New and unexpected routes for ultrafast electron transfer in photosynthetic reaction centers

Shedding Light on Primary Donors in Photosynthetic Reaction Centers

Chlorophylls (Chl)s exist in a variety of flavors and are ubiquitous in both the energy and electron transfer processes of photosynthesis. The functions they perform often occur on the ultrafast (fs-ns) time scale and until recently, these have been difficult to measure in real time. Further, the complexity of the binding pockets and the resulting protein-matrix effects that alter the respective electronic properties have rendered theoretical modeling of these states difficult. Recent advances in experimental methodology, computational modeling, and emergence of new reaction center (RC) structures have renewed interest in these processes and allowed researchers to elucidate previously ambiguous functions of Chls and related pheophytins. This is complemented by a wealth of experimental data obtained from decades of prior research. Studying the electronic properties of Chl molecules has advanced our understanding of both the nature of the primary charge separation and subsequent electron transfer processes of RCs. In this review, we examine the structures of primary electron donors in Type I and Type II RCs in relation to the vast body of spectroscopic research that has been performed on them to date. Further, we present density functional theory calculations on each oxidized primary donor to study both their electronic properties and our ability to model experimental spectroscopic data. This allows us to directly compare the electronic properties of hetero- and homodimeric RCs.

Vibronic Coherence in the Charge Separation Process of the Rhodobacter sphaeroides Reaction Center

Two-dimensional electronic spectroscopy was applied to a variant of the reaction center (RC) of purple bacterium Rhodobacter sphaeroides lacking the primary acceptor ubiquinone in order to understand the ultrafast separation and transfer of charge between the bacteriochlorin cofactors. For the first time, characteristic 2D spectra were obtained for the participating excited and charge-transfer states, and the electron-transfer cascade (including two different channels, the P* and B* channels) was fully mapped. By analyzing quantum beats using 2D frequency maps, excited-state vibrational modes at 153 and 33 cm-1 were identified. We speculate that these modes couple to the charge separation (CS) process and collectively optimize the CS and are responsible for the superhigh efficiency.

Femtosecond Relaxation Processes in Rhodobacter sphaeroides Reaction Centers

Energy relaxation was studied with difference femtosecond spectroscopy in reaction centers of the YM210L mutant of the purple photosynthetic bacterium Rhodobacter sphaeroides at low temperature (90 K). A dynamical long-wavelength shift of stimulated emission of the excited state of the bacteriochlorophyll dimer P was found, which starts simultaneously with P* formation and is accompanied by a change in the spectral shape of this emission. The characteristic value of this shift was about 30 nm, and the characteristic time about 200 fs. Difference kinetics ΔA measured at fixed wavelengths demonstrate the femtosecond shift of the P* stimulated emission appearing as a dependence of these kinetics on wavelength. We found that the reported long-wavelength shift can be explained in terms of electron-vibrational relaxation of the P* excited state with time constants of vibrational and electronic relaxation of 100 and 50 fs, respectively. Alternative mechanisms of the dynamical shift of the P* stimulated emission spectrum are also discussed in terms of energy redistribution between vibrational modes or coherent excitation of the modes.

Structure of a symmetric photosynthetic reaction center-photosystem

Reaction centers are pigment-protein complexes that drive photosynthesis by converting light into chemical energy. It is believed that they arose once from a homodimeric protein. The symmetry of a homodimer is broken in heterodimeric reaction-center structures, such as those reported previously. The 2.2-angstrom resolution x-ray structure of the homodimeric reaction center-photosystem from the phototroph Heliobacterium modesticaldum exhibits perfect C₂ symmetry. The core polypeptide dimer and two small subunits coordinate 54 bacteriochlorophylls and 2 carotenoids that capture and transfer energy to the electron transfer chain at the center, which performs charge separation and consists of 6 (bacterio)chlorophylls and an iron-sulfur cluster; unlike other reaction centers, it lacks a bound quinone. This structure preserves characteristics of the ancestral reaction center, providing insight into the evolution of photosynthesis.

Spectral exhibition of electron-vibrational relaxation in P* state of Rhodobacter sphaeroides reaction centers

Electron-vibrational relaxation in the excited state of the primary electron donor, bacteriochlorophyll dimer P, in the reaction centers (RCs) of purple photosynthetic bacteria Rhodobacter sphaeroides is modeled. A multimode model of three states (i.e., the ground state Pg, initially excited P1*, and relaxed excited P2*) is used to calculate the incoherent dynamics of the difference (ΔA) spectra on a femtosecond timescale for the YM210 W mutant RCs. The relaxation processes are described by the step-ladder model. The model shows that the electron-vibrational relaxation in the excited state of P is visualized by the transient red shift of the stimulated emission from P*. The dynamics of this shift is observed as a change in the ΔA spectrum shape in its red-most part, within a few hundreds of femtoseconds after excitation. As a result, an initial rise in the red-side ΔA kinetics is delayed with respect to the blue-side kinetics. The time constant of the P1* → P2* electronic relaxation (54 fs) and the Pg, P1*, and P2* vibrational relaxations (120 fs), used in the model, provided the best fit of the experimental time-resolved ΔA spectra and kinetics at 90 and 293 K. The possible nature of the P1* → P2* electronic relaxation is discussed.

Ultrafast infrared spectroscopy in photosynthesis

In recent years visible pump/mid-infrared (IR) probe spectroscopy has established itself as a key technology to unravel structure-function relationships underlying the photo-dynamics of complex molecular systems. In this contribution we review the most important applications of mid-infrared absorption difference spectroscopy with sub-picosecond time-resolution to photosynthetic complexes. Considering several examples, such as energy transfer in photosynthetic antennas and electron transfer in reaction centers and even more intact structures, we show that the acquisition of ultrafast time resolved mid-IR spectra has led to new insights into the photo-dynamics of the considered systems and allows establishing a direct link between dynamics and structure, further strengthened by the possibility of investigating the protein response signal to the energy or electron transfer processes. This article is part of a Special Issue entitled: Vibrational spectroscopies and bioenergetic systems.

