Electron Transfer In Reaction Centers Research Articles

Two pathways to understanding electron transfer in reaction centers from photosynthetic bacteria: A comparison of Rhodobacter sphaeroides and Rhodobacter capsulatus mutants

The rates, yields, mechanisms and directionality of electron transfer (ET) are explored in twelve pairs of Rhodobacter (R.) sphaeroides and R. capsulatus mutant RCs designed to defeat ET from the excited primary donor (P*) to the A-side cofactors and re-direct ET to the normally inactive mirror-image B-side cofactors. In general, the R. sphaeroides variants have larger P+HB− yields (up to ∼90%) than their R. capsulatus analogs (up to ∼60%), where HB is the B-side bacteriopheophytin. Substitution of Tyr for Phe at L-polypeptide position L181 near BB primarily increases the contribution of fast P* → P+BB− → P+HB− two-step ET, where BB is the “bridging” B-side bacteriochlorophyll. The second step (∼6–8 ps) is slower than the first (∼3–4 ps), unlike A-side two-step ET (P* → P+BA− → P+HA−) where the second step (∼1 ps) is faster than the first (∼3–4 ps) in the native RC. Substitutions near HB, at L185 (Leu, Trp or Arg) and at M-polypeptide site M133/131 (Thr, Val or Glu), strongly affect the contribution of slower (20–50 ps) P* → P+HB− one-step superexchange ET. Both ET mechanisms are effective in directing electrons “the wrong way” to HB and both compete with internal conversion of P* to the ground state (∼200 ps) and ET to the A-side cofactors. Collectively, the work demonstrates cooperative amino-acid control of rates, yields and mechanisms of ET in bacterial RCs and how A- vs. B-side charge separation can be tuned in both species.

Model quantification of the light-induced thylakoid membrane processes in Synechocystis sp. PCC 6803 in vivo and after exposure to radioactive irradiation

Measurements of OJIP-SMT patterns of fluorescence induction (FI) in Synechocystis sp. PCC 6803 (Synechocystis) cells on a time scale up to several minutes were mathematically treated within the framework of thylakoid membrane (T-M) model (Belyaeva et al., Photosynth Res 140:1-19, 2019) that was renewed to account for the state transitions effects. Principles of describing electron transfer in reaction centers of photosystems II and I (PSII and PSI) and cytochrome b6f complex remained unchanged, whereas parameters for dissipative reactions of non-radiative charge recombination were altered depending on the oxidation state of QB-site (neutral, reduced by one electron, empty, reduced by two electrons). According to our calculations, the initial content of plastoquinol (PQH2) in the total quinone pool of Synechocystis cells adapted to darkness for 10min ranged between 20 and 40%. The results imply that the PQ pool mediates photosynthetic and respiratory charge flows. The redistribution of PBS antenna units responsible for the increase of Chl fluorescence in cyanobacteria (qT2 → 1) upon state 2 → 1 transition or the fluorescence lowering (qT1 → 2) due to state 1 → 2 transition were described in the model by exponential functions. Parameters of dynamically changed effective cross section were found by means of simulations of OJIP-SMT patterns observed on Synechocystis cells upon strong (3000μmol photons m-2s-1) and moderate (1000μmol photons m-2s-1) actinic light intensities. The corresponding light constant values kLΣAnt = 1.2ms-1 and 0.4ms-1 define the excitation of total antenna pool dynamically redistributed between PSII and PSI reaction centers. Although the OCP-induced quenching of antenna excitation is not involved in the model, the main features of the induction signals have been satisfactorily explained. In the case of strong illumination, the effective cross section decreases by approximately 33% for irradiated Synechocystis cells as compared to untreated cells. Under moderate light, the irradiated Synechocystis cells showed in simulations the same cross section as the untreated cells. The thylakoid model renewed with state transitions description allowed simulation of fluorescence induction OJIP-SMT curves detected on time scale from microseconds to minutes.

Thermodynamic View of Proton Activated Electron Transfer in the Reaction Center of Photosynthetic Bacteria.

