Primary Electron Transfer Processes Research Articles

Single Amino Acid Mutation Decouples Photochemistry of the BLUF Domain from the Enzymatic Function of OaPAC and Drives the Enzyme to a Switched-on State

Photoactivated adenylate cyclases (PACs) are light-activated enzymes that combine a BLUF (blue-light using flavin) domain and an adenylate cyclase domain that are able to increase the levels of the important second messenger cAMP (cyclic adenosine monophosphate) upon blue-light excitation. The light-induced changes in the BLUF domain are transduced to the adenylate cyclase domain via a mechanism that has not yet been established. One critical residue in the photoactivation mechanism of BLUF domains, present in the vicinity of the flavin is the glutamine amino acid close to the N5 of the flavin. The role of this residue has been investigated extensively both experimentally and theoretically. However, its role in the activity of the photoactivated adenylate cyclase, OaPAC has never been addressed. In this work, we applied ultrafast transient visible and infrared spectroscopies to study the photochemistry of the Q48E OaPAC mutant. This mutation altered the primary electron transfer process and switched the enzyme into a permanent ‘on’ state, able to increase the cAMP levels under dark conditions compared to the cAMP levels of the dark-adapted state of the wild-type OaPAC. Differential scanning calorimetry measurements point to a less compact structure for the Q48E OaPAC mutant. The ensemble of these findings provide insight into the important elements in PACs and how their fine tuning may help in the design of optogenetic devices.

Photocycle alteration and increased enzymatic activity in genetically modified photoactivated adenylate cyclase OaPAC

Photoactivable adenylate cyclases (PAC) are light activated enzymes that combine blue light sensing capacity with the ability to convert adenosine triphosphate (ATP) to cyclic adenosine monophosphate (cAMP) and pyrophosphate (PPi) in a light-dependent manner. In most of the known PACs blue light regulation is provided by a BLUF domain which undergoes a structural reorganization after blue-light absorption. This minor structural change then is translated towards the C-terminal of the protein, inducing a larger conformational change that results in the ATP conversion to cAMP. As cAMP is a key second messenger in numerous signal transduction pathways regulating various cellular functions, photoactivable adenylate cyclases are of great interest in optogenetic studies. The optimal optogenetic device must be "silent" in the dark and highly responsive upon light illumination. OaPAC is a very good candidate as its basal activity is very small in the dark and the conversion rates increase 20-fold upon light illumination. We studied the effect of replacing D67 to N, in the BLUF domain. This mutation was found to accelerate the primary electron transfer process in the photosensing domain of the protein, as has been predicted. Furthermore, it resulted in a longer lived signalling state, which was formed with a lower quantum yield. Our studies show that the overall effects of the D67N mutation lead to a slightly higher conversion of ATP to cAMP, which points in the direction that by fine tuning the kinetic properties more responsive PACs and optogenetic devices can be generated.

High Yield of B-Side Electron Transfer at 77 K in the Photosynthetic Reaction Center Protein from Rhodobacter sphaeroides.

The primary electron transfer (ET) processes at 295 and 77 K are compared for the Rhodobacter sphaeroides reaction center (RC) pigment-protein complex from 13 mutants including a wild-type control. The engineered RCs bear mutations in the L and M polypeptides that largely inhibit ET from the excited state P* of the primary electron donor (P, a bacteriochlorophyll dimer) to the normally photoactive A-side cofactors and enhance ET to the C2-symmetry related, and normally photoinactive, B-side cofactors. P* decay is multiexponential at both temperatures and modeled as arising from subpopulations that differ in contributions of two-step ET (e.g., P* → P+BB- → P+HB-), one-step superexchange ET (e.g., P* → P+HB-), and P* → ground state. [HB and BB are monomeric bacteriopheophytin and bacteriochlorophyll, respectively.] The relative abundances of the subpopulations and the inherent rate constants of the P* decay routes vary with temperature. Regardless, ET to produce P+HB- is generally faster at 77 K than at 295 K by about a factor of 2. A key finding is that the yield of P+HB-, which ranges from ∼5% to ∼90% among the mutant RCs, is essentially the same at 77 K as at 295 K in each case. Overall, the results show that ET from P* to the B-side cofactors in these mutants does not require thermal activation and involves combinations of ET mechanisms analogous to those operative on the A side in the native RC.