Detection of a charge-separated catalyst precursor state in a linked photosensitizer-catalyst assembly

We have designed two new supramolecular assemblies based on Co(ii)-templated coordination of Ru(bpy)3(2+) (bpy = 2,2'-bipyridyl) analogues as photosensitizers and electron donors to a cobaloxime macrocycle, which are of interest as proton reduction catalysts. The self-assembled photocatalyst precursors were structurally characterized by Co K-edge X-ray absorption spectroscopy and solution-phase X-ray scattering. Visible light excitation of one of the assemblies has yielded instantaneous electron transfer and charge separation to form a transient Co(i) state which persists for 26 ps. The development of a linked photosensitizer-cobaloxime architecture supporting efficient Co(i) charge transfer is significant since it is mechanistically critical as the first photo-induced electron transfer step for hydrogen production, and has not been detected in previous photosensitizer-cobaloxime linked dyad assemblies. X-band EPR spectroscopy has revealed that the Co(ii) centres of both assemblies are high spin, in contrast to most previously described cobaloximes, and likely plays an important role in facilitating photoinduced charge separation. Based on the results obtained from ultrafast and nanosecond transient absorption optical spectroscopies, we propose that charge recombination occurs through multiple ligand states present within the photosensitizer modules. The studies presented here will enhance our understanding of supramolecular photocatalyst assembly and direct new designs for artificial photosynthesis.

Excited state dynamics in photosynthetic reaction center and light harvesting complex 1

Key to efficient harvesting of sunlight in photosynthesis is the first energy conversion process in which electronic excitation establishes a trans-membrane charge gradient. This conversion is accomplished by the photosynthetic reaction center (RC) that is, in case of the purple photosynthetic bacterium Rhodobacter sphaeroides studied here, surrounded by light harvesting complex 1 (LH1). The RC employs six pigment molecules to initiate the conversion: four bacteriochlorophylls and two bacteriopheophytins. The excited states of these pigments interact very strongly and are simultaneously influenced by the surrounding thermal protein environment. Likewise, LH1 employs 32 bacteriochlorophylls influenced in their excited state dynamics by strong interaction between the pigments and by interaction with the protein environment. Modeling the excited state dynamics in the RC as well as in LH1 requires theoretical methods, which account for both pigment-pigment interaction and pigment-environment interaction. In the present study we describe the excitation dynamics within a RC and excitation transfer between light harvesting complex 1 (LH1) and RC, employing the hierarchical equation of motion method. For this purpose a set of model parameters that reproduce RC as well as LH1 spectra and observed oscillatory excitation dynamics in the RC is suggested. We find that the environment has a significant effect on LH1-RC excitation transfer and that excitation transfers incoherently between LH1 and RC.

Mixed exciton-charge-transfer states in photosystem II: Stark spectroscopy on site-directed mutants

We investigated the electronic structure of the photosystem II reaction center (PSII RC) in relation to the light-induced charge separation process using Stark spectroscopy on a series of site-directed PSII RC mutants from the cyanobacterium Synechocystis sp. PCC 6803. The site-directed mutations modify the protein environment of the cofactors involved in charge separation (P(D1), P(D2), Chl(D1), and Phe(D1)). The results demonstrate that at least two different exciton states are mixed with charge-transfer (CT) states, yielding exciton states with CT character: (P(D2)(δ)(+)P(D1)(δ)(-)Chl(D1)) (673 nm) and (Chl(D1)(δ)(+)Phe(D1)(δ)(-)) (681 nm) (where the subscript indicates the wavelength of the electronic transition). Moreover, the CT state P(D2)(+)P(D1)(-) acquires excited-state character due to its mixing with an exciton state, producing (P(D2)(+)P(D1)(-))(δ) (684 nm). We conclude that the states that initiate charge separation are mixed exciton-CT states, and that the degree of mixing between exciton and CT states determines the efficiency of charge separation. In addition, the results reveal that the pigment-protein interactions fine-tune the energy of the exciton and CT states, and hence the mixing between these states. This mixing ultimately controls the selection and efficiency of a specific charge separation pathway, and highlights the capacity of the protein environment to control the functionality of the PSII RC complex.

Resolving the Electron Transfer Kinetics in the Bacterial Reaction Center by Pulse Polarized 2-D Photon Echo Spectroscopy

At the heart of photosynthesis is excitation energy transfer toward and charge separation within highly conserved reaction centers (RCs). The function principles of RCs in purple bacteria offer a blueprint for an optoelectronic device, which efficiently utilizes the near-IR region of the solar spectrum. We present theoretical modeling of the nonlinear optical response of the bacterial RC B. viridis incorporating electron and energy transfer on equal footing. The splitting of special pair excitons P is the origin of distinct cross peaks, which allow monitoring of the kinetics of charge separation. The xxyy - xyxy signal, obtained from sequences of orthogonal polarized laser pulses, highlights the kinetics of the secondary, subpicosecond electron transfer from the accessory bacteriochlorophyll BClL to the bacteriopheophytine BPL. The increased selectivity is explained by the relative orientation of exciton transitions. The technique can resolve complex kinetics in congested signals of photosynthetic complexes that are otherwise hardly accessible.