The temperature dependence of the sequential coupling of proton transfer to the second interquinone electron transfer is studied in the reaction center proteins of photosynthetic bacteria modified by different mutations and treatment by divalent cations. The Eyring plots of kinetics were evaluated by the Marcus theory of electron and proton transfer. In mutants of electron transfer limitation (including the wild type), the observed thermodynamic parameters had to be corrected for those of the fast proton pre-equilibrium. The electron transfer is nonadiabatic with transmission coefficient 6 × 10-4, and the reorganization energy amounts to 1.2 eV. If the proton transfer is the rate limiting step, the reorganization energy and the works terms fall in the range of 200-500 meV, depending on the site of damage in the proton transfer chain. The product term is 100-150 meV larger than the reactant term. While the electron transfer mutants have a low free energy of activation (∼200 meV), the proton transfer variants show significantly elevated levels of the free energy barrier (∼500 meV). The second electron transfer in the bacterial reaction center can serve as a model system of coupled electron and proton transfer in other proteins or ion channels.

Effect of Leucine M196 Substitution by Histidine on Electronic Structure of the Primary Electron Donor and Electron Transfer in Reaction Centers from Rhodobacter sphaeroides

In our recent X-ray study, we demonstrated that substitution of the natural leucine residue M196 with histidine in the reaction center (RC) from Rhodobacter (Rba.) sphaeroides leads to formation of a close contact between the genetically introduced histidine and the primary electron donor P (bacteriochlorophylls (BChls) PA and PB dimer) creating a novel pigment-protein interaction that is not observed in native RCs. In the present work, the possible nature of this novel interaction and its effects on the electronic properties of P and the photochemical charge separation in isolated mutant RCs L(M196)H are investigated at room temperature using steady-state absorption spectroscopy, light-induced difference FTIR spectroscopy, and femtosecond transient absorption spectroscopy. The results are compared with the data obtained for the RCs from Rba. sphaeroides pseudo-wild type strain. It is shown that the L(M196)H mutation results in a decrease in intensity and broadening of the long-wavelength Qy absorption band of P at ~865nm. Due to the mutation, there is also weakening of the electronic coupling between BChls in the radical cation P+ and increase in the positive charge localization on the PA molecule. Despite the significant perturbations of the electronic structure of P, the mutant RCs retain high electron transfer rates and quantum yield of the P+QA- state (QA is the primary quinone acceptor), which is close to the one observed in the native RCs. Comparison of our results with the literature data suggests that the imidazole group of histidine M196 forms a π-hydrogen bond with the π-electron system of the PB molecule in the P dimer. It is likely that the specific (T-shaped) spatial organization of the π-hydrogen interaction and its potential heterogeneity in relation to the bonding energy is, at least partially, the reason that this type of interaction between the protein and the pigment and quinone cofactors is not realized in the native RCs.

Both electronic and vibrational coherences are involved in primary electron transfer in bacterial reaction center

Understanding the mechanism behind the near-unity efficiency of primary electron transfer in reaction centers is essential for designing performance-enhanced artificial solar conversion systems to fulfill mankind’s growing demands for energy. One of the most important challenges is distinguishing electronic and vibrational coherence and establishing their respective roles during charge separation. In this work we apply two-dimensional electronic spectroscopy to three structurally-modified reaction centers from the purple bacterium Rhodobacter sphaeroides with different primary electron transfer rates. By comparing dynamics and quantum beats, we reveal that an electronic coherence with dephasing lifetime of ~190 fs connects the initial excited state, P*, and the charge-transfer intermediate {mathrm{P}}_{mathrm{A}}^ + {mathrm{P}}_{mathrm{B}}^ -; this {mathrm{P}}^ ast to {mathrm{P}}_{mathrm{A}}^ + {mathrm{P}}_{mathrm{B}}^ - step is associated with a long-lived quasi-resonant vibrational coherence; and another vibrational coherence is associated with stabilizing the primary photoproduct, {mathrm{P}}^ + {mathrm{B}}_{mathrm{A}}^ -. The results show that both electronic and vibrational coherences are involved in primary electron transfer process and they correlate with the super-high efficiency.

