Manipulating the Energetics and Rates of Electron Transfer in Rhodobacter capsulatus Reaction Centers with Asymmetric Pigment Content

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.

Unravelling the Roles of Integral Polypeptides in Excitation Energy Transfer of Photosynthetic RC-LH1 Supercomplexes

Elucidating the photosynthetic processes that occur within the reaction center-light-harvesting 1 (RC-LH1) supercomplexes from purple bacteria is crucial for uncovering the assembly and functional mechanisms of natural photosynthetic systems and underpinning the development of artificial photosynthesis. Here, we examined excitation energy transfer of various RC-LH1 supercomplexes of Rhodobacter sphaeroides using transient absorption spectroscopy, coupled with lifetime density analysis, and studied the roles of the integral transmembrane polypeptides, PufX and PufY, in energy transfer within the RC-LH1 core complex. Our results show that the absence of PufX increases both the LH1 → RC excitation energy transfer lifetime and distribution due to the role of PufX in defining the interaction and orientation of the RC within the LH1 ring. While the absence of PufY leads to the conformational shift of several LH1 subunits toward the RC, it does not result in a marked change in the excitation energy transfer lifetime.

Primary charge separation in native and plant pheophytin a-modified reaction centers of Chloroflexus aurantiacus: Ultrafast transient absorption measurements at low temperature

Ultrafast transient absorption (TA) spectroscopy was used to study electron transfer (ET) at 100 K in native (as isolated) reaction centers (RCs) of the green filamentous photosynthetic bacterium Chloroflexus (Cfl.) aurantiacus. The rise and decay of the 1028 nm anion absorption band of the monomeric bacteriochlorophyll a molecule at the B_A binding site were monitored as indicators of the formation and decay of the P⁺B_A^- state, respectively (P is the primary electron donor, a dimer of bacteriochlorophyll a molecules). Global analysis of the TA data indicated the presence of at least two populations of the P^⁎ excited state, which decay by distinct means, forming the state P⁺H_A^- (H_A is a photochemically active bacteriopheophytin a molecule). In one population (~65 %), P^⁎ decays in ~2 ps with the formation of P⁺H_A^- via a short-lived P⁺B_A^- intermediate in a two-step ET process P^⁎ → P⁺B_A^- → P⁺H_A^-. In another population (~35 %), P^⁎ decays in ~20 ps to form P⁺H_A^- via a superexchange mechanism without producing measurable amounts of P⁺B_A^-. Similar TA measurements performed on chemically modified RCs of Cfl. aurantiacus containing plant pheophytin a at the H_A binding site also showed the presence of two P^⁎ populations (~2 and ~20 ps), with P^⁎ decaying through P⁺B_A^- only in the ~2 ps population. At 100 K, the quantum yield of primary charge separation in native RCs is determined to be close to unity. The results are discussed in terms of involving a one-step P^⁎ → P⁺H_A^- superexchange process as an alternative highly efficient ET pathway in Cfl. aurantiacus RCs.

Rhodobacter capsulatus forms a compact crescent-shaped LH1-RC photocomplex

Rhodobacter (Rba.) capsulatus has been a favored model for studies of all aspects of bacterial photosynthesis. This purple phototroph contains PufX, a polypeptide crucial for dimerization of the light-harvesting 1-reaction center (LH1-RC) complex, but lacks protein-U, a U-shaped polypeptide in the LH1-RC of its close relative Rba. sphaeroides. Here we present a cryo-EM structure of the Rba. capsulatus LH1-RC purified by DEAE chromatography. The crescent-shaped LH1-RC exhibits a compact structure containing only 10 LH1 αβ-subunits. Four αβ-subunits corresponding to those adjacent to protein-U in Rba. sphaeroides were absent. PufX in Rba. capsulatus exhibits a unique conformation in its N-terminus that self-associates with amino acids in its own transmembrane domain and interacts with nearby polypeptides, preventing it from interacting with proteins in other complexes and forming dimeric structures. These features are discussed in relation to the minimal requirements for the formation of LH1-RC monomers and dimers, the spectroscopic behavior of both the LH1 and RC, and the bioenergetics of energy transfer from LH1 to the RC.

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⁺B_B^- → P⁺H_B^-), one-step superexchange ET (e.g., P* → P⁺H_B^-), and P* → ground state. [H_B and B_B 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⁺H_B^- 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⁺H_B^-, 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.

Hidden vibronic and excitonic structure and vibronic coherence transfer in the bacterial reaction center

We report two-dimensional electronic spectroscopy (2DES) experiments on the bacterial reaction center (BRC) from purple bacteria, revealing hidden vibronic and excitonic structure. Through analysis of the coherent dynamics of the BRC, we identify multiple quasi-resonances between pigment vibrations and excitonic energy gaps, and vibronic coherence transfer processes that are typically neglected in standard models of photosynthetic energy transfer and charge separation. We support our assignment with control experiments on bacteriochlorophyll and simulations of the coherent dynamics using a reduced excitonic model of the BRC. We find that specific vibronic coherence processes can readily reveal weak exciton transitions. While the functional relevance of such processes is unclear, they provide a spectroscopic tool that uses vibrations as a window for observing excited state structure and dynamics elsewhere in the BRC via vibronic coupling. Vibronic coherence transfer reveals the upper exciton of the “special pair” that was weakly visible in previous 2DES experiments.