Cofactor-specific photochemical function resolved by ultrafast spectroscopy in photosynthetic reaction center crystals

High-resolution mapping of cofactor-specific photochemistry in photosynthetic reaction centers (RCs) from Rhodobacter sphaeroides was achieved by polarization selective ultrafast spectroscopy in single crystals at cryogenic temperature. By exploiting the fixed orientation of cofactors within crystals, we isolated a single transition within the multicofactor manifold, and elucidated the site-specific photochemical functions of the cofactors associated with the symmetry-related active A and inactive B branches. Transient spectra associated with the initial excited states were found to involve a set of cofactors that differ depending upon whether the monomeric bacteriochlorophylls, BChl(A), BChl(B), or the special pair bacteriochlorophyll dimer, P, was chosen for excitation. Proceeding from these initial excited states, characteristic photochemical functions were resolved. Specifically, our measurements provide direct evidence for an alternative charge separation pathway initiated by excitation of BChl(A) that does not involve P*. Conversely, the initial excited state produced by excitation of BChl(B) was found to decay by energy transfer to P. A clear sequential kinetic resolution of BChl(A) and the A-side bacteriopheophytin, BPh(A), in the electron transfer proceeding from P* was achieved. These experiments demonstrate the opportunity to resolve photochemical function of individual cofactors within the multicofactor RC complexes using single crystal spectroscopy.

Charge separation in photosystem II: a comparative and evolutionary overview

Our current understanding of the PSII reaction centre owes a great deal to comparisons to the simpler and better understood, purple bacterial reaction centre. Here we provide an overview of the similarities with a focus on charge separation and the electron acceptors. We go on to discuss some of the main differences between the two kinds of reaction centres that have been highlighted by the improving knowledge of PSII. We attempt to relate these differences to functional requirements of water splitting. Some are directly associated with that function, e.g. high oxidation potentials, while others are associated with regulation and protection against photodamage. The protective and regulatory functions are associated with the harsh chemistry performed during its normal function but also with requirements of the enzyme while it is undergoing assembly and repair. Key aspects of PSII reaction centre evolution are also addressed. This article is part of a Special Issue entitled: Photosystem II.

Optical properties, excitation energy and primary charge transfer in photosystem II: theory meets experiment

In this review we discuss structure-function relationships of the core complex of photosystem II, as uncovered from analysis of optical spectra of the complex and its subunits. Based on descriptions of optical difference spectra including site directed mutagenesis we propose a revision of the multimer model of the symmetrically arranged reaction center pigments, described by an asymmetric exciton Hamiltonian. Evidence is provided for the location of the triplet state, the identity of the primary electron donor, the localization of the cation and the secondary electron transfer pathway in the reaction center. We also discuss the stationary and time-dependent optical properties of the CP43 and CP47 subunits and the excitation energy transfer and trapping-by-charge-transfer kinetics in the core complex.

Effect of the P700 pre-oxidation and point mutations near A(0) on the reversibility of the primary charge separation in Photosystem I from Chlamydomonas reinhardtii

Time-resolved fluorescence studies with a 3-ps temporal resolution were performed in order to: (1) test the recent model of the reversible primary charge separation in Photosystem I (Müller et al., 2003; Holwzwarth et al., 2005, 2006), and (2) to reconcile this model with a mechanism of excitation energy quenching by closed Photosystem I (with P700 pre-oxidized to P700+). For these purposes, we performed experiments using Photosystem I core samples isolated from Chlamydomonas reinhardtii wild type, and two mutants in which the methionine axial ligand to primary electron acceptor, A(0), has been change to either histidine or serine. The temporal evolution of fluorescence spectra was recorded for each preparation under conditions where the "primary electron donor," P700, was either neutral or chemically pre-oxidized to P700+. For all the preparations under study, and under neutral and oxidizing conditions, we observed multiexponential fluorescence decay with the major phases of approximately 7 ps and approximately 25 ps. The relative amplitudes and, to a minor extent the lifetimes, of these two phases were modulated by the redox state of P700 and by the mutations near A(0): both pre-oxidation of P700 and mutations caused slight deceleration of the excited state decay. These results are consistent with a model in which P700 is not the primary electron donor, but rather a secondary electron donor, with the primary charge separation event occurring between the accessory chlorophyll, A, and A(0). We assign the faster phase to the equilibration process between the excited state of the antenna/reaction center ensemble and the primary radical pair, and the slower phase to the secondary electron transfer reaction. The pre-oxidation of P700 shifts the equilibrium between the excited state and the primary radical pair towards the excited state. This shift is proposed to be induced by the presence of the positive charge on P700+. The same charge is proposed to be responsible for the fast A+A(0)(-)-->AA(0) charge recombination to the ground state and, in consequence, excitation quenching in closed reaction centers. Mutations of the A(0) axial ligand shift the equilibrium in the same direction as pre-oxidation of P700 due to the up-shift of the free energy level of the state A+A(0)(-).

Photosystem II: The machinery of photosynthetic water splitting

This review summarizes our current state of knowledge on the structural organization and functional pattern of photosynthetic water splitting in the multimeric Photosystem II (PS II) complex, which acts as a light-driven water: plastoquinone-oxidoreductase. The overall process comprises three types of reaction sequences: (1) photon absorption and excited singlet state trapping by charge separation leading to the ion radical pair [Formula: see text] formation, (2) oxidative water splitting into four protons and molecular dioxygen at the water oxidizing complex (WOC) with P680+* as driving force and tyrosine Y(Z) as intermediary redox carrier, and (3) reduction of plastoquinone to plastoquinol at the special Q(B) binding site with Q(A)-* acting as reductant. Based on recent progress in structure analysis and using new theoretical approaches the mechanism of reaction sequence (1) is discussed with special emphasis on the excited energy transfer pathways and the sequence of charge transfer steps: [Formula: see text] where (1)(RC-PC)* denotes the excited singlet state (1)P680* of the reaction centre pigment complex. The structure of the catalytic Mn(4)O(X)Ca cluster of the WOC and the four step reaction sequence leading to oxidative water splitting are described and problems arising for the electronic configuration, in particular for the nature of redox state S(3), are discussed. The unravelling of the mode of O-O bond formation is of key relevance for understanding the mechanism of the process. This problem is not yet solved. A multistate model is proposed for S(3) and the functional role of proton shifts and hydrogen bond network(s) is emphasized. Analogously, the structure of the Q(B) site for PQ reduction to PQH(2) and the energetic and kinetics of the two step redox reaction sequence are described. Furthermore, the relevance of the protein dynamics and the role of water molecules for its flexibility are briefly outlined. We end this review by presenting future perspectives on the water oxidation process.

