Triplet energy transfer in bacterial photosynthetic reaction centres

Anomalous Temperature Dependence of the Triplet-Triplet Energy Transfer in Cereibacter sphaeroides I(L177)H Mutant Reaction Centers

In photosynthetic reaction centers, quenching of the primary donor triplet state by energy transfer to the carotenoid molecule provides efficient suppression of generation of singlet-excited oxygen, potent chemical oxidant. This process in the Cereibacter sphaeroides reaction centers is thermoactivated, and discontinues at temperatures below 40 K. In these reaction centers, substitution of amino acid residue isoleucine at the 177 position of the L-subunit with histidine results in the sharp decrease of activation energy, so that the carotenoid triplets are populated even at 10 K. Activation energy of the T-T energy transfer was estimated as 7.5 cm-1, which is more than 10-fold lower than activation energy in the wild type reaction centers. At certain temperatures, the energy transfer in the mutant is decelerated, which is related to the increase of effective distance of the triplet-triplet transfer. To the best of our knowledge, the described mutation presents the first reaction center modification leading to the significant decrease in activation energy of the T-T energy transfer to carotenoid molecule. The I(L177)H mutant reaction centers present a considerable interest for further studies of the triplet state quenching mechanisms, and of other photophysical and photochemical processes in the reaction centers of bacterial photosynthesis.

Influence of the Electrochemical Properties of the Bacteriochlorophyll Dimer on Triplet Energy-Transfer Dynamics in Bacterial Reaction Centers

Energetics, protein dynamics, and electronic coupling are the key factors in controlling both electron and energy transfer in photosynthetic bacterial reaction centers (RCs). Here, we examine the rates and mechanistic pathways of the P⁺H_A^- radical-pair charge recombination, triplet state formation, and subsequent triplet energy transfer from the triplet state of the bacteriochlorophyll dimer (P) to the carotenoid in a series of mutant RCs (L131LH + M160LH (D1), L131LH + M197FH (D2), and L131LH + M160LH + M197FH (T1)) of Rhodobacter sphaeroides. In these mutants, the electronic structure of P is perturbed and the P/P⁺ midpoint potential is systematically increased due to addition of hydrogen bonds between P and the introduced residues. High-resolution, broad-band, transient absorption spectroscopy on the femtosecond to microsecond timescale shows that the charge recombination rate increases and the triplet energy transfer rate decreases in these mutants relative to the wild type (WT). The increase of the charge recombination rate is correlated to the increase in the energy level of P⁺H_A^- and the increase in the P/P⁺ midpoint potential. On the other hand, the decrease in rate of triplet energy transfer in the mutants can be explained in terms of a lower energy of ³P and a shift in the electron spin density distribution in the bacteriochlorophylls of P. The triplet energy-transfer rate follows the order of WT > L131LH + M197FH > L131LH + M160LH > L131LH + M160LH + M197FH, both at room temperature and at 77 K. A pronounced temperature dependence of the rate is observed for all of the RC samples. The activation energy associated to this process is increased in the mutants relative to WT, consistent with a lower ³P energy due to the addition of hydrogen bonds between P and the introduced residues.

Modelling of the cathodic and anodic photocurrents from Rhodobacter sphaeroides reaction centres immobilized on titanium dioxide

As one of a number of new technologies for the harnessing of solar energy, there is interest in the development of photoelectrochemical cells based on reaction centres (RCs) from photosynthetic organisms such as the bacterium Rhodobacter (Rba.) sphaeroides. The cell architecture explored in this report is similar to that of a dye-sensitized solar cell but with delivery of electrons to a mesoporous layer of TiO₂ by natural pigment-protein complexes rather than an artificial dye. Rba. sphaeroides RCs were bound to the deposited TiO₂ via an engineered extramembrane peptide tag. Using TMPD (N,N,N',N'-tetramethyl-p-phenylenediamine) as an electrolyte, these biohybrid photoactive electrodes produced an output that was the net product of cathodic and anodic photocurrents. To explain the observed photocurrents, a kinetic model is proposed that includes (1) an anodic current attributed to injection of electrons from the triplet state of the RC primary electron donor (P^T) to the TiO₂ conduction band, (2) a cathodic current attributed to reduction of the photooxidized RC primary electron donor (P⁺) by surface states of the TiO₂ and (3) transient cathodic and anodic current spikes due to oxidation/reduction of TMPD/TMPD⁺ at the conductive glass (FTO) substrate. This model explains the origin of the photocurrent spikes that appear in this system after turning illumination on or off, the reason for the appearance of net positive or negative stable photocurrents depending on experimental conditions, and the overall efficiency of the constructed cell. The model may be a used as a guide for improvement of the photocurrent efficiency of the presented system as well as, after appropriate adjustments, other biohybrid photoelectrodes.