Influence of Ligand Structure on Excited State Surface Chemistry of Lead Sulfide Quantum Dots.

The ligand-nanocrystal boundaries of colloidal quantum dots (QDs) mediate the primary energy and electron transfer processes that underpin photochemical and photocatalytic transformations at their surfaces. We use mid-infrared transient absorption spectroscopy to reveal the influence that ligand structure and bonding to nanocrystal surfaces have on the changes of the excited state surface chemistry of this boundary in PbS QDs and the corresponding impact on charge transfer processes between nanocrystals. We demonstrate that oleate ligands undergo marked changes in their bonding to surfaces in the excitonic excited states of the nanocrystals, indicating that oleate passivated PbS surfaces undergo significant structural changes following photoexcitation. These changes can impact the surface mobility of the ligands and the ability of redox shuttles to approach the nanocrystal surfaces to undergo charge transfer in photocatalytic reactions. In contrast, markedly different transient vibrational features are observed in iodide/mercaptoproprionic acid passivated PbS QD films that result from charge transfer between neighboring nanocrystals and localization of holes at the nanocrystal surfaces near MPA ligands. This ability to distinguish the influence that excitonic excited states vs charge transfer processes have on the surface chemistry of the ligand-nanocrystal boundary lays the groundwork for exploration of how this boundary can be understood and controlled for the design of nanocrystalline materials tailored for specific applications in solar energy harvesting and photocatalytic reactions.

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.

Time-resolved fluorescence measurements on leaves: principles and recent developments

Photosynthesis starts when a pigment in the photosynthetic antennae absorbs a photon. The electronic excitation energy is then transferred through the network of light-harvesting pigments to special chlorophyll (Chl) molecules in the reaction centres, where electron transfer is initiated. Energy transfer and primary electron transfer processes take place on timescales ranging from femtoseconds to nanoseconds, and can be monitored in real time via time-resolved fluorescence spectroscopy. This method is widely used for measurements on unicellular photosynthetic organisms, isolated photosynthetic membranes, and individual complexes. Measurements on intact leaves remain a challenge due to their high structural heterogeneity, high scattering, and high optical density, which can lead to optical artefacts. However, detailed information on the dynamics of these early steps, and the underlying structure–function relationships, is highly informative and urgently required in order to get deeper insights into the physiological regulation mechanisms of primary photosynthesis. Here, we describe a current methodology of time-resolved fluorescence measurements on intact leaves in the picosecond to nanosecond time range. Principles of fluorescence measurements on intact leaves, possible sources of alterations of fluorescence kinetics and the ways to overcome them are addressed. We also describe how our understanding of the organisation and function of photosynthetic proteins and energy flow dynamics in intact leaves can be enriched through the application of time-resolved fluorescence spectroscopy on leaves. For that, an example of a measurement on Zea mays leaves is presented.

Regulated Electron Tunneling of Photoinduced Primary Charge-Separated State in the Photosystem II Reaction Center.

In initial events of the photosynthesis by higher plants, the photosystem II (PSII) generates photoinduced primary charge-separated (CS) state composed of reduced pheophytin (PheoD1-•) and oxidized special pair (P+•) in chlorophyll a (Chla) PD1/PD2 in the D1/D2 heterodimer, ultimately leading to the water oxidation at the oxygen-evolving Mn4CaO5 cluster by P+•. To understand the mechanism of the efficient generation of initially localized CS state (PD1+• PheoD1-•), we have characterized cofactor geometries and electronic coupling of the photoinduced primary CS state in quinone prereduced membrane of PSII from spinach using the time-resolved electron paramagnetic resonance method. It has been revealed that the electronic coupling between the charges is significantly weak in the CS state separated by 1.5 nm, showing the importance of regulated cofactor-cofactor electronic interaction between a vinyl substituent in PheoD1 and an accessory chlorophyll to inhibit the energy-wasting charge recombination after the primary electron-transfer processes.