Spectroscopic properties of reaction center pigments in photosystem II core complexes: revision of the multimer model

Absorbance difference spectra associated with the light-induced formation of functional states in photosystem II core complexes from Thermosynechococcus elongatus and Synechocystis sp. PCC 6803 (e.g., P(+)Pheo(-),P(+)Q(A)(-),(3)P) are described quantitatively in the framework of exciton theory. In addition, effects are analyzed of site-directed mutations of D1-His(198), the axial ligand of the special-pair chlorophyll P(D1), and D1-Thr(179), an amino-acid residue nearest to the accessory chlorophyll Chl(D1), on the spectral properties of the reaction center pigments. Using pigment transition energies (site energies) determined previously from independent experiments on D1-D2-cytb559 complexes, good agreement between calculated and experimental spectra is obtained. The only difference in site energies of the reaction center pigments in D1-D2-cytb559 and photosystem II core complexes concerns Chl(D1). Compared to isolated reaction centers, the site energy of Chl(D1) is red-shifted by 4 nm and less inhomogeneously distributed in core complexes. The site energies cause primary electron transfer at cryogenic temperatures to be initiated by an excited state that is strongly localized on Chl(D1) rather than from a delocalized state as assumed in the previously described multimer model. This result is consistent with earlier experimental data on special-pair mutants and with our previous calculations on D1-D2-cytb559 complexes. The calculations show that at 5 K the lowest excited state of the reaction center is lower by approximately 10 nm than the low-energy exciton state of the two special-pair chlorophylls P(D1) and P(D2) which form an excitonic dimer. The experimental temperature dependence of the wild-type difference spectra can only be understood in this model if temperature-dependent site energies are assumed for Chl(D1) and P(D1), reducing the above energy gap from 10 to 6 nm upon increasing the temperature from 5 to 300 K. At physiological temperature, there are considerable contributions from all pigments to the equilibrated excited state P*. The contribution of Chl(D1) is twice that of P(D1) at ambient temperature, making it likely that the primary charge separation will be initiated by Chl(D1) under these conditions. The calculations of absorbance difference spectra provide independent evidence that after primary electron transfer the hole stabilizes at P(D1), and that the physiologically dangerous charge recombination triplets, which may form under light stress, equilibrate between Chl(D1) and P(D1).

Electron transfer in the reaction center of the Rb. sphaeroides R-26 studied by transient absorption

Electron transfer at the reaction center of the purple photosynthetic bacterium Rb. sphaeroides R-26 was measured at room temperature by the time-resolved transient absorption spectroscopy technique with 200 fs temporal resolution. The absorbance changes characteristic of the excited state of the primary donor and extending over the whole spectral range investigated from 350 nm up to 720 nm appeared after excitation with a laser pulse of about 100 fs duration at 800 nm. The time evolution of the spectra reflected the excitation of bacteriochlorophylls (BChl) M and L and the subsequent transfer of this excitation to the primary electron donor (P), with the time constant shorter than 1 ps. The decay time constant of the excited primary donor P was determined as about 3 ps, and it was faster than the rise of the reduced intermediary acceptor bacteriopheophytin (BPhe(L)). Photoreduction of BPhe(L) and its further reoxidation was clearly observed as an increase in its bleaching band intensity at around 540 nm in about 4 ps and its decrease in about 200 ps. Our findings support the theoretical model assuming the involvement of the intermediate state P(+)BChl- in the so-called "two-step" model. In this model an electron is transferred in a sequence from the excited special pair P* to bacteriochlorophyll, BChl(L), then to bacteriopheophytin, BPhe(L), and further on to quinone, Q(A). The branched charge separation, partially via P and partially via BChl(L), was also observed.

Mixing of exciton and charge-transfer states in Photosystem II reaction centers: modeling of Stark spectra with modified Redfield theory

We propose an exciton model for the Photosystem II reaction center (RC) based on a quantitative simultaneous fit of the absorption, linear dichroism, circular dichroism, steady-state fluorescence, triplet-minus-singlet, and Stark spectra together with the spectra of pheophytin-modified RCs, and so-called RC5 complexes that lack one of the peripheral chlorophylls. In this model, the excited state manifold includes a primary charge-transfer (CT) state that is supposed to be strongly mixed with the pure exciton states. We generalize the exciton theory of Stark spectra by 1), taking into account the coupling to a CT state (whose static dipole cannot be treated as a small parameter in contrast to usual excited states); and 2), expressing the line shape functions in terms of the modified Redfield approach (the same as used for modeling of the linear responses). This allows a consistent modeling of the whole set of experimental data using a unified physical picture. We show that the fluorescence and Stark spectra are extremely sensitive to the assignment of the primary CT state, its energy, and coupling to the excited states. The best fit of the data is obtained supposing that the initial charge separation occurs within the special-pair PD1PD2. Additionally, the scheme with primary electron transfer from the accessory chlorophyll to pheophytin gave a reasonable quantitative fit. We show that the effectiveness of these two pathways is strongly dependent on the realization of the energetic disorder. Supposing a mixed scheme of primary charge separation with a disorder-controlled competition of the two channels, we can explain the coexistence of fast sub-ps and slow ps components of the Phe-anion formation as revealed by different ultrafast spectroscopic techniques.

Photosynthetic reaction center functionalized nano-composite films: effective strategies for probing and exploiting the photo-induced electron transfer of photosensitive membrane protein

Photosynthetic reaction center (RC), a robust transmembrane pigment-protein complex, works as the crucial component participating the primary event of the photo-electrochemical conversion in bacteria. Sparked by the high photo-induced charge separation yield (ca. 100%) of RC, great interests have been aroused to fabricate versatile RC-functionalized nano-composite films for exploring the initial photosynthetic electron transfer (ET) of RC, and thus exploiting well-designed bio-photoelectric converters. In this review, we classify and summarize the current status about the concepts and methods of constructing RC-immobilized nano-composite films or devices for probing the photo-induced ET, and applying to novel bioelectronics if it is possible.

