Internal Electrostatic Control of the Primary Charge Separation and Recombination in Reaction Centers from Rhodobacter sphaeroides Revealed by Femtosecond Transient Absorption

Pigment-modified reaction centers of Chloroflexus aurantiacus: chemical exchange of bacteriopheophytins with plant-type pheophytins

The pigment composition of isolated reaction centers (RCs) of the green filamentous bacterium Chloroflexus (Cfl.) aurantiacus was changed by chemical exchange of native bacteriopheophytin a (BPheo) molecules with externally added pheophytin a (Pheo) or [3-acetyl]-Pheo upon incubation of RC/pheophytin mixtures at room temperature and 45 °C. The modified RCs were characterized by Vis/NIR absorption spectroscopy, and the effect of pigment exchange on RC photochemical activity was assessed by measuring the photoaccumulation of the reduced pigment at the binding site H_A. It is shown that both pheophytins can be exchanged into the H_A site instead of BPheo by incubation at room temperature. While the newly introduced Pheo molecule is not active in electron transfer, the [3-acetyl]-Pheo molecule is able to replace functionally the photoreducible H_A BPheo molecule with the formation of the [3-acetyl]-Pheo^- radical anion instead of the BPheo^-. After incubation at 45 °C, the majority (~ 90%) of H_A BPheo molecules is replaced by both Pheo and [3-acetyl]-Pheo. Only a partial replacement of inactive BPheo molecules with pheophytins is observed even when the incubation temperature is raised to 50 °C. The results are discussed in terms of (i) differences in the accessibility of BPheo binding sites for extraneous pigments depending on structural constraints and incubation temperature and (ii) the effect of the reduction potential of pigments introduced into the H_A site on the energetics of the charge separation process. The possible implication of Pheo-exchanged preparations for studying early electron-transfer events in Cfl. aurantiacus RCs is considered.

Antagonistic Effects of Point Mutations on Charge Recombination and a New View of Primary Charge Separation in Photosynthetic Proteins

Light-induced electron-transfer reactions were investigated in wild-type and three mutant Rhodobacter sphaeroides reaction centers with the secondary electron acceptor (ubiquinone Q_A) either removed or permanently reduced. Under such conditions, charge separation between the primary electron donor (bacteriochlorophyll dimer, P) and the electron acceptor (bacteriopheophytin, H_A) was followed by P⁺H_A^– → PH_A charge recombination. Two reaction centers were used that had different single amino-acid mutations that brought about either a 3-fold acceleration in charge recombination compared to that in the wild-type protein, or a 3-fold deceleration. In a third mutant in which the two single amino-acid mutations were combined, charge recombination was similar to that in the wild type. In all cases, data from transient absorption measurements were analyzed using similar models. The modeling included the energetic relaxation of the charge-separated states caused by protein dynamics and evidenced the appearance of an intermediate charge-separated state, P⁺B_A^–, with B_A being the bacteriochlorophyll located between P and H_A. In all cases, mixing of the states P⁺B_A^– and P⁺H_A^– was observed and explained in terms of electron delocalization over B_A and H_A. This delocalization, together with picosecond protein relaxation, underlies a new view of primary charge separation in photosynthesis.

Temperature dependence of nanosecond charge recombination in mutant Rhodobacter sphaeroides reaction centers: modelling of the protein dynamics

We investigated the influence of a range of factors-temperature, redox midpoint potential of an electron carrier, and protein dynamics-on nanosecond electron transfer within a protein. The model reaction was back electron transfer from a bacteriopheophytin anion, H_A^-, to an oxidized primary electron donor, P⁺, in a wild type Rhodobacter sphaeroides reaction center (RC) with a permanently reduced secondary electron acceptor (quinone, Q_A^-). Also used were two modified RCs with single amino acid mutations near the monomeric bacteriochlorophyll, B_A, located between P and H_A. Both mutant RCs showed significant slowing down of this back electron transfer reaction with decreasing temperature, similar to that observed with the wild type RC, but contrasting with a number of single point mutant RCs studied previously. The observed similarities and differences are explained in the framework of a (P⁺B_A^- ↔ P⁺H_A^-) equilibrium model with an important role played by protein relaxation. The major cause of the observed temperature dependence, both in the wild type RC and in the mutant proteins, is a limitation in access to the thermally activated pathway of charge recombination via the state P⁺B_A^- at low temperatures. The data indicate that in all RCs both charge recombination pathways, the thermally activated one and a direct one without involvement of the P⁺B_A^- state, are controlled by the protein dynamics. It is concluded that the modifications of the protein environment affect the overall back electron transfer kinetics primarily by changing the redox potential of B_A and not by changing the protein relaxation dynamics.