Dynamics of Interfacial Charge Transfer States and Carriers Separation in Dye-Sensitized Solar Cells: A Time-Resolved Terahertz Spectroscopy Study

Electron injection from a photoexcited molecular sensitizer into a wide-bandgap semiconductor is the primary step toward charge separation in dye-sensitized solar cells (DSSCs). According to the current understanding of DSSCs functioning mechanism, charges are separated directly during this primary electron transfer process, yielding hot conduction band electrons in the semiconductor and positive holes localized on oxidized dye molecules at the surface. Comparing results of ultrafast transient absorption and time-resolved terahertz measurements, we show here that intermediate interfacial charge transfer states (CTSs) are rather formed upon ultrafast injection from photoexcited Ru(II)–bipyridyl dye-sensitizer molecules into mesoporous TiO2 films. Formation and dissociation of these CTSs were found to strongly depend on their ionic environment and excess excitation energy. This finding establishes a new mechanism for charge separation in DSSCs. It also offers a rationale for the effect of electrolyte compos...

Primary electron transfer processes in photosynthetic reaction centers from oxygenic organisms.

This minireview is written in honor of Vladimir A. Shuvalov, a pioneer in the area of primary photochemistry of both oxygenic and anoxygenic photosyntheses (See a News Report: Allakhverdiev et al. 2014). In the present paper, we describe the current state of the formation of the primary and secondary ion-radical pairs within photosystems (PS) II and I in oxygenic organisms. Spectral-kinetic studies of primary events in PS II and PS I, upon excitation by ~20fs laser pulses, are now available and reviewed here; for PS II, excitation was centered at 710nm, and for PS I, it was at 720nm. In PS I, conditions were chosen to maximally increase the relative contribution of the direct excitation of the reaction center (RC) in order to separate the kinetics of the primary steps of charge separation in the RC from that of the excitation energy transfer in the antenna. Our results suggest that the sequence of the primary electron transfer reactions is P680→ChlD1→PheD1→QA (PS II) and P700→A 0A/A 0B→A 1A/A 1B (PS I). However, alternate routes of charge separation in PS II, under different excitation conditions, are not ruled out.

Type I reaction center from the green sulfur bacterium Chlorobium tepidum: is Chl a a primary electron acceptor?

The green sulfur bacterium Chlorobium tepidum has one of the simplest type I reaction center (RC) complexes. While its structure is still unknown, biochemical and protein sequence analyses suggest that it is similar to photosystem I (PS I), with two BChl a forming a special pair P840, four Chl a serving as pairs of accessory and primary electron acceptor (A0) pigments and 14 BChl a constituting as an immediate RC antenna. This is a dramatic simplification compared to PS I RC, where 90 Chl a antenna pigments serve as antenna and 6 additional Chl a molecules function as electron transfer cofactors. The resulting spectral congestion has prevented direct visualization of ultrafast electron transfer processes within PS I RC and even the sequence of primary electron transfer processes in PS I is still under debate. The suggested presence of two types of pigments in RC from Chlorobium tepidum removes spectral congestion and opens a way to directly visualize electron transfer steps in type I RC using ultrafast spectroscopy, since the Chl a and BChl a pigments absorb at ∼670 nm and ∼800 nm, respectively. To confirm the proposed functional role of Chl a as electron transfer cofactor we performed extensive ultrafast optical pump-probe experiments on different preparations of RC complexes from Chlorobium tepidum, revealing energy/electron transfer rates between different groups of pigments. Surprisingly, we found that ∼3 out of 4 Chl a pigments do not transfer excitation energy to the BChl a antenna or to P840, which indicates that these pigments must be >20A away from any other BChl a pigment and thus argues against the suggested presence of 4 Chl a in the reaction center core complex.