Ultrafast transient absorption studies on photosystem I reaction centers from Chlamydomonas reinhardtii. 2: mutations near the P700 reaction center chlorophylls provide new insight into the nature of the primary electron donor

The energy transfer and charge separation kinetics in several core Photosystem I particles of Chlamydomonas reinhardtii with point mutations around the PA and PB reaction center chlorophylls (Chls) have been studied using ultrafast transient absorption spectroscopy in the femtosecond to nanosecond time range to characterize the influence on the early electron transfer processes. The data have been analyzed in terms of kinetic compartment models. The adequate description of the transient absorption kinetics requires three different radical pairs in the time range up to approximately 100 ps. Also a charge recombination process from the first radical pair back to the excited state is present in all the mutants, as already shown previously for the wild-type (Müller, M. G., J. Niklas, W. Lubitz, and A. R. Holzwarth. 2003. Biophys. J. 85:3899-3922; and Holzwarth, A. R., M. G. Müller, J. Niklas, and W. Lubitz. 2005. J. Phys. Chem. B. 109:5903-59115). In all mutants, the primary charge separation occurs with the same effective rate constant within the error limits as in the wild-type (>>350 ns(-1)), which implies an intrinsic rate constant of charge separation of <1 ps(-1). The rate constant of the secondary electron transfer process is slowed down by a factor of approximately 2 in the mutant B-H656C, which lacks the ligand to the central metal of Chl PB. For the mutant A-T739V, which breaks the hydrogen bond to the keto carbonyl of Chl PA, only a slight slowing down of the secondary electron transfer is observed. Finally for mutant A-W679A, which has the Trp near the PA Chl replaced, either no pronounced effect or, at best, a slight increase on the secondary electron transfer rate constants is observed. The effective charge recombination rate constant is modified in all mutants to some extent, with the strongest effect observed in mutant B-H656C. Our data strongly suggest that the Chls of the PA and PB pair, constituting what is traditionally called the "primary electron donor P700", are not oxidized in the first electron transfer process, but rather only in the secondary electron transfer step. We thus propose a new electron transfer mechanism for Photosystem I where the accessory Chl(s) function as the primary electron donor(s) and the A0 Chl(s) are the primary electron acceptor(s). This new mechanism also resolves in a straightforward manner the difficulty with the previous mechanism, where an electron would have to overcome a distance of approximately 14 A in <1 ps in a single step. If interpreted within a scheme of single-sided electron transfer, our data suggest that the B-branch is the active branch, although parallel A-branch activity cannot be excluded. All the mutations do affect to a varying extent the energy difference between the reaction center excited state RC* and the first radical pair and thus affect the rate constant of charge recombination. It is interesting to note that the new mechanism proposed is in fact analogous to the electron transfer mechanism in Photosystem II, where the accessory Chl also plays the role of the primary electron donor, rather than the special Chl pair P680 (Prokhorenko, V. and A. R. Holzwarth. 2000. J. Phys. Chem. B. 104:11563-11578).

Excitonic States in Photosystem II Reaction Center

The excited states of a structurally well-determined photosystem II (PSII) reaction center are obtained using an effective Hamiltonian for the interaction between the Q(y) states. The latter are calculated using the time-dependent density functional theory (DFT) method in DFT-optimized geometries, but with conserved side group orientations. Of particular importance is the orientation of the vinyl group of ring I. Couplings are calculated using actual transition charge distributions via the INDO/S model. Good agreement with experimental spectra is obtained. The lowest excited state is mainly located on the inactive B-side, but with a large component on P(A) too, making charge separation to H(A) possible at low temperature. The "trap state" and triplet state are localized on the inactive B-side. Since the spin singlet Q(y) states of the reaction center are all within a rather small energy range, the state with the highest component of B(A)*, on the blue side of the Q(y) absorption, has a rather high Boltzmann population at room temperature. The charge-transfer states, however, have a rather large spread and cannot be calculated accurately at present. The orientation of the phytyl chains is important and has as a consequence that the energy for the charge-separated B(A)+ H(A)- state is significantly lower than the corresponding state on the B-side. It follows that the B(A)* and P(A)* states are both possible origins for a fast charge separation in PSII.

EPR characterisation of the triplet state in photosystem II reaction centers with singly reduced primary acceptor Q(A)

The triplet states of photosystem II core particles from spinach were studied using time-resolved cw EPR technique at different reduction states of the iron--quinone complex of the reaction center primary electron acceptor. With doubly reduced primary acceptor, the well-known photosystem II triplet state characterised by zero-field splitting parameters |D|=0.0286 cm(-1), |E|=0.0044 cm(-1) was detected. When the primary acceptor was singly reduced either chemically or photochemically, a triplet state of a different spectral shape was observed, bearing the same D and E values and characteristic spin polarization pattern arising from RC radical pair recombination. The latter triplet state was strongly temperature dependent disappearing at T=100 K, and had a much faster decay than the former one. Based on its properties, this triplet state was also ascribed to the photosystem II reaction center. A sequence of electron-transfer events in the reaction centers is proposed that explains the dependence of the triplet state properties on the reduction state of the iron--quinone primary acceptor complex.

Initial electron donor and acceptor in isolated Photosystem II reaction centers identified with femtosecond mid-IR spectroscopy

Despite the apparent similarity between the plant Photosystem II reaction center (RC) and its purple bacterial counterpart, we show in this work that the mechanism of charge separation is very different for the two photosynthetic RCs. By using femtosecond visible-pump-mid-infrared probe spectroscopy in the region of the chlorophyll ester and keto modes, between 1,775 and 1,585 cm(-1), with 150-fs time resolution, we show that the reduction of pheophytin occurs on a 0.6- to 0.8-ps time scale, whereas P+, the precursor state for water oxidation, is formed after approximately 6 ps. We conclude therefore that in the Photosystem II RC the primary charge separation occurs between the "accessory chlorophyll" Chl(D1) and the pheophytin on the so-called active branch.