Insight into Electron Transfer from a Redox Polymer to a Photoactive Protein

Biohybrid photoelectrochemical systems in photovoltaic or biosensor applications have gained considerable attention in recent years. While the photoactive proteins engaged in such systems usually maintain an internal charge separation quantum yield of nearly 100%, the subsequent steps of electron and hole transfer beyond the protein often limit the overall system efficiency and their kinetics remain largely uncharacterized. To reveal the dynamics of one of such charge-transfer reactions, we report on the reduction of Rhodobacter sphaeroides reaction centers (RCs) by Os-complex-modified redox polymers (P-Os) characterized using transient absorption spectroscopy. RCs and P-Os were mixed in buffered solution in different molar ratios in the presence of a water-soluble quinone as an electron acceptor. Electron transfer from P-Os to the photoexcited RCs could be described by a three-exponential function, the fastest lifetime of which was on the order of a few microseconds, which is a few orders of magnitude faster than the internal charge recombination of RCs with fully separated charge. This was similar to the lifetime for the reduction of RCs by their natural electron donor, cytochrome c2. The rate of electron donation increased with increasing ratio of polymer to protein concentrations. It is proposed that P-Os and RCs engage in electrostatic interactions to form complexes, the sizes of which depend on the polymer-to-protein ratio. Our findings throw light on the processes within hydrogel-based biophotovoltaic devices and will inform the future design of materials optimally suited for this application.

Primary electron transfer in Rhodobacter sphaeroides R-26 reaction centers under dehydration conditions

The photoinduced charge separation in Q_B-depleted reaction centers (RCs) from Rhodobacter sphaeroides R-26 in solid air-dried and vacuum-dried (~10^-2 Torr) films, obtained in the presence of detergent n-dodecyl-β-D-maltoside (DM), is characterized using ultrafast transient absorption spectroscopy. It is shown that drying of RC-DM complexes is accompanied by reversible blue shifts of the ground-state absorption bands of the pigment ensemble, which suggest that no dehydration-induced structural destruction of RCs occurs in both types of films. In air-dried films, electron transfer from the excited primary electron donor P^⁎ to the photoactive bacteriopheophytin H_A proceeds in 4.7 ps to form the P⁺H_A^- state with essentially 100% yield. P⁺H_A^- decays in 260 ps both by electron transfer to the primary quinone Q_A to give the state P⁺Q_A^- (87% yield) and by charge recombination to the ground state (13% yield). In vacuum-dried films, P^⁎ decay is characterized by two kinetic components with time constants of 4.1 and 46 ps in a proportion of ~55%/45%, and P⁺H_A^- decays about 2-fold slower (462 ps) than in air-dried films. Deactivation of both P^⁎ and P⁺H_A^- to the ground state effectively competes with the corresponding forward electron-transfer reactions in vacuum-dried RCs, reducing the yield of P⁺Q_A^- to 68%. The results are compared with the data obtained for fully hydrated RCs in solution and are discussed in terms of the presence in the RC complexes of different water molecules, the removal/displacement of which affects spectral properties of pigment cofactors and rates and yields of the electron-transfer reactions.

Weak temperature dependence of P (+) H A (-) recombination in mutant Rhodobacter sphaeroides reaction centers

In contrast with findings on the wild-type Rhodobacter sphaeroides reaction center, biexponential P (+) H A (-)  → PH A charge recombination is shown to be weakly dependent on temperature between 78 and 298 K in three variants with single amino acids exchanged in the vicinity of primary electron acceptors. These mutated reaction centers have diverse overall kinetics of charge recombination, spanning an average lifetime from ~2 to ~20 ns. Despite these differences a protein relaxation model applied previously to wild-type reaction centers was successfully used to relate the observed kinetics to the temporal evolution of the free energy level of the state P (+) H A (-) relative to P (+) B A (-) . We conclude that the observed variety in the kinetics of charge recombination, together with their weak temperature dependence, is caused by a combination of factors that are each affected to a different extent by the point mutations in a particular mutant complex. These are as follows: (1) the initial free energy gap between the states P (+) B A (-) and P (+) H A (-) , (2) the intrinsic rate of P (+) B A (-)  → PB A charge recombination, and (3) the rate of protein relaxation in response to the appearance of the charge separated states. In the case of a mutant which displays rapid P (+) H A (-) recombination (ELL), most of this recombination occurs in an unrelaxed protein in which P (+) B A (-) and P (+) H A (-) are almost isoenergetic. In contrast, in a mutant in which P (+) H A (-) recombination is relatively slow (GML), most of the recombination occurs in a relaxed protein in which P (+) H A (-) is much lower in energy than P (+) H A (-) . The weak temperature dependence in the ELL reaction center and a YLH mutant was modeled in two ways: (1) by assuming that the initial P (+) B A (-) and P (+) H A (-) states in an unrelaxed protein are isoenergetic, whereas the final free energy gap between these states following the protein relaxation is large (~250 meV or more), independent of temperature and (2) by assuming that the initial and final free energy gaps between P (+) B A (-) and P (+) H A (-) are moderate and temperature dependent. In the case of the GML mutant, it was concluded that the free energy gap between P (+) B A (-) and P (+) H A (-) is large at all times.