Terahertz radiation from bacteriorhodopsin reveals correlated primary electron and proton transfer processes

The kinetics of electrogenic events associated with the different steps of the light-induced proton pump of bacteriorhodopsin is well studied in a wide range of time scales by direct electric methods. However, the investigation of the fundamental primary charge translocation phenomena taking place in the functional energy conversion process of this protein, and in other biomolecular assemblies using light energy, has remained experimentally unfeasible because of the lack of proper detection technique operating in the 0.1- to 20-THz region. Here, we show that extending the concept of the familiar Hertzian dipole emission into the extreme spatial and temporal range of intramolecular polarization processes provides an alternative way to study ultrafast electrogenic events on naturally ordered biological systems. Applying a relatively simple experimental arrangement based on this idea, we were able to observe light-induced coherent terahertz radiation from bacteriorhodopsin with femtosecond time resolution. The detected terahertz signal was analyzed by numerical simulation in the framework of different models for the elementary polarization processes. It was found that the principal component of the terahertz emission can be well described by excited-state intramolecular electron transfer within the retinal chromophore. An additional slower process is attributed to the earliest phase of the proton pump, probably occurring by the redistribution of a H bond near the retinal. The correlated electron and proton translocation supports the concept, assigning a functional role to the light-induced sudden polarization in retinal proteins.

Probing the Compound I-like Reactivity of a Bare High-Valent Oxo Iron Porphyrin Complex: The Oxidation of Tertiary Amines

The mechanisms of oxidative N-dealkylation of amines by heme enzymes including peroxidases and cytochromes P450 and by functional models for the active Compound I species have long been studied. A debated issue has concerned in particular the character of the primary step initiating the oxidation sequence, either a hydrogen atom transfer (HAT) or an electron transfer (ET) event, facing problems such as the possible contribution of multiple oxidants and complex environmental effects. In the present study, an oxo iron(IV) porphyrin radical cation intermediate 1, [(TPFPP)*+ Fe(IV)=O]+ (TPFPP = meso-tetrakis (pentafluorophenyl)porphinato dianion), functional model of Compound I, has been produced as a bare species. The gas-phase reaction with amines (A) studied by ESI-FT-ICR mass spectrometry has revealed for the first time the elementary steps and the ionic intermediates involved in the oxidative activation. Ionic products are formed involving ET (A*+, the amine radical cation), formal hydride transfer (HT) from the amine ([A(-H)]+, an iminium ion), and oxygen atom transfer (OAT) to the amine (A(O), likely a carbinolamine product), whereas an ionic product involving a net initial HAT event is never observed. The reaction appears to be initiated by an ET event for the majority of the tested amines which included tertiary aliphatic and aromatic amines as well as a cyclic and a secondary amine. For a series of N,N-dimethylanilines the reaction efficiency for the ET activated pathways was found to correlate with the ionization energy of the amine. A stepwise pathway accounts for the C-H bond activation resulting in the formal HT product, namely a primary ET process forming A*+, which is deprotonated at the alpha-C-H bond forming an N-methyl-N-arylaminomethyl radical, A(-H)*, readily oxidized to the iminium ion, [A(-H)]+. The kinetic isotope effect (KIE) for proton transfer (PT) increases as the acidity of the amine radical cation increases and the PT reaction to the base, the ferryl group of (TPFPP)Fe(IV)=O, approaches thermoneutrality. The ET reaction displayed by 1 with gaseous N,N-dimethylaniline finds a counterpart in the ET reactivity of FeO+, reportedly a potent oxidant in the gas phase, and with the barrierless ET process for a model (P)*+ Fe(IV)=O species (where P is the porphine dianion) as found by theoretical calculations. Finally, the remarkable OAT reactivity of 1 with C6F5N(CH3)2 may hint to a mechanism along a route of diverse spin multiplicity.