Pathways and timescales of primary charge separation in the photosystem II reaction center as revealed by a simultaneous fit of time-resolved fluorescence and transient absorption

We model the dynamics of energy transfer and primary charge separation in isolated photosystem II (PSII) reaction centers. Different exciton models with specific site energies of the six core pigments and two peripheral chlorophylls (Chls) in combination with different charge transfer schemes have been compared using a simultaneous fit of the absorption, linear dichroism, circular dichroism, steady-state fluorescence, transient absorption upon different excitation wavelengths, and time-resolved fluorescence. To obtain a quantitative fit of the data we use the modified Redfield theory, with the experimental spectral density including coupling to low-frequency phonons and 48 high-frequency vibrations. The best fit has been obtained with a model implying that the final charge separation occurs via an intermediate state with charge separation within the special pair (RP(1)). This state is weakly dipole-allowed, due to mixing with the exciton states, and can be populated directly or via 100-fs energy transfer from the core-pigments. The RP(1) and next two radical pairs with the electron transfer to the accessory Chl (RP(2)) and to the pheophytin (RP(3)) are characterized by increased electron-phonon coupling and energetic disorder. In the RP(3) state, the hole is delocalized within the special pair, with a predominant localization at the inactive-branch Chl. The intrinsic time constants of electron transfer between the three radical pairs vary from subpicoseconds to several picoseconds (depending on the realization of the disorder). The equilibration between RP(1) and RP(2) is reached within 5 ps at room temperature. During the 5-100-ps period the equilibrated core pigments and radical pairs RP(1) and RP(2) are slowly populated from peripheral chlorophylls and depopulated due to the formation of the third radical pair, RP(3). The effective time constant of the RP(3) formation is 7.5 ps. The calculated dynamics of the pheophytin absorption at 545 nm displays an instantaneous bleach (30% of the total amplitude) followed by a slow increase of the bleaching amplitude with time constants of 15 and 12 ps for blue (662 nm) and red (695 nm) excitation, respectively.

Evidence for localisation of accumulated chlorophyll cation on the D1-accessory chlorophyll in the reaction centre of photosystem II

Absorption and circular dichroism spectra of Photosystem II (PS II) reaction centres (RC) were studied and compared with spectra calculated on the basis of point-dipole approximation. Chlorophyll cation was accumulated during a light treatment of PS II RC in the presence of artificial electron acceptor silicomolybdate. Light-induced difference spectra and their calculated counterparts revealed the location of accumulated cation at the accessory chlorophyll of the D1 protein subunit.

Photoelectric performance of bacteria photosynthetic proteins entrapped on tailored mesoporous WO3-TiO2 films

Novel three-dimensional wormlike mesoporous WO(3)-TiO(2) films with tailored pore size (approximately 7.1 nm) were applied to prepare the bio-photoelectrodes (Bio-PEs) through direct entrapping the bacteria photosynthetic reaction center (RC) proteins. These mesoporous WO(3)-TiO(2) films exhibited unique characteristics in the specific loading of RC with high activity retained. Moreover, well-matched energy levels of WO(3)-TiO(2) and RC contributed to the photoelectric performance, especially in the red to near-infrared (NIR) region, of the derived Bio-PEs. Such strategy of manipulating the Bio-PEs based on well-designed mesoporous metal oxides and RC provides an alternative system to probe the photoinduced multiple-pathway electron transfer of photosensitive chromophores, which may open a new perspective to develop versatile bio-photoelectric devices.

Tuning electron transfer by ester-group of chlorophylls in bacterial photosynthetic reaction center

Accessory chlorophylls (B(A/B)) in bacterial photosynthetic reaction center play a key role in charge-separation. Although light-exposed and dark-adapted bRC crystal structures are virtually identical, the calculated B(A) redox potentials for one-electron reduction differ. This can be traced back to different orientations of the B(A) ester-group. This tuning ability of chlorophyll redox potentials modulates the electron transfer from SP* to B(A).

Theory of optical spectra of photosystem II reaction centers: location of the triplet state and the identity of the primary electron donor

Based on the structural analysis of photosystem II of Thermosynechococcus elongatus, a detailed calculation of optical properties of reaction-center (D1-D2) complexes is presented applying a theory developed previously. The calculations of absorption, linear dichroism, circular dichroism, fluorescence spectra, all at 6 K, and the temperature-dependence of the absorption spectrum are used to extract the local optical transition energies of the reaction-center pigments, the so-called site energies, from experimental data. The site energies are verified by calculations and comparison with seven additional independent experiments. Exciton relaxation and primary electron transfer in the reaction center are studied using the site energies. The calculations are used to interpret transient optical data. Evidence is provided for the accessory chlorophyll of the D1-branch as being the primary electron donor and the location of the triplet state at low temperatures.

Energy and electron transfer in photosystem II reaction centers with modified pheophytin composition

Energy and electron transfer in Photosystem II reaction centers in which the photochemically inactive pheophytin had been replaced by 13(1)-deoxo-13(1)-hydroxy pheophytin were studied by femtosecond transient absorption-difference spectroscopy at 77 K and compared to the dynamics in untreated reaction center preparations. Spectral changes induced by 683-nm excitation were recorded both in the Q(Y) and in the Q(X) absorption regions. The data could be described by a biphasic charge separation. In untreated reaction centers the major component had a time constant of 3.1 ps and the minor component 33 ps. After exchange, time constants of 0.8 and 22 ps were observed. The acceleration of the fast phase is attributed in part to the redistribution of electronic transitions of the six central chlorin pigments induced by replacement of the inactive pheophytin. In the modified reaction centers, excitation of the lowest energy Q(Y) transition produces an excited state that appears to be localized mainly on the accessory chlorophyll in the active branch (B(A) in bacterial terms) and partially on the active pheophytin H(A). This state equilibrates in 0.8 ps with the radical pair. B(A) is proposed to act as the primary electron donor also in untreated reaction centers. The 22-ps (pheophytin-exchanged) or 33-ps (untreated) component may be due to equilibration with the secondary radical pair. Its acceleration by H(B) exchange is attributed to a faster reverse electron transfer from B(A) to. After exchange both and are nearly isoenergetic with the excited state.