Photosynthetic reaction center variants made via genetic code expansion show Tyr at M210 tunes the initial electron transfer mechanism

Photosynthetic reaction centers (RCs) from Rhodobacter sphaeroides were engineered to vary the electronic properties of a key tyrosine (M210) close to an essential electron transfer component via its replacement with site-specific, genetically encoded noncanonical amino acid tyrosine analogs. High fidelity of noncanonical amino acid incorporation was verified with mass spectrometry and X-ray crystallography and demonstrated that RC variants exhibit no significant structural alterations relative to wild type (WT). Ultrafast transient absorption spectroscopy indicates the excited primary electron donor, P*, decays via a ∼4-ps and a ∼20-ps population to produce the charge-separated state P⁺H_A ^- in all variants. Global analysis indicates that in the ∼4-ps population, P⁺H_A ^- forms through a two-step process, P*→ P⁺B_A ^-→ P⁺H_A ^-, while in the ∼20-ps population, it forms via a one-step P* → P⁺H_A ^- superexchange mechanism. The percentage of the P* population that decays via the superexchange route varies from ∼25 to ∼45% among variants, while in WT, this percentage is ∼15%. Increases in the P* population that decays via superexchange correlate with increases in the free energy of the P⁺B_A ^- intermediate caused by a given M210 tyrosine analog. This was experimentally estimated through resonance Stark spectroscopy, redox titrations, and near-infrared absorption measurements. As the most energetically perturbative variant, 3-nitrotyrosine at M210 creates an ∼110-meV increase in the free energy of P⁺B_A ^- along with a dramatic diminution of the 1,030-nm transient absorption band indicative of P⁺B_A ^- formation. Collectively, this work indicates the tyrosine at M210 tunes the mechanism of primary electron transfer in the RC.

In Situ, Protein-Mediated Generation of a Photochemically Active Chlorophyll Analogue in a Mutant Bacterial Photosynthetic Reaction Center

All possible natural amino acids have been substituted for the native LeuL185 positioned near the B-side bacteriopheophytin (H_B) in the bacterial reaction center (RC) from Rhodobacter sphaeroides. Additional mutations that enhance electron transfer to the normally inactive B-side cofactors are present. Approximately half of the isolated RCs with Glu at L185 contain a magnesium chlorin (C_B) in place of H_B. The chlorin is not the common BChl a oxidation product 3-desvinyl-3-acetyl chlorophyll a with a C-C bond in ring D and a C═C bond in ring B but has properties consistent with reversal of these bond orders, giving 17,18-didehydro BChl a. In such RCs, charge-separated state P⁺C_B^- forms in ∼5% yield. The other half of the GluL185-containing RCs have a bacteriochlorophyll a (BChl a) denoted β_B in place of H_B. Residues His, Asp, Asn, and Gln at L185 yield RCs with ≥85% β_B in the H_B site, while most other amino acids result in RCs that retain H_B (≥95%). To the best of our knowledge, neither bacterial RCs that harbor five BChl a molecules and one chlorophyll analogue nor those with six BChl a molecules have been reported previously. The finding that altering the local environment within a cofactor binding site of a transmembrane complex leads to in situ generation of a photoactive chlorin with an unusual ring oxidation pattern suggests new strategies for amino acid control over pigment type at specific sites in photosynthetic proteins.

Structures of Rhodopseudomonas palustris RC-LH1 complexes with open or closed quinone channels

The reaction-center light-harvesting complex 1 (RC-LH1) is the core photosynthetic component in purple phototrophic bacteria. We present two cryo-electron microscopy structures of RC-LH1 complexes from Rhodopseudomonas palustris A 2.65-Å resolution structure of the RC-LH1₁₄-W complex consists of an open 14-subunit LH1 ring surrounding the RC interrupted by protein-W, whereas the complex without protein-W at 2.80-Å resolution comprises an RC completely encircled by a closed, 16-subunit LH1 ring. Comparison of these structures provides insights into quinone dynamics within RC-LH1 complexes, including a previously unidentified conformational change upon quinone binding at the RC Q_B site, and the locations of accessory quinone binding sites that aid their delivery to the RC. The structurally unique protein-W prevents LH1 ring closure, creating a channel for accelerated quinone/quinol exchange.

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.

