energy lambda = 600 +/- 200 cm(-1) and a free energy gap Delta G ranging from -600 cm(-1) to 800 cm(-1). However, this analysis fails to describe the temperature dependence of the reaction rates. 2) A more realistic description of the temperature dependence of the primary electron transfer requires different values for the energetics and specific variations of the electronic coupling upon mutation. Apparently the mutations also lead to pronounced changes in the electronic coupling, which may even dominate the change in the reaction rate. One main message of the paper is that a simple relationship between mutation and a change in one reaction parameter cannot be given and that at the very least the electronic coupling is changed upon mutation.

Selective perturbation of the second electron transfer step in mutant bacterial reaction centers

In order to specifically perturb the primary electron acceptor B(A) -- a monomeric bacteriochlorophyll (BChl) a -- involved in bacterial photosynthetic charge separation (CS), the protein environment of B(A) in the reaction center (RC) of Rhodobacter sphaeroides was modified by site-directed mutagenesis. Isolated RCs were characterized by redox titrations, low temperature optical spectroscopy, ENDOR/TRIPLE resonance spectroscopy and femtosecond time-resolved spectroscopy. Two mutations were studied: In the GS(M203) mutant a serine is introduced near the ring E keto group of B(A), while in FY(L146) a phenylalanine near the ring A acetyl group of B(A) is replaced by tyrosine. In all mutations the oxidation potential of the primary electron donor P as well as the electronic structure of both the P(*+) radical cation and the radical anion of the secondary electron acceptor, H(A)(*-), are not significantly altered compared to the wild type (WT), while changes of the optical absorption spectra at 77 K in the BChl Q(X) and Q(Y) regions are observed. The GS(M203) mutation only leads to a minor retardation of the CS reactions at room temperature, whereas for FY(L146) significant deviations from the native electron transfer (ET) rates could be detected: In addition to a faster first (2.9 ps) and a slower second (1 ps) ET step, a new 8-ps time constant was found in the FY(L146) mutant, which can be ascribed to a fraction of RCs with slowed down secondary ET. The results allow us to address the functional role of the acetyl group of B(A) and question the role of the free energy changes as the main determining factor of ET rates in RCs. It is concluded that structural rearrangements alter the electronic coupling between the pigments and thereby influence the rate of fast CS.