Geometric phase and quantum interference in photosynthetic reaction center: Regulation of electron transfer

Photosynthesis is driven by electron transfer in reaction centers in which the functional unit is composed of several simple molecules C2-symmetrically arranged into two branches. In view of quantum mechanism, both branches are possible pathways traversed by the transferred electron. Due to different evolution of spin state along two pathways in transmembrane electric potential (TEP), quantum state of the transferred electron at the bridged site acquires a geometric phase difference dependent on TEP, the most efficient electron transport takes place in a specific range of TEP beyond which electron transfer is dramatically suppressed. What’s more, reaction center acts like elaborately designed quantum device preparing polarized spin dependent on TEP for the transferred electron to regulate the reduction potential at bridged site. In brief, electron transfer generates the TEP, reversely, TEP modulates the efficiency of electron transfer. This may be an important approach to maintaining an appreciable pH environment in photosynthesis.

The rate of second electron transfer to QB− in bacterial reaction center of impaired proton delivery shows hydrogen-isotope effect

The 2nd electron transfer in reaction center of photosynthetic bacterium Rhodobacter sphaeroides is a two step process in which protonation of QB− precedes interquinone electron transfer. The thermal activation and pH dependence of the overall rate constants of different RC variants were measured and compared in solvents of water (H2O) and heavy water (D2O). The electron transfer variants where the electron transfer is rate limiting (wild type and M17DN, L210DN and H173EQ mutants) do not show solvent isotope effect and the significant decrease of the rate constant of the second electron transfer in these mutants is due to lowering the operational pKa of QB−/QBH: 4.5 (native), 3.9 (L210DN), 3.7 (M17DN) and 3.1 (H173EQ) at pH7. On the other hand, the proton transfer variants where the proton transfer is rate limiting demonstrate solvent isotope effect of pH-independent moderate magnitude (2.11±0.26 (WT+Ni2+), 2.16±0.35 (WT+Cd2+) and 2.34±0.44 (L210DN/M17DN)) or pH-dependent large magnitude (5.7 at pH4 (L213DN)). Upon deuteration, the free energy and the enthalpy of activation increase in all proton transfer variants by about 1kcal/mol and the entropy of activation becomes negligible in L210DN/M17DN mutant. The results are interpreted as manifestation of equilibrium and kinetic solvent isotope effects and the structural, energetic and kinetic possibility of alternate proton delivery pathways are discussed.

Primary Electron Transfer Reactions in Membrane-Bound Open and Closed Reaction Centers from Purple Bacterium Rhodobacter sphaeroides

Most ultrafast transient absorption studies of primary electron transfer in reaction centers from purple bacteria have been performed in complexes isolated from their natural lipid membrane environment using detergent. In this contribution we present near-UV-vis transient absorption studies of reaction centers embedded in their natural membrane environment. The evolution of absorption spectra recorded with subpicosecond resolution and re ecting primarily electron transfer reactions has been compared to data obtained previously for isolated reaction centers. We conclude that the overall spectral evolution in both types of samples is similar, and the environment of the reaction center protein has only a minor e ect on the primary electron transfer reactions. The di erences between the two samples are explained in terms of di erent energetic levels (and their di erent temporal evolution) of the two initial charge separated states PBA and P HA , with P being the primary electron donor and BA and HA the two consecutive electron acceptors. Additionally, in the electric eld generated by P HA , BA in membrane-bound reaction centers undergoes a stronger electrochromic shift than in isolated reaction centers.

