The Photosynthetic Reaction Center from the Purple Bacterium Rhodopseudomonas viridis

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.

Neutron and light scattering studies of light-harvesting photosynthetic antenna complexes

Small-angle neutron scattering (SANS) and dynamic light scattering (DLS) have been employed in studying the structural information of various biological systems, particularly in systems without high-resolution structural information available. In this report, we briefly present some principles and biological applications of neutron scattering and DLS, compare the differences in information that can be obtained with small-angle X-ray scattering (SAXS), and then report recent studies of SANS and DLS, together with other biophysical approaches, for light-harvesting antenna complexes and reaction centers of purple and green phototrophic bacteria.

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.

Synthesis and electrochemical investigation of covalently linked porphyrin dimers containing a β-brominated subunit

Ten meso-tetraphenylporphyrin-type heterodimers containing a partly or completely β-brominated subunit were synthesized and characterized by UV-visible spectroscopy, cyclic voltammetry and spectroelectrochemistry, showing the presence of low electronic interactions between the two subunits. The investigated compounds are represented as M [( tripp - tpp ( Br 4)] M and M [ tripp - tpp ( Br 8)] M ( M = 2 H , Zn , Ni , Co and Cu ) where tripp - tpp ( Br 4) is the tetraanion of 1-[5-(10,15,20-triphenylporphyrinyl)]-4-[10-(2,3,12,13-tetrabromoporphyrinyl)]-benzene and tripp - tpp ( Br 8) is the tetraanion of 1-[5-(10,15,20-triphenylporphyrinyl)]-4-[10-(2,3,7,8,12,13,17,18-octabromoporphyrinyl)]-benzene. One of the synthesized dimers, H 2[ tripp - tpp ( Br 8)] H 2, was characterized by a single-crystal X-ray investigation.

Photophysical properties of composite film of dendron-appended porphyrin and fullerene[60]

Dendron-appended porphyrin (den-por) and C 60 (den- C 60) were synthesized for the application of photovoltaic materials. The composite films of den-por and den- C 60 were subjected to absorption and photoluminescence spectroscopic measurements. We determined that solid-state film, spin-coated with a blend of den-por and den- C 60 at weight ratio 1:0.5, showed an efficient photoluminescence quenching via donor-to-acceptor electron transfer.

Comparison of the photophysical properties of a planar, PtOEP, and a nonplanar, PtOETPP, porphyrin in solution and doped films

The absorption and emission spectroscopic properties of planar (2,3,7,8,12,13,17,18-octaethylporphyrinato)platinum(II) (PtOEP) and nonplanar (2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrinato)platinum(II) (PtOETPP) complexes have been studied at room temperature. Liquid solutions and doped films, in polystyrene (PS) and epoxy (EPO) polymers, have been investigated. In dilute liquid solution, the photophysical properties of the nonplanar complex are substantially perturbed compared to the planar analogue. Strong ligating solvents further affect the photophysical behavior of both Pt(II) complexes via axial ligation to the central metal ion. At high concentrations, ground state aggregation and excimer formation is observed for PtOEP films in PS and EPO hosts. Incorporation of the nonplanar PtOETPP complex in PS results in enhanced coplanarity of the meso-phenyl groups, leading to a more extended conjugation between the meso-substituents and the π-conjugated system of the macrocycle. A more planar conformer for the nonplanar PtOETPP is present in the EPO host.

Proton coupled electron transfer and redox active tyrosines in Photosystem II

In this article, progress in understanding proton coupled electron transfer (PCET) in Photosystem II is reviewed. Changes in acidity/basicity may accompany oxidation/reduction reactions in biological catalysis. Alterations in the proton transfer pathway can then be used to alter the rates of the electron transfer reactions. Studies of the bioenergetic complexes have played a central role in advancing our understanding of PCET. Because oxidation of the tyrosine results in deprotonation of the phenolic oxygen, redox active tyrosines are involved in PCET reactions in several enzymes. This review focuses on PCET involving the redox active tyrosines in Photosystem II. Photosystem II catalyzes the light-driven oxidation of water and reduction of plastoquinone. Photosystem II provides a paradigm for the study of redox active tyrosines, because this photosynthetic reaction center contains two tyrosines with different roles in catalysis. The tyrosines, YZ and YD, exhibit differences in kinetics and midpoint potentials, and these differences may be due to noncovalent interactions with the protein environment. Here, studies of YD and YZ and relevant model compounds are described.

