Ground State and Femtosecond Transient Absorption Spectroscopy of a Mutant of Rhodobacter capsulatus which Lacks the Initial Electron Acceptor Bacteriopheophytin. In: Michel-beyerle M, editor. Reaction Centers of Photosynthetic Bacteria. Berlin: Springer Berlin Heidelberg; 1990. pp. 293-302. [DOI: 10.1007/978-3-642-61297-8_29]

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.

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 (HB) 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 (CB) in place of HB. 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+CB- forms in ∼5% yield. The other half of the GluL185-containing RCs have a bacteriochlorophyll a (BChl a) denoted βB in place of HB. Residues His, Asp, Asn, and Gln at L185 yield RCs with ≥85% βB in the HB site, while most other amino acids result in RCs that retain HB (≥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.

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.

Engineering opposite electronic polarization of singlet and triplet states increases the yield of high-energy photoproducts

Efficient photosynthetic energy conversion requires quantitative, light-driven formation of high-energy, charge-separated states. However, energies of high-lying excited states are rarely extracted, in part because the congested density of states in the excited-state manifold leads to rapid deactivation. Conventional photosystem designs promote electron transfer (ET) by polarizing excited donor electron density toward the acceptor ("one-way" ET), a form of positive design. Curiously, negative design strategies that explicitly avoid unwanted side reactions have been underexplored. We report here that electronic polarization of a molecular chromophore can be used as both a positive and negative design element in a light-driven reaction. Intriguingly, prudent engineering of polarized excited states can steer a "U-turn" ET-where the excited electron density of the donor is initially pushed away from the acceptor-to outcompete a conventional one-way ET scheme. We directly compare one-way vs. U-turn ET strategies via a linked donor-acceptor (DA) assembly in which selective optical excitation produces donor excited states polarized either toward or away from the acceptor. Ultrafast spectroscopy of DA pinpoints the importance of realizing donor singlet and triplet excited states that have opposite electronic polarizations to shut down intersystem crossing. These results demonstrate that oppositely polarized electronically excited states can be employed to steer photoexcited states toward useful, high-energy products by routing these excited states away from states that are photosynthetic dead ends.

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.

Genetic Code Expansion in Rhodobacter sphaeroides to Incorporate Noncanonical Amino Acids into Photosynthetic Reaction Centers

Photosynthetic reaction centers (RCs) are the membrane proteins responsible for the initial charge separation steps central to photosynthesis. As a complex and spectroscopically complicated membrane protein, the RC (and other associated photosynthetic proteins) would benefit greatly from the insight offered by site-specifically encoded noncanonical amino acids in the form of probes and an increased chemical range in key amino acid analogues. Toward that goal, we developed a method to transfer amber codon suppression machinery developed for E. coli into the model bacterium needed to produce RCs, Rhodobacter sphaeroides. Plasmids were developed and optimized to incorporate 3-chlorotyrosine, 3-bromotyrosine, and 3-iodotyrosine into RCs. Multiple challenges involving yield and orthogonality were overcome to implement amber suppression in R. sphaeroides, providing insights into the hurdles that can be involved in host transfer of amber suppression systems from E. coli. In the process of verifying noncanonical amino acid incorporation, characterization of this membrane protein via mass spectrometry (which has been difficult previously) was substantially improved. Importantly, the ability to incorporate noncanonical amino acids in R. sphaeroides expands research capabilities in the photosynthetic field.

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*.

Putative hydrogen bond to tyrosine M208 in photosynthetic reaction centers from Rhodobacter capsulatus significantly slows primary charge separation

Slow, ∼50 ps, P* → P⁺H_A^– electron transfer is observed in Rhodobacter capsulatus reaction centers (RCs) bearing the native Tyr residue at M208 and the single amino acid change of isoleucine at M204 to glutamic acid. The P* decay kinetics are unusually homogeneous (single exponential) at room temperature. Comparative solid-state NMR of [4′-¹³C]Tyr labeled wild-type and M204E RCs show that the chemical shift of Tyr M208 is significantly altered in the M204E mutant and in a manner consistent with formation of a hydrogen bond to the Tyr M208 hydroxyl group. Models based on RC crystal structure coordinates indicate that if such a hydrogen bond is formed between the Glu at M204 and the M208 Tyr hydroxyl group, the −OH would be oriented in a fashion expected (based on the calculations by Alden et al., J. Phys. Chem.1996, 100, 16761–16770) to destabilize P⁺B_A^– in free energy. Alteration of the environment of Tyr M208 and B_A by Glu M204 via this putative hydrogen bond has a powerful influence on primary charge separation.

Escape probability and trapping mechanism in purple bacteria: revisited

Despite intensive research for decades, the trapping mechanism in the core complex of purple bacteria is still under discussion. In this article, it is attempted to derive a conceptionally simple model that is consistent with all basic experimental observations and that allows definite conclusions on the trapping mechanism. Some experimental data reported in the literature are conflicting or incomplete. Therefore we repeated two already published experiments like the time-resolved fluorescence decay in LH1-only purple bacteria Rhodospirillum rubrum and Rhodopseudomonas viridis chromatophores with open and closed (Q(A)(-)) reaction centers. Furthermore, we measured fluorescence excitation spectra for both species under the two redox-conditions. These data, all measured at room temperature, were analyzed by a target analysis based on a three-state model (antenna, primary donor, and radical pair). All states were allowed to react reversibly and their decay channels were taken into consideration. This leads to seven rate constants to be determined. It turns out that a unique set of numerical values of these rate constants can be found, when further experimental constraints are met simultaneously, i.e. the ratio of the fluorescence yields in the open and closed (Q(A)(-)) states F(m)/F(o) approximately 2 and the P(+)H(-)-recombination kinetics of 3-6 ns. The model allows to define and to quantify escape probabilities and the transfer equilibrium. We conclude that trapping in LH1-only purple bacteria is largely transfer-to-the-trap-limited. Furthermore, the model predicts properties of the reaction center (RC) in its native LH1-environment. Within the framework of our model, the predicted P(+)H(-)-recombination kinetics are nearly indistinguishable for a hypothetically isolated RC and an antenna-RC complex, which is in contrast to published experimental data for physically isolated RCs. Therefore RC preparations may display modified kinetic properties.

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.

Structural model of the photosynthetic reaction center of Rhodobacter capsulatus

The reaction center (RC) from the photosynthetic bacterium Rhodobacter (Rb.) capsulatus has been the subject of a considerable amount of molecular biological and spectroscopic work aimed at improving our understanding of the primary steps of photosynthesis. However, no three-dimensional structure is available for this protein. We present here a model obtained by combining information from the structure of the highly homologous RC from Rhodopseudomonas (Rps.) viridis with molecular mechanics and simulated annealing calculations. In the Rb. capsulatus model the orientations of the bacteriochlorophyll monomer and the bacteriopheophytin on the branch inactive in electron transfer differ significantly from those in the RCs of Rps. viridis and Rb. sphaeroides. The bacteriopheophytin orientational difference is in good accord with previous linear dichroism measurements. A comparison is made of interactions between the pigments and the protein environment that may be of functional significance in Rps. viridis, Rb. sphaeroides, and Rb. capsulatus.