The effects of protein crowding in bacterial photosynthetic membranes on the flow of quinone redox species between the photochemical reaction center and the ubiquinol-cytochrome c2 oxidoreductase

Atomic force microscopy (AFM) of the native architecture of the intracytoplasmic membrane (ICM) of a variety of species of purple photosynthetic bacteria, obtained at submolecular resolution, shows a tightly packed arrangement of light harvesting (LH) and reaction center (RC) complexes. Since there are no unattributed structures or gaps with space sufficient for the cytochrome bc(1) or ATPase complexes, they are localized in membrane domains distinct from the flat regions imaged by AFM. This has generated a renewed interest in possible long-range pathways for lateral diffusion of UQ redox species that functionally link the RC and the bc(1) complexes. Recent proposals to account for UQ flow in the membrane bilayer are reviewed, along with new experimental evidence provided from an analysis of intrinsic near-IR fluorescence emission that has served to test these hypotheses. The results suggest that different mechanism of UQ flow exist between species such as Rhodobacter sphaeroides, with a highly organized arrangement of LH and RC complexes and fast RC electron transfer turnover, and Phaeospirillum molischianum with a more random organization and slower RC turnover. It is concluded that packing density of the peripheral LH2 antenna in the Rba. sphaeroides ICM imposes constraints that significantly slow the diffusion of UQ redox species between the RC and cytochrome bc(1) complex, while in Phs. molischianum, the crowding of the ICM with LH3 has little effect upon UQ diffusion. This supports the proposal that in this type of ICM, a network of RC-LH1 core complexes observed in AFM provides a pathway for long-range quinone diffusion that is unaffected by differences in LH complex composition or organization.

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 285 K, 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 285 K. At 77 K, 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 + 390 mV vs. SHE. For the tetraheme Cyt two distinct midpoint potentials of + 85 and + 265 mV 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.

Primary electron transfer in reaction centers of YM210L and YM210L/HL168L mutants of Rhodobacter sphaeroides

The role of tyrosine M210 in charge separation and stabilization of separated charges was studied by analyzing of the femtosecond oscillations in the kinetics of decay of stimulated emission from P* and of a population of the primary charge separated state P(+)B(A)(-) in YM210L and YM210L/HL168L mutant reaction centers (RCs) of Rhodobacter sphaeroides in comparison with those in native Rba. sphaeroides RCs. In the mutant RCs, TyrM210 was replaced by Leu. The HL168L mutation placed the redox potential of the P(+)/P pair 123 mV below that of native RCs, thus creating a theoretical possibility of P(+)B(A)(-) stabilization. Kinetics of P* decay at 940 nm of both mutants show a significant slowing of the primary charge separation reaction in comparison with native RCs. Distinct damped oscillations in these kinetics with main frequency bands in the range of 90-150 cm(-1) reflect mostly nuclear motions inside the dimer P. Formation of a very small absorption band of B(A)(-) at 1020 nm is registered in RCs of both mutants. The formation of the B(A)(-) band is accompanied by damped oscillations with main frequencies from ~10 to ~150 cm(-1). Only a partial stabilization of the P(+)B(A)(-) state is seen in the YM210L/HL168L mutant in the form of a small non-oscillating background of the 1020-nm kinetics. A similar charge stabilization is absent in the YM210L mutant. A model of oscillatory reorientation of the OH-group of TyrM210 in the electric fields of P(+) and B(A)(-) is proposed to explain rapid stabilization of the P(+)B(A)(-) state in native RCs. Small oscillatory components at ~330-380 cm(-1) in the 1020-nm kinetics of native RCs are assumed to reflect this reorientation. We conclude that the absence of TyrM210 probably cannot be compensated by lowering of the P(+)B(A)(-) free energy that is expected for the double YM210L/HL168L mutant. An oscillatory motion of the HOH55 water molecule under the influence of P(+) and B(A)(-) is assumed to be another potential contributor to the mechanism of P(+)B(A)(-) stabilization.