Light-induced quinone reduction in photosystem II

The photosystem II core complex is the water:plastoquinone oxidoreductase of oxygenic photosynthesis situated in the thylakoid membrane of cyanobacteria, algae and plants. It catalyzes the light-induced transfer of electrons from water to plastoquinone accompanied by the net transport of protons from the cytoplasm (stroma) to the lumen, the production of molecular oxygen and the release of plastoquinol into the membrane phase. In this review, we outline our present knowledge about the "acceptor side" of the photosystem II core complex covering the reaction center with focus on the primary (Q(A)) and secondary (Q(B)) quinones situated around the non-heme iron with bound (bi)carbonate and a comparison with the reaction center of purple bacteria. Related topics addressed are quinone diffusion channels for plastoquinone/plastoquinol exchange, the newly discovered third quinone Q(C), the relevance of lipids, the interactions of quinones with the still enigmatic cytochrome b559 and the role of Q(A) in photoinhibition and photoprotection mechanisms. This article is part of a Special Issue entitled: Photosystem II.

Structural basis of cyanobacterial photosystem II Inhibition by the herbicide terbutryn

Herbicides that target photosystem II (PSII) compete with the native electron acceptor plastoquinone for binding at the Q(B) site in the D1 subunit and thus block the electron transfer from Q(A) to Q(B). Here, we present the first crystal structure of PSII with a bound herbicide at a resolution of 3.2 Å. The crystallized PSII core complexes were isolated from the thermophilic cyanobacterium Thermosynechococcus elongatus. The used herbicide terbutryn is found to bind via at least two hydrogen bonds to the Q(B) site similar to photosynthetic reaction centers in anoxygenic purple bacteria. Herbicide binding to PSII is also discussed regarding the influence on the redox potential of Q(A), which is known to affect photoinhibition. We further identified a second and novel chloride position close to the water-oxidizing complex and in the vicinity of the chloride ion reported earlier (Guskov, A., Kern, J., Gabdulkhakov, A., Broser, M., Zouni, A., and Saenger, W. (2009) Nat. Struct. Mol. Biol. 16, 334-342). This discovery is discussed in the context of proton transfer to the lumen.

Carboxylate shifts steer interquinone electron transfer in photosynthesis

Understanding the mechanisms of electron transfer (ET) in photosynthetic reaction centers (RCs) may inspire novel catalysts for sunlight-driven fuel production. The electron exit pathway of type II RCs comprises two quinone molecules working in series and in between a non-heme iron atom with a carboxyl ligand (bicarbonate in photosystem II (PSII), glutamate in bacterial RCs). For decades, the functional role of the iron has remained enigmatic. We tracked the iron site using microsecond-resolution x-ray absorption spectroscopy after laser-flash excitation of PSII. After formation of the reduced primary quinone, Q(A)(-), the x-ray spectral changes revealed a transition (t½ ≈ 150 μs) from a bidentate to a monodentate coordination of the bicarbonate at the Fe(II) (carboxylate shift), which reverted concomitantly with the slower ET to the secondary quinone Q(B). A redox change of the iron during the ET was excluded. Density-functional theory calculations corroborated the carboxylate shift both in PSII and bacterial RCs and disclosed underlying changes in electronic configuration. We propose that the iron-carboxyl complex facilitates the first interquinone ET by optimizing charge distribution and hydrogen bonding within the Q(A)FeQ(B) triad for high yield Q(B) reduction. Formation of a specific priming intermediate by nuclear rearrangements, setting the stage for subsequent ET, may be a common motif in reactions of biological redox cofactors.