Switching sides-Reengineered primary charge separation in the bacterial photosynthetic reaction center

We report 90% yield of electron transfer (ET) from the singlet excited state P* of the primary electron-donor P (a bacteriochlorophyll dimer) to the B-side bacteriopheophytin (H_B) in the bacterial photosynthetic reaction center (RC). Starting from a platform Rhodobacter sphaeroides RC bearing several amino acid changes, an Arg in place of the native Leu at L185-positioned over one face of H_B and only ∼4 Å from the 4 central nitrogens of the H_B macrocycle-is the key additional mutation providing 90% yield of P⁺H_B ^- This all but matches the near-unity yield of A-side P⁺H_A ^- charge separation in the native RC. The 90% yield of ET to H_B derives from (minimally) 3 P* populations with distinct means of P* decay. In an ∼40% population, P* decays in ∼4 ps via a 2-step process involving a short-lived P⁺B_B ^- intermediate, analogous to initial charge separation on the A side of wild-type RCs. In an ∼50% population, P* → P⁺H_B ^- conversion takes place in ∼20 ps by a superexchange mechanism mediated by B_B An ∼10% population of P* decays in ∼150 ps largely by internal conversion. These results address the long-standing dichotomy of A- versus B-side initial charge separation in native RCs and have implications for the mechanism(s) and timescale of initial ET that are required to achieve a near-quantitative yield of unidirectional charge separation.

Consequences of saturation mutagenesis of the protein ligand to the B-side monomeric bacteriochlorophyll in reaction centers from Rhodobacter capsulatus

In bacterial reaction centers (RCs), photon-induced initial charge separation uses an A-side bacteriochlorophyll (BChl, B_A) and bacteriopheophytin (BPh, H_A), while the near-mirror image B-side B_B and H_B cofactors are inactive. Two new sets of Rhodobacter capsulatus RC mutants were designed, both bearing substitution of all amino acids for the native histidine M180 (M-polypeptide residue 180) ligand to the core Mg ion of B_B. Residues are identified that largely result in retention of a BChl in the B_B site (Asp, Ser, Pro, Gln, Asn, Gly, Cys, Lys, and Thr), ones that largely harbor the Mg-free BPh in the B_B site (Leu and Ile), and ones for which isolated RCs are comprised of a substantial mixture of these two RC types (Ala, Glu, Val, Met and, in one set, Arg). No protein was isolated when M180 is Trp, Tyr, Phe, or (in one set) Arg. These findings are corroborated by ground state spectra, pigment extractions, ultrafast transient absorption studies, and the yields of B-side transmembrane charge separation. The changes in coordination chemistries did not reveal an RC with sufficiently precise poising of the redox properties of the B_B-site cofactor to result in a high yield of B-side electron transfer to H_B. Insights are gleaned into the amino acid properties that support BChl in the B_B site and into the widely observed multi-exponential decay of the excited state of the primary electron donor. The results also have direct implications for tuning free energies of the charge-separated intermediates in RCs and mimetic systems.

Mixed Upper Exciton State of the Special Pair in Bacterial Reaction Centers

Excitonic interactions between two closely separated bacteriochlorophyll a molecules (BChls) in the special pair of the reaction center (RC) of purple bacteria determine the positions and relative oscillator strengths of its two excitonic components. While the absorption of the lower excitonic band is well-defined, the position and the intensity of the upper excitonic band ( P_Y+) are still under debate. Recent 77 K two-dimensional electronic spectroscopy data on Rba. capsulatus suggested that the P_Y+ component absorbs at ∼840 nm, i.e., at a significantly lower energy than previously suggested. In the present work, we argue that the P_Y+ state is mixed with the excited states of the accessory BChls ( B*/ P _Y+) leading to excitons contributing to the 785-825 nm spectral region which is consistent with previously published data. This conclusion is based on hole-burning/linear dichroism data and modeling studies of the excitonic structure of the RC using a non-Markovian reduced density matrix approach.

Primary processes in the bacterial reaction center probed by two-dimensional electronic spectroscopy

In the initial steps of photosynthesis, reaction centers convert solar energy to stable charge-separated states with near-unity quantum efficiency. The reaction center from purple bacteria remains an important model system for probing the structure-function relationship and understanding mechanisms of photosynthetic charge separation. Here we perform 2D electronic spectroscopy (2DES) on bacterial reaction centers (BRCs) from two mutants of the purple bacterium Rhodobacter capsulatus, spanning the Q _y absorption bands of the BRC. We analyze the 2DES data using a multiexcitation global-fitting approach that employs a common set of basis spectra for all excitation frequencies, incorporating inputs from the linear absorption spectrum and the BRC structure. We extract the exciton energies, resolving the previously hidden upper exciton state of the special pair. We show that the time-dependent 2DES data are well-represented by a two-step sequential reaction scheme in which charge separation proceeds from the excited state of the special pair (P*) to P⁺H_A^- via the intermediate P⁺B_A^- When inhomogeneous broadening and Stark shifts of the B* band are taken into account we can adequately describe the 2DES data without the need to introduce a second charge-separation pathway originating from the excited state of the monomeric bacteriochlorophyll B_A*.