Femtosecond stage of electron transfer in reaction centers of the triple mutant SL178K/GM203D/LM214H of Rhodobacter sphaeroides

Coherent processes in an initial phase of charge transfer in reaction centers (RCs) of the triple mutant S(L178)K/G(M203)D/L(M214)H of Rhodobacter sphaeroides were investigated by difference (light - dark) absorption spectroscopy with 18 fsec time resolution. Electron transfer in the B cofactor branch is activated in this mutant, while the A-branch electron transfer is slowed in comparison with native RCs of Rba. sphaeroides. A bulk of absorption difference spectra was analyzed in the 940-1060 nm range (stimulated emission of excited bacteriochlorophyll dimer P* and absorption of bacteriochlorophyll anions B(A)(-) and beta(-), where beta is a bacteriochlorophyll substituting the native bacteriopheophytin H(A)) and in the 735-775 nm range (bleaching of the absorption band of the bacteriopheophytin H(B) in the B-branch) in the -0.1 to 4 psec range of delays with respect to the moment of photoexcitation of P at 870 nm. Spectra were measured at 293 and 90 K. The kinetics of P* stimulated emission at 940 nm shows its decay with a time constant of approximately 14 psec at 90 K and approximately 18 psec at 293 K, which is accompanied by oscillations with a frequency of approximately 150 cm(-1). A weak absorption band is found at 1018 nm that is formed approximately 100 fsec after excitation of P and reflects the electron transfer from P* to beta and/or B(A) with accumulation of the P(+)beta(-) and/or P(+)B(A)(-) states. The kinetics of DeltaA at 1018 nm contains the oscillations at approximately 150 cm(-1) and distinct low-frequency oscillations at 20-100 cm(-1); also, the amplitude of the oscillations at 150 cm(-1) is much smaller at 293 than at 90 K. The oscillations in the kinetics of the 1018 nm band do not contain a 32 cm(-1) mode that is characteristic for native Rba. sphaeroides RCs having water molecule HOH55 in their structure. The DeltaA kinetics at 751 nm reflects the electron transfer to H(B) with formation of the P(+)H(B)(-) state. The oscillatory part of this kinetics has the form of a single peak with a maximum at ~50 fsec completely decaying at ~200 fsec, which might reflect a reversible electron transfer to the B-branch. The results are analyzed in terms of coherent nuclear wave packet motion induced in the P* excited state by femtosecond light pulses, of an influence of the incorporated mutations on the mutual position of the energy levels of charge separated states, and of the role of water HOH55 in the dynamics of the initial electron transfer.

Rates of the initial two steps of electron transfer in reaction centers from Rhodobacter sphaeroides as determined by singular-value decomposition followed by global fitting

The pump–probe spectral data matrices of reaction centers from Rhodobacter sphaeroides R26.1 and 2.4.1 in the 650–1265 nm region, that have been obtained by excitation of P at 868 nm with ∼0.1 ps pulses, were analyzed by singular-value decomposition followed by global fitting by the use of a three-component sequential model. The species-associated difference spectra showed the P∗BH → P +B −H → P +BH − sequential electron transfer and the rates of electron transfer were determined to be (1.9 ps) −1 and (2.7 ps) −1 for the first and the second steps, respectively. The P∗BH ⇌ P +B −H reversible electron transfer was also identified.

Application of the standard theory of electronic transitions to the description of oscillations in the kinetics of electron transfer in reaction centers of purple bacteria

The standard theory of the electron transfer between donor and acceptor molecules was used to describe oscillations in the reduction kinetics of the intermediate electron acceptor BA and the primary electron acceptor HA. The kinetics of the reduction of BA and HA were simulated on the basis of the model in which one and two accepting modes were used. A principal experiment is offered for the selection of the suitable theory for adequate description of oscillations in the kinetics of electron transfer in the reaction centers of purple bacteria.

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.

Protein dynamics control of electron transfer in reaction centers from Rps. viridis