Excited-state vibrational coherence in methanol solution of Zn(II) tetrakis(N-methylpyridyl)porphyrin: charge-dependent intermolecular mode frequencies and implications for electron-transfer dynamics in photosynthetic reaction centers

The nature of the intermolecular vibrational modes between the redox-active chromophores and the protein medium in the photosynthetic reaction center is central to an understanding of the structural origin of the quantum efficiency of the light-driven charge-separation reactions that result in storage of solar energy. In recent work on this issue, we have characterized the low-frequency vibrational coherence of Zn(II) meso-tetrakis(N-methylpyridyl)porphyrin (ZnTMPyP) and compared it to that from bacteriochlorophyll a in polar solution and in the small light-harvesting subunits B820 and B777. The charge-transfer character of ZnTMPyP's π* excited states afford us the opportunity to characterize how the intermolecular vibrational modes and potential with the surrounding medium are affected by the charge on the porphyrin macrocycle. The excited-state vibrational coherence observed with Q-band (S(1) state) excitation at 625 nm of ZnTMPyP in methanol solution contains dominant contributions from a pair of rapidly damped (effective damping time γ < 400 fs) components that are assigned to the hindered translational and librational porphyrin-solvent intermolecular modes. The 256 cm(-1) mean frequency of the intermolecular modes is significantly higher than that observed previously in the ground state, 79 cm(-1), with Soret-band excitation at 420 nm [Dillman et al., J. Phys. Chem. B. 2009, 113, 6127-6139]. The increased mode frequency arises from the activation of the ion-dipole and ion-induced-dipole terms in the intermolecular potential. In the ground state, the π-electron density of ZnTMPyP is mostly confined to the region of the porphyrin macrocycle. In the excited state, the π-electron density is extensively delocalized from the porphyrin out to two of the peripheral N-methylpyridyl rings, each of which carries a single formal charge. The charge-dependent terms contribute to a significant stabilization of the equilibrium geometry of the porphyrin-solvent complex in the excited state. In the photosynthetic reaction center, these terms will play an important role in trapping the charged products of the forward, charge-separation reactions, and the location of the bacteriopheophytin acceptor in a nonpolar region of the structure enhances the rate of the secondary charge-separation reaction.

Biogenesis of bacterial inner-membrane proteins

All cells must traffic proteins into and across their membranes. In bacteria, several pathways have evolved to enable protein transfer across the inner membrane, the periplasm, and the outer membrane. The major route of protein translocation in and across the cytoplasmic membrane is the general secretion pathway (Sec-pathway). The biogenesis of membrane proteins not only requires protein translocation but also coordinated targeting to the membrane beforehand and folding and assembly into their protein complexes afterwards to function properly in the cell. All these processes are responsible for the biogenesis of membrane proteins that mediate essential functions of the cell such as selective transport, energy conversion, cell division, extracellular signal sensing, and motility. This review will highlight the most recent developments on the structure and function of bacterial membrane proteins, focusing on the journey that integral membrane proteins take to find their final destination in the inner membrane.

The semiquinone-iron complex of photosystem II: structural insights from ESR and theoretical simulation; evidence that the native ligand to the non-heme iron is carbonate

The semiquinone-iron complex of photosystem II was studied using electron spin resonance (ESR) spectroscopy and density functional theory calculations. Two forms of the signal were investigated: 1), the native g approximately 1.9 form; and 2), the g approximately 1.84 form, which is well known in purple bacterial reaction centers and occurs in photosystem II when treated with formate. The g approximately 1.9 form shows low- and high-field edges at g approximately 3.5 and g < 0.8, respectively, and resembles the g approximately 1.84 form in terms of shape and width. Both types of ESR signal were simulated using the theoretical approach used previously for the BRC complex, a spin Hamiltonian formalism in which the semiquinone radical magnetically interacts (J approximately 1 cm(-1)) with the nearby high-spin Fe(2+). The two forms of ESR signal differ mainly by an axis rotation of the exchange coupling tensor (J) relative to the zero-field tensor (D) and a small increase in the zero-field parameter D ( approximately 6 cm(-1)). Density functional theory calculations were conducted on model semiquinone-iron systems to identify the physical nature of these changes. The replacement of formate (or glutamate in the bacterial reaction centers) by bicarbonate did not result in changes in the coupling environment. However, when carbonate (CO(3)(2-)) was used instead of bicarbonate, the exchange and zero-field tensors did show changes that matched those obtained from the spectral simulations. This indicates that 1), the doubly charged carbonate ion is responsible for the g approximately 1.9 form of the semiquinone-iron signal; and 2), carbonate, rather than bicarbonate, is the ligand to the iron.