Bridging the gap between structural and lattice models: a parameterization of energy transfer and trapping in Photosystem I

In the absence of an accurate structural model, the excited state dynamics of energy-transferring systems are often modeled using lattice models. To demonstrate the validity and other potential merits of such an approach we present the results of the modeling of the energy transfer and trapping in Photosystem I based upon the 2.5 A structural model, and show that these results can be reproduced in terms of a lattice model with only a few parameters. It has recently been shown that at room temperature the dynamics of a hypothetical Photosystem I particle, not containing any red chlorophylls (chls), are characterized by a longest (trapping) lifetime of 18 ps. The structure-based modeling of the dynamics of this particle yields an almost linear relationship between the possible values of the intrinsic charge-separation time at P700, 1/gamma, and the average single-site lifetime in the antenna, tauss. Lattice-based modeling, using the approach of a perturbed two-level model, reproduces this linear relation between tauss and 1/gamma. Moreover, this approach results in a value of the (modified) structure-function corresponding to a structure exhibiting a mixture of the characteristics of both a square and a cubic lattice, consistent with the structural model. These findings demonstrate that the lattice model describes the dynamics of the system appropriately. In the lattice model, the total trapping time is the sum of the delivery time to the reaction center and the time needed to quench the excitation after delivery. For the literature value of tauss=150 fs, both these times contribute almost equally to the total trapping time of 18 ps, indicating that the system is neither transfer- nor trap-limited. The value of approximately 9 ps for the delivery time is basically equal to the excitation-transfer time from the bulk chls to the red chls in Synechococcus elongatus, indicating that energy transfer from the bulk to the reaction center and to the red chls are competing processes. These results are consistent with low-temperature time-resolved and steady-state fluorescence measurements. We conclude that lattice models can be used to describe the global energy-transfer properties in complex chromophore networks, with the advantage that such models deal with only a few global, intuitive parameters rather than the many microscopic parameters obtained in structure-based modeling.

Ultrafast transient absorption studies on Photosystem I reaction centers from Chlamydomonas reinhardtii. 1. A new interpretation of the energy trapping and early electron transfer steps in Photosystem I

The energy transfer and charge separation kinetics in core Photosystem I (PSI) particles of Chlamydomonas reinhardtii has been studied using ultrafast transient absorption in the femtosecond-to-nanosecond time range. Although the energy transfer processes in the antenna are found to be generally in good agreement with previous interpretations, we present evidence that the interpretation of the energy trapping and electron transfer processes in terms of both kinetics and mechanisms has to be revised substantially as compared to current interpretations in the literature. We resolved for the first time i), the transient difference spectrum for the excited reaction center state, and ii), the formation and decay of the primary radical pair and its intermediate spectrum directly from measurements on open PSI reaction centers. It is shown that the dominant energy trapping lifetime due to charge separation is only 6-9 ps, i.e., by a factor of 3 shorter than assumed so far. The spectrum of the first radical pair shows the expected strong bleaching band at 680 nm which decays again in the next electron transfer step. We show furthermore that the early electron transfer processes up to approximately 100 ps are more complex than assumed so far. Several possibilities are discussed for the intermediate redox states and their sequence which involve oxidation of P700 in the first electron transfer step, as assumed so far, or only in the second electron transfer step, which would represent a fundamental change from the presently assumed mechanism. To explain the data we favor the inclusion of an additional redox state in the electron transfer scheme. Thus we distinguish three different redox intermediates on the timescale up to 100 ps. At this level no final conclusion as to the exact mechanism and the nature of the intermediates can be drawn, however. From comparison of our data with fluorescence kinetics in the literature we also propose a reversible first charge separation step which has been excluded so far for open PSI reaction centers. For the first time an ultrafast 150-fs equilibration process, occurring among exciton states in the reaction center proper, upon direct excitation of the reaction center at 700 nm, has been resolved. Taken together the data call for a fundamental revision of the present understanding of the energy trapping and early electron transfer kinetics in the PSI reaction center. Due to the fact that it shows the fastest trapping time observed so far of any intact PSI particle, the PSI core of C. reinhardtii seems to be best suited to further characterize the electron transfer steps and mechanisms in the reaction center of PSI.

Modeling charge transfer in the photosynthetic reaction center

In this work, we present a model to elucidate the unidirectionality of the primary charge-separation process in the bacterial reaction centers. We have used a model of three sites/molecules with electron transfer beginning at site 1 with an option to proceed to site 2 or site 3. We used a stochastic model with arbitrary correlation functions. We get the quantum yields of electron escape via the sites 2,3 in two limiting cases that correspond to a spectral density of underdamped and overdamped Brownian oscillator. In the fast modulation limit of an overdamped regime we get the effect, which was named "fear of death," in which for strong enough sink parameters the electron has a tendency to avoid the place with greater sink. The presented model was used to provide a plausible explanation of the temperature dependence of the quantum yields of the Rhodobacter sphaeroides photosynthetic reaction center in the high-temperature regime.