Electron transfer (ET) in the reaction center from Rhodopseudomonas viridis has been studied experimentally by our group in the range of temperatures of 153–295 K. The kinetics of ET reaction from the proximal heme of cytochrome to the special pair was found to be non-exponential. The degree of non-exponentiality strongly depends on temperature, with increasing non-exponentiality at lower temperatures. Here, the experimental kinetic data for ET are analyzed in the frames of a theoretical Sumi–Marcus-type model, which allows establishing a connection between the observed kinetics and local structural dynamics of the protein in a close vicinity of the donor and acceptor sites. The phenomenological model subdivides the multi-time-scale dynamics of the protein into two groups: fast and slow. The division is determined by the rate of ET, which is dynamically controlled by the slow modes of the protein medium. The slow modes are described by a phenomenological collective coordinate , for which a diffusion type of dynamics is assumed. The analysis of the temperature dependence of the kinetic curves for ET allows a complete characterization of the slow time-scale protein dynamics: we find the corresponding activation barrier for the dynamics of , around 0.5 eV and the exponential pre-factor, which is in the range of 10− 16 s. The pre-exponential factor is lower than is expected for a typical activation process described by the transition state theory. We discuss the nature of the collective modes , the nature of its activation barrier (breaking of hydrogen bonds) and possible explanations for a low-pre-exponential factor of its dynamics. The main conclusion of the paper is that the kinetics of ET at low-temperatures can be used as a probe of protein structural dynamics in the microsecond time-scale, the time-scale that is not easily accessible by the computer simulations methods and can be complementary to other experimental techniques.

The femtosecond dynamics of electron transfer in a modified photosynthesis reaction center: Quantum, classical, and kinetic analyses

The dynamics of electron transfer in a modified photosynthesis reaction center in which electron transfer from the bridge to the acceptor is blocked is considered. A microscopic model of the process is suggested. Within this model, the diabatic electronic states of the donor and bridge are described by one-dimensional displaced harmonic oscillators. The dynamics of the population of electronic states is calculated by the quantum method of wave packets and classical and kinetic modeling. The suggested model is used to study the qualitative dependence of the dynamics of electron transfer on the nonadiabatic interaction potential. The parameters of the model are determined by comparing the experimental and calculated absorption spectra of the product of electron transfer. It is shown that kinetic models can be used to approximately describe the dynamics of electron transfer in reaction centers. The boundaries of the applicability of the kinetic method are considered.

The structure and function of the cytochrome c2: reaction center electron transfer complex from Rhodobacter sphaeroides

In the photosynthetic bacterium, Rhodobacter sphaeroides, the mobile electron carrier, cytochrome c2 (cyt c2) transfers an electron from reduced heme to the photooxidized bacteriochlorophyll dimer in the membrane bound reaction center (RC) as part of the light induced cyclic electron transfer chain. A complex between these two proteins that is active in electron transfer has been crystallized and its structure determined by X-ray diffraction. The structure of the cyt:RC complex shows the cyt c2 (cyt c2) positioned at the center of the periplasmic surface of the RC. The exposed heme edge from cyt c2 is in close tunneling contact with the electron acceptor through an intervening bridging residue, Tyr L162 located on the RC surface directly above the bacteriochlorophyll dimer. The binding interface between the two proteins can be divided into two regions: a short-range interaction domain and a long-range interaction domain. The short-range domain includes residues immediately surrounding the tunneling contact region around the heme and Tyr L162 that display close intermolecular contacts optimized for electron transfer. These include a small number of hydrophobic interactions, hydrogen bonds and a pi-cation interaction. The long-range interaction domain consists of solvated complementary charged residues; positively charged residues from the cyt and negatively charged residues from the RC that provide long range electrostatic interactions that can steer the two proteins into position for rapid association.

Advancements in the local reaction center electron transfer theory and the transition state structure in the first step of oxygen reduction over platinum

This paper demonstrates the importance of calculating both the reduction and oxidation electrode potential-dependent activation energy curves, E a( U), and then evaluating the respective derived E a( U) curves from them when using constrained variation theory for electron transfer to a local reaction center, an approach developed in this lab. A structure parameter averaging is introduced for calculating improved results if the calculated and derived results are not in satisfactory agreement. This averaging technique is used to improve upon a previous study of the first step of O 2 reduction to hydrogen peroxide or water when bonded to platinum and to show the role that twisting of Pt–O–O–H + can play by increasing its electron affinity and lowering the activation energy. By the same token, oxidation of peroxide proceeds with a higher barrier when the molecule is allowed to be twisted, which is its stable structure.

Metal Ion Modulated Electron Transfer in Photosynthetic Proteins

AbstractFor Abstract see ChemInform Abstract in Full Text.