Unified Model of Nanosecond Charge Recombination in Closed Reaction Centers from Rhodobacter sphaeroides: Role of Protein Polarization Dynamics

Ongoing questions surround the influence of protein dynamics on rapid processes such as biological electron transfer. Such questions are particularly addressable in light-activated systems. In Rhodobacter sphaeroides reaction centers, charge recombination or back electron transfer from the reduced bacteriopheophytin, HA(-), to the oxidized dimeric bacteriochlorophyll, P(+), may be monitored by both transient absorption spectroscopy and transient fluorescence spectroscopy. Signals measured with both these techniques decay in a similar three-exponential fashion with lifetimes of ∼0.6-0.7, ∼2-4, and ∼10-20 ns, revealing the complex character of this electron transfer reaction. In this study a single kinetic model was developed to connect lifetime and amplitude data from both techniques. The model took into account the possibility that electron transfer from HA(-) to P(+) may occur with transient formation of the state P(+)BA(-). As a result it was possible to model the impact of nanosecond protein relaxation on the free energy levels of both P(+)HA(-) and P(+)BA(-) states relative to that of the singlet excited state of P, P*. Surprisingly, whereas the free energy gap between P* and P(+)HA(-) increased with time in response to protein reorganization, the free energy gap between P* and P(+)BA(-) decreased. This finding may be accounted for by a gradual polarization of the protein environment which stabilizes the state P(+)HA(-) and destabilizes the state P(+)BA(-), favoring productive charge separation over unproductive charge recombination.

Analysis of the temperature-dependence of P(+)HA(-) charge recombination in the Rhodobacter sphaeroides reaction center suggests nanosecond temperature-independent protein relaxation

The temperature dependence of charge recombination of the pair P(+)HA(-) in isolated reaction centers from the purple bacterium Rhodobacter sphaeroides with prereduced quinone QA was studied by sub-nanosecond to microsecond time-scale transient absorption. Overall, the kinetics slowed down substantially upon cooling from room temperature to ∼200 K, and then remained virtually unchanged down to 77 K, indicating the coexistence of two competitive pathways of charge recombination, a thermally-activated pathway appearing only above ~200 K and a temperature-independent pathway. In our modelling, the thermally activated pathway includes an uphill electron transfer from HA(-) to BA(-) leading to transient formation of the state P(+)BA(-), whereas the temperature-independent pathway is due to direct downhill electron transfer from HA(-) to P(+). At all temperatures studied, the kinetics could be approximated by a four-component decay. Detailed analysis of the lifetimes and amplitudes of particular phases over the range of temperatures suggests that the kinetically resolved phases reveal the consecutive appearance of three conformational states characterized by an increasing free energy gap between the states P(+)BA(-) and P(+)HA(-). The initial gap between these states was estimated to be only ~8 meV, the intermediate gap being ~92 meV, and the final gap ~135 meV, with no dependence on temperature. It was also calculated through a very straightforward approach that the relaxation process from the initial to the intermediate state occurs within 0.6 ± 0.1 ns, whereas the second step of relaxation from the intermediate to the final state takes 11 ± 2 ns. Both phases of the protein relaxation process are essentially temperature-independent. Possible alternative models to describe the experimental data that cannot be definitely excluded are also discussed.

Arbitrary-detuning asynchronous optical sampling pump-probe spectroscopy of bacterial reaction centers

A recently reported variant of asynchronous optical sampling compatible with arbitrary unstabilized laser repetition rates is applied to pump-probe spectroscopy. This makes possible the use of a 5.1 MHz chirped pulse oscillator as the pump laser, thus extending the available time window to almost 200 ns with a time resolution as good as about 320 fs. The method is illustrated with the measurement in a single experiment of the complete charge transfer dynamics of the reaction center from Rhodobacter sphaeroides.