Length, time, and energy scales of photosystems

The design of photosynthetic systems reflects the length scales of the fundamental physical processes. Energy transfer is rapid at the few angstrom scale and continues to be rapid even at the 50-A scale of the membrane thickness. Electron tunneling is nearly as rapid at the shortest distances, but becomes physiologically too slow well before 20 A. Diffusion, which starts out at a relatively slow nanosecond time scale, has the most modest slowing with distance and is physiologically competent at all biologically relevant distances. Proton transfer always operates on the shortest angstrom scale. The structural consequences of these distance dependencies are that energy transfer networks can extend over large, multisubunit and multicomplex distances and take leaps of 20 A before entering the domain of charge separating centers. Electron transfer systems are effectively limited to individual distances of 15 A or less and span the 50 A dimensions of the bioenergetic membrane by use of redox chains. Diffusion processes are generally used to cover the intercomplex electron transfer distances of 50 A and greater and tend to compensate for the lack of directionality by restricting the diffusional space to the membrane or the membrane surface, and by multiplying the diffusing species through the use of pools. Proton transfer reactions act over distances larger than a few angstroms through the use of clusters or relays, which sometimes rely on water molecules and which may only be dynamically assembled. Proteins appear to place a premium on robustness of design, which is relatively easily achieved in the long-distance physical processes of energy transfer and electron tunneling. By placing cofactors close enough, the physical process is relatively rapid compared to decay processes. Thus suboptimal conditions such as cofactor orientation, energy level, or redox potential level can be tolerated and generally do not have to be finely tuned. The most fragile regions of design tend to come in areas of complex formation and catalysis involving proton management, where relatively small changes in distance or mutations can lead to a dramatic decrease in turnover, which may already be limiting the overall speed of energy conversion in these proteins. Light-activated systems also face a challenge to robust function from the ever-present dangers of high redox potential chemistry. This can turn the protein matrix and wandering oxygen molecules into unintentional redox partners, which in the case of PSII requires the frequent, costly replacement of protein subunits.

Electric field effects on the chlorophylls, pheophytins, and beta-carotenes in the reaction center of photosystem II

We present an electric field modulated absorption spectroscopy (Stark effect) study of isolated photosystem II reaction center complexes, including a preparation in which the inactive pheophytin H(B) was exchanged for 13(1)-deoxo-13(1)-hydroxy-pheophytin. The results reveal that the Stark spectrum of the Q(x) and Q(y) transitions of the pheophytins has a second-derivative line shape, indicating that the Stark effect is dominated by differences in the dipole moment between the ground and the electronically excited states of these transitions (Delta mu). The Delta mu values for the Q(x) and Q(y) transitions of H(B) are small (Delta mu = 0.6-1.0 D f(-1)), whereas that of the Q(x) transition of the active pheophytin H(A) is remarkably large (Delta mu = 3 D f(-1)). The Stark spectrum of the red-most absorbing pigments also shows a second-derivative line shape, but this spectrum is considerably red-shifted as compared to the second derivative of the absorption spectrum. This situation is unusual but has been observed before in heterodimer special pair mutants of purple bacterial reaction centers [Moore, L. J., Zhou, H., and Boxer, S. G. (1999) Biochemistry 38, 11949-11960]. The red-shifted Stark spectra can be explained by a mixing of exciton states with a charge-transfer state of about equal energy. We conclude that the charge transfer state involves H(A) and its immediate chlorophyll neighbor (B(A)), and we suggest that this (B(A)(delta+)H(A)(delta-)) charge transfer state plays a crucial role in the primary charge separation reaction in photosystem II. In contrast to most other carotenes, the two beta-carotene molecules of the photosystem II reaction center display a very small Delta mu, which can most easily be explained by excitonic coupling of both molecules. These results favor a model that locates both beta-carotene molecules at the same side of the complex.

Energy equilibration and primary charge separation in chlorophyll d-based photosystem I reaction center isolated from Acaryochloris marina

Primary photochemistry in photosystem I (PS I) reaction center complex from Acaryochloris marina that uses chlorophyll d instead of chlorophyll a has been studied with a femtosecond spectroscopy. Upon excitation at 630 nm, almost full excitation equilibration among antenna chlorophylls and 40% of the excitation quenching by the reaction center are completed with time constants of 0.6(+/-0.1) and 4.9(+/-0.6) ps, respectively. The rise and decay of the primary charge-separated state proceed with apparent time constants of 7.2(+/-0.9) and 50(+/-10) ps, suggesting the reduction of the primary electron acceptor chlorophyll (A(0)) and its reoxidation by phylloquinone (A(1)), respectively.

Excitonic interactions in the reaction centre of photosystem II studied by using circular dichroism

Changes in excitonic interactions of photosystem II (PSII) reaction centre (RC) pigments upon light-induced oxidation of primary donor (P680) or reduction of primary acceptor (pheophytin (Pheo)) were analysed using circular dichroism (CD). The CD spectrum of PSII RC shows positive bands at 417, 435 and 681 and negative bands at 447 and 664 nm. Oxidation of the primary donor by illuminating the sample in the presence of silicomolybdate resulted in nearly symmetric decrease of CD amplitudes at 664 and 684 nm. In the Soret region, the maximum bleaching of CD signal was detected at 449 and 440 nm. Accumulation of reduced Pheo in the presence of dithionite brought about much lower changes in CD amplitudes than P680 oxidation. In this case, only a small asymmetric bleaching at 680 and 668 nm in the red region and a bleaching at 445, 435 and 416 nm in the Soret region has been detected. Therefore, we suppose that the contribution of the Pheo of the primary acceptor to the total CD signal of RC is negligible. In contrast to the oxidation of primary donor, the light-induced change in the CD spectrum upon primary acceptor reduction was strongly temperature-dependent. The reversible CD bleaching was completely inhibited below 200 K, although the reduced Pheo was accumulated even at a temperature of 77 K. Since the temperature does not influence the excitonic interaction, the temperature dependence of the CD changes upon Pheo reduction does not support the model of Pheo excitonically interacting with the other chlorophylls (Chl) of the RC. We propose that Pheo should not be considered as a part of a multimer model.