Detergent effects on primary charge separation in wild-type and mutant Rhodobacter capsulatus reaction centers

The primary electron-transfer processes in reaction centers (RCs) from wild-type and several mutants of Rhodobacter capsulatus have been investigated as a function of the detergent used to extract the RC protein from the membrane. Wild-type and L(M212)H mutant RCs that have been isolated using the detergent Deriphat 160-C both display somewhat slower initial charge separation (longer P* lifetimes) than the same RCs isolated using the detergent LDAO. For the F(L181)Y/Y(M208)F/L(M212)H triple mutant, the differences in the initial charge separation events for Deriphat-versus LDAO-isolated RCs are more significant. In all cases, use of Deriphat 160-C to extract the protein from the membrane yields RCs in which the QY band of P is retained at its native position near 865 nm, whereas LDAO extraction yields RCs that have the P band near 850 nm. Origins of the differences in both the ground state spectrum and the photochemistry, including possible RC–lipid associations, are considered.

Investigations of the heavy atom effect occurring in bianthryl and 10,10′-dibromobianthryl. Fluorescence, cyclovoltamperometric and actinometric studiesDedicated to Professor Dr Z. R. Grabowski and Professor Dr J. Wirz on the occasions of their 75th and 60th birthdays.

A preliminary study of photophysical and photochemical properties of 9,9′-bianthryl (BA) and 10,10′-dibromo,9,9′-bianthryl (DBrBA) is presented. Dual fluorescence occurring in bianthryl (BA) has been investigated in solvents containing heavy atoms, such as chlorine, bromine and iodine. The presence of heavy atoms reduces strongly the fluorescence lifetimes and the fluorescence quantum yields of BA, the effect is strongest in ethyl iodide, i.e. in the solvent containing the heaviest atom, iodine. On the other hand, it has been found that introduction of two heavy atoms (bromine) into the bianthryl molecule modifies noticeably its fluorescence properties, which indicates existance of an internal heavy atom effect. Again, a reduction of the fluorescence lifetime and the fluorescence quantum yield, compared to the parent molecule, has been observed. On the basis of these observations the mechanism of 1CT ⇒ 3LE intersystem crossing has been discussed. In BA the rate determining process is a reversible spin inversion within the radical pair. In DBrBA where the spin flip is accelerated by the presence of heavy atoms (bromines) the spin-allowed electron transfer 3CT ⇒ 3LE becomes important in the overall intersystem crossing process. It has been found that DBrBA undergoes a photoreduction in the presence of aromatic donors, such as amines, leading finally to BA. Similar reaction seems to be observed in electrochemical measurement. The reduction of DBrBA originates from the primary electron transfer process either photoinduced or electrochemical leading to the free anion radicals of DBrBA.

Photophysical properties of the ReI and RuII complexes of a new C60-substituted bipyridine ligand.

The rhenium(I) and ruthenium(II) complexes of a fullerene-substituted bipyridine ligand have been prepared. Electrochemical studies indicate that some ground state electronic interaction between the fullerene subunit and the metal-complexed moiety are present in the Re(I) but not the Ru(II) complex. The photophysical properties have been investigated by steady-state and time-resolved UV/Vis-NIR luminescence spectroscopy and nanosecond laser flash photolysis in CH2Cl2 solution, and compared to those of the corresponding model compounds. Excitation of the methanofullerene moiety in the dyads does not lead to excited state intercomponent interactions. Instead, excitation of the metal-complexed unit shows that the lowest triplet metal-to-ligand-charge-transfer excited state ((3)MLCT) centered on the Re(I)- or Ru(II)-type unit is quenched with a rate constant of about 2.5 x 10(8) s(-1). The quenching is attributed to an electron-transfer (ElT) process leading to the reduction of the carbon sphere, as determined by luminescence spectroscopy for the Ru(II) dyad. Experimental detection of electron transfer in the Re(I) dyad is prevented due to the unfavorable absorption of the metal-complexed moiety relative to the fullerene unit. However, it can be postulated on the basis of energetic/kinetic arguments and by comparison with the Ru(II)-type array. The primary ElT process is followed by charge-recombination to give the lowest-lying fullerene triplet excited state (3C60) with quantitative yield, as determined by sensitized singlet oxygen luminescence experiments. Direct (3)MLCT-->3C60 triplet-triplet energy-transfer (EnT) does not successfully compete with ElT since it is highly exoergonic and located in the Marcus inverted region. The quantum yield of singlet oxygen sensitization (Phi(delta)) of the Re(I)-based dyad is found to be lower (0.80) than for the corresponding Ru(II) derivative (1.0). This is likely to be the consequence of different conformational structures for the two dyads, rather than a different yield of 3C60 formation.