Protein dielectric environment modulates the electron-transfer pathway in photosynthetic reaction centers

The replacement of tyrosine by aspartic acid at position M210 in the photosynthetic reaction center of Rhodobacter sphaeroides results in the generation of a fast charge recombination pathway that is not observed in the wild-type. Apparently, the initially formed charge-separated state (cation of the special pair, P, and anion of the A-side bacteriopheophytin, H(A)) can decay rapidly via recombination through the neighboring bacteriochlorophyll (B(A)) soon after formation. The charge-separated state then relaxes over tens of picoseconds and recombination slows to the hundreds-of-picoseconds or nanosecond timescale. This dielectric relaxation results in a time-dependent blue shift of B(A)(-) absorption, which can be monitored using transient absorbance measurements. Protein dynamics also appear to modulate the electron transfer between H(A) and the next electron carrier, Q(A) (a ubiquinone). The kinetics of this reaction are complex in the mutant, requiring two kinetic terms, and the spectra associated with the two terms are distinct; a red shift of the H(A) ground-state bleaching is observed between the shorter and longer H(A)-to-Q(A) electron-transfer phases. The kinetics appears to be pH-independent, suggesting a negligible contribution of static heterogeneity originating from protonation/deprotonation in the ground state. A dynamic model based on the energy levels of the two early charge-separated states, P(+)B(A)(-) and P(+)H(A)(-), has been developed in which the energetics of these states is modulated by fast protein dielectric relaxations and this in turn alters both the kinetic complexity of the reaction and the reaction pathway.

Analysis of the kinetics of P+ HA- recombination in membrane-embedded wild-type and mutant Rhodobacter sphaeroides reaction centers between 298 and 77 K indicates that the adjacent negatively charged QA ubiquinone modulates the free energy of P+ HA- and may influence the rate of the protein dielectric response

Time-resolved spectroscopic studies of recombination of the P(+)HA(-) radical pair in photosynthetic reaction centers (RCs) from Rhodobacter sphaeroides give an opportunity to study protein dynamics triggered by light and occurring over the lifetime of P(+)HA(-). The state P(+)HA(-) is formed after the ultrafast light-induced electron transfer from the primary donor pair of bacteriochlorophylls (P) to the acceptor bacteriopheophytin (HA). In order to increase the lifetime of this state, and thus increase the temporal window for the examination of protein dynamics, it is possible to block forward electron transfer from HA(-) to the secondary electron acceptor QA. In this contribution, the dynamics of P(+)HA(-) recombination were compared at a range of temperatures from 77 K to room temperature, electron transfer from HA(-) to QA being blocked either by prereduction of QA or by genetic removal of QA. The observed P(+)HA(-) charge recombination was significantly slower in the QA-deficient RCs, and in both types of complexes, lowering the temperature from RT to 77 K led to a slowing of charge recombination. The effects are explained in the frame of a model in which charge recombination occurs via competing pathways, one of which is thermally activated and includes transient formation of a higher-energy state, P(+)BA(-). An internal electrostatic field supplied by the negative charge on QA increases the free energy levels of the state P(+)HA(-), thus decreasing its energetic distance to the state P(+)BA(-). In addition, the dielectric response of the protein environment to the appearance of the state P(+)HA(-) is accelerated from ∼50-100 ns in the QA-deficient mutant RCs to ∼1-16 ns in WT RCs with a negatively charged QA(-). In both cases, the temperature dependence of the protein dynamics is weak.

Hopping Maps for Photosynthetic Reaction Centers()

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

Mechanism of recombination of the P+H(A)- radical pair in mutant Rhodobacter sphaeroides reaction centers with modified free energy gaps between P+B(A)- and P+H(A)-

The kinetics of recombination of the P(+)H(A)(-) radical pair were compared in wild-type reaction centers from Rhodobacter sphaeroides and in seven mutants in which the free energy gap, ΔG, between the charge separated states P(+)B(A)(-) and P(+)H(A)(-) was either increased or decreased. Five of the mutant RCs had been described previously, and X-ray crystal structures of two newly constructed complexes were determined by X-ray crystallography. The charge recombination reaction was accelerated in all mutants with a smaller ΔG than in the wild-type, and was slowed in a mutant having a larger ΔG. The free energy difference between the state P(+)H(A)(-) and the PH(A) ground state was unaffected by most of these mutations. These observations were consistent with a model in which the P(+)H(A)(-) → PH(A) charge recombination is thermally activated and occurs via the intermediate state P(+)B(A)(-), with a mean rate related to the size of the ΔG between the states P(+)B(A)(-) and P(+)H(A)(-) and not the ΔG between P(+)H(A)(-) and the ground state. A more detailed analysis of charge recombination in the mutants showed that the kinetics of the reaction were multiexponential, and characterized by ~0.5, ~1-3, and 7-17 ns lifetimes, similar to those measured for wild-type reaction centers. The exact lifetimes and relative amplitudes of the three components were strongly modulated by the mutations. Two models were considered in order to explain the observed multiexponentiality and modulation, involving heterogeneity or relaxation of P(+)H(A)(-) states, with the latter model giving a better fit to the experimental results.

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.