Theoretical studies on the mechanism of primary electron transfer in the photosynthetic reaction center of Rhodobacter sphaeroides.

The mechanism of the primary electron transfer (ET) process in the photosynthetic reaction center (PRC) of Rhodobacter sphaeroides has been studied with quantum chemistry method of ab initio density functional theory (DFT) (B3LYP/6-31G) based on the optimized X-ray crystallographic structure. The calculation was carried out on different structural levels. The electronic structure of pigment molecules was first studied, and then the influence of the neighboring protein was taken into account at three approximation levels: (a) the surrounding proteins were treated as a homogeneous medium with a uniform dielectric constant (SCRF); (b) both the influence of axial coordination of His to the special pair P and ABChl as, and the hydrogen bonds between related residues and P and also BPhas were included; and (c) the influence of the electronic structure of the protein subunit chains as a whole was studied. The results suggest that: (1) according to the composition of the HOMO and LUMO of P, there might be a charge-separated state of (BChl(L) (+)BChl(M) (-)) for the excited state of P; (2) to treat the protein surroundings as a homogeneous medium is not sufficient. Different interactions between pigment molecules and related residues play different roles in the ET process; (3) the axial coordination of His to P raises the E (LUMO) of P greatly, and it is very important for the ET process to occur in the PRC of wild-type bacterium; the axial coordination of His to ABChl as also raises their E (LUMO) significantly; (4) the hydrogen-bonds between amino acid residues and P and also BPh as depress the E (LUMO) of the pigment molecules to some extent, which makes the E (LUMO) of P lower than those of ABChlas, and the E (LUMO) of BPh a (L) lower than that of BPh a (M). Consequently, the ET process from P to BPh a (L) does not, according to our calculation model, occur via ABChl a (L). The possibility of the ET pathway from P to BPh a (L) via ABChl a (L) was discussed; (5) the frontier orbitals of protein subunit chains L and M are localized at the random coil area and the alpha-helix areas, respectively. Results mentioned above support the fact that the ET process proceeds in favourable circumstances along the branch L.

Salt effects on the reactions of radical ion pairs formed by electron transfer quenching of triplet 2-methyl-1,4-naphthoquinone by amines. Optical flash photolysis and step-scan FTIR investigations.

Electron transfer quenching of triplet 2-methyl-1,4-naphthoquinone by amines in acetonitrile was investigated by means of nanosecond flash photolysis and step-scan infrared spectroscopy. With 2,5-di-tert-butylaniline as the electron donor, proton transfer following the primary electron transfer process forms neutral radicals, which were identified by comparing the resulting transient IR absorption spectra with those obtained in the presence of phenol as the quencher. Addition of lithium perchlorate promotes the formation of free radical ions. The salt effect on the reactions of the nascent radical ion pairs formed by electron transfer is discussed in terms of Eigen theory. The quinone radical anion strongly associates with lithium cations. The salt retards the diffusional recombination of the free radical ions, as predicted by the Debye-Smoluchowski equation. The observed, predictable salt effects provide a useful diagnostic tool for elucidation of the reaction mechanism.

Investigation of the primary electron transfer processes in some rigidly bridged dyads and triads by (sub)picosecond pump–probe experiments

The electron transfer kinetics of several completely rigid dyads and triads which contain N,N-dimethylaniline (DMA) and dimethoxynaphthalene (DMN) as possible donors and the dicyanovinyl group (DCV) as acceptor were studied by means of (sub)picosecond time-resolved transient absorption spectroscopy. In the dyads DMN[n]DCV, the rate of charge separation decreases exponentially with the number of σ-bonds n in the bridge, the ‘‘damping factor’’ being 0.8 per bond in tetrahydrofuran solvent. In the triads DMA[4]DMN[8]DCV, the primary electron transfer from DMN to DCV occurs in solvents of low and medium polarity within 10 ps in both isomers (syn and anti). The rates of the secondary electron transfer step (formation of the fully charge separated state, DMA+[4]DMN[8]DCV−) and the following deactivation processes depend, however, strongly on the conformation. In acetonitrile, the primary electron transfer involves the two donor groups yielding preferentially DMA+[4]DMN−[8]DCV. In the anti-conformer this state is fairly long-lived; in the syn-conformer, however, it decays rapidly, in part to locally excited triplet states.

M-side electron transfer in reaction center mutants with a lysine near the nonphotoactive bacteriochlorophyll.

We report the primary charge separation events in a series of Rhodobacter capsulatus reaction centers (RCs) that have been genetically modified to contain a lysine near the bacteriochlorophyll molecule, BChl(M), on the nonphotoactive M-side of the RC. Using wild type and previously constructed mutants as templates, we substituted Lys for the native Ser residue at position 178 on the L polypeptide to make the S(L178)K single mutant, the S(L178)K/G(M201)D and S(L178)K/L(M212)H double mutants, and the S(L178)K/G(M201)D/L(M212)H triple mutant. In the triple mutant, the decay of the photoexcited primary electron donor (P) occurs with a time constant of 15 ps and is accompanied by 15% return to the ground state, 62% electron transfer to the L-side bacteriopheophytin, BPh(L), and 23% electron transfer to the M-side analogue, BPh(M). The data supporting electron transfer to the M-side include bleaching of the Q(X) band of BPh(M) at 528 nm and a spectrally and kinetically resolved anion band with a maximum at 640 nm assigned to BPh(M)(-). The decay of these features and concomitant approximately 20% decay of bleaching of the 850 nm band of P give a P(+)BPh(M)(-) lifetime on the order of 1-2 ns that reflects deactivation to give the ground state. These data and additional findings are compared to those from parallel experiments on the G(M201)D/L(M212)H double mutant, in which 15% electron transfer to BPh(M) has been reported previously and is reproduced here. We also compare the above results with the primary electron-transfer processes in S(L178)K, S(L178)K/G(M201)D, and S(L178)K /L(M212)H RCs and with those for the L(M212)H and G(M201)D single mutants and wild-type RCs. The comparison of extensive results that track the primary events in these eight RCs helps to elucidate key factors underlying the directionality and high yield of charge separation in the bacterial photosynthetic RC.

Fluctuational tunnel effects and vibrational dispersion in primary electron transfer processes of bacterial photosynthesis

We have investigated forward electron transfer (ET) from reduced bacteriopheophytin (bph −) to the primary quinone (Q A), and the recombination ET from Q A − to the oxidized special pair (bch) 2 + in the photosynthetic reaction centre of Rh. sphaeroides. The same reactant, Q A, is here engaged in two ET reactions of widely different distances. The processes can also be followed over broad temperature ranges. Focus is on two elements not commonly considered in ET theory. One is continuous environmental dielectric dispersion which is both formally and physically important. The other one is environmental fluctuational modulation of the tunnel factor which is important when the donor and acceptor are separated by environmental matter. The analysis shows, first that a broad dielectric dispersion is in line with the ET data up to ≈180°K. Fluctuational modulation of the tunnel factor is, secondly, unimportant in shorter-range forward ET from bph − to Q A (13 Å centre-to-centre distance) but could be important in the much longer-range recombination ET from Q A − to (bch) 2 + (28 Å).