Bacteriopheophytin triplet state in Rhodobacter sphaeroides reaction centers

It is well established that photoexcitation of Rhodobacter sphaeroides reaction centers (RC) with reduced quinone acceptors results in the formation of a triplet state localized on the primary electron donor P with a significant yield. The energy of this long-lived and therefore potentially damaging excited state is then efficiently quenched by energy transfer to the RC spheroidenone carotenoid, with its subsequent decay to the ground state by intersystem crossing. In this contribution, we present a detailed transient absorption study of triplet states in a set of mutated RCs characterized by different efficiencies of triplet formation that correlate with lifetimes of the initial charge-separated state P(+)H A (-) . On a microsecond time scale, two types of triplet state were detected: in addition to the well-known spheroidenone triplet state with a lifetime of ~4 μs, in some RCs we discovered a bacteriopheophytin triplet state with a lifetime of ~40 μs. As expected, the yield of the carotenoid triplet increased approximately linearly with the lifetime of P(+)H A (-) , reaching the value of 42 % for one of the mutants. However, surprisingly, the yield of the bacteriopheophytin triplet was the highest in RCs with the shortest P(+)H A (-) lifetime and the smallest yield of carotenoid triplet. For these the estimated yield of bacteriopheophytin triplet was comparable with the yield of the carotenoid triplet, reaching a value of ~7 %. Possible mechanisms of formation of the bacteriopheophytin triplet state are discussed.

Ultrafast Electron Transfer Kinetics in the LM Dimer of Bacterial Photosynthetic Reaction Center from Rhodobacter sphaeroides

It has become increasingly clear that dynamics plays a major role in the function of many protein systems. One system that has proven particularly facile for studying the effects of dynamics on protein-mediated chemistry is the bacterial photosynthetic reaction center from Rhodobacter sphaeroides. Previous experimental and computational analysis have suggested that the dynamics of the protein matrix surrounding the primary quinone acceptor, QA, may be particularly important in electron transfer involving this cofactor. One can substantially increase the flexibility of this region by removing one of the reaction center subunits, the H-subunit. Even with this large change in structure, photoinduced electron transfer to the quinone still takes place. To evaluate the effect of H-subunit removal on electron transfer to QA, we have compared the kinetics of electron transfer and associated spectral evolution for the LM dimer with that of the intact reaction center complex on picosecond to millisecond time scales. The transient absorption spectra associated with all measured electron transfer reactions are similar, with the exception of a broadening in the QX transition and a blue-shift in the QY transition bands of the special pair of bacteriochlorophylls (P) in the LM dimer. The kinetics of the electron transfer reactions not involving quinones are unaffected. There is, however, a 4-fold decrease in the electron transfer rate from the reduced bacteriopheophytin to QA in the LM dimer compared to the intact reaction center and a similar decrease in the recombination rate of the resulting charge-separated state (P(+)QA(-)). These results are consistent with the concept that the removal of the H-subunit results in increased flexibility in the region around the quinone and an associated shift in the reorganization energy associated with charge separation and recombination.

Low-temperature pulsed EPR study at 34 GHz of the triplet states of the primary electron Donor P865 and the carotenoid in native and mutant bacterial reaction centers of Rhodobacter sphaeroides

The photosynthetic charge separation in bacterial reaction centers occurs predominantly along one of two nearly symmetric branches of cofactors. Low-temperature EPR spectra of the triplet states of the chlorophyll and carotenoid pigments in the reaction center of Rhodobacter sphaeroides R-26.1, 2.4.1 and two double-mutants GD(M203)/AW(M260) and LH(M214)/AW(M260) have been recorded at 34 GHz to investigate the relative activities of the "A" and "B" branches. The triplet states are found to derive from radical pair and intersystem crossing mechanisms, and the rates of formation are anisotropic. The former mechanism is operative for Rb. sphaeroides R-26.1, 2.4.1, and mutant GD(M203)/AW(M260) and indicates that A-branch charge separation proceeds at temperatures down to 10 K. The latter mechanism, derived from the spin polarization and operative for mutant LH(M214)/AW(M260), indicates that no long-lived radical pairs are formed upon direct excitation of the primary donor and that virtually no charge separation at the B-branch occurs at low temperatures. When the temperature is raised above 30 K, B-branch charge separation is observed, which is at most 1% of A-branch charge separation. B-branch radical pair formation can be induced at 10 K with low yield by direct excitation of the bacteriopheophytin of the B-branch at 590 nm. The formation of a carotenoid triplet state is observed. The rate of formation depends on the orientation of the reaction center in the magnetic field and is caused by a magnetic field dependence of the oscillation frequency by which the singlet and triplet radical pair precursor states interchange. Combination of these findings with literature data provides strong evidence that the thermally activated transfer step on the B-branch occurs between the primary donor, P865, and the accessory bacteriochlorophyll, whereas this step is barrierless down to 10 K along the A-branch.

Design and engineering of photosynthetic light-harvesting and electron transfer using length, time, and energy scales

Decades of research on the physical processes and chemical reaction-pathways in photosynthetic enzymes have resulted in an extensive database of kinetic information. Recently, this database has been augmented by a variety of high and medium resolution crystal structures of key photosynthetic enzymes that now include the two photosystems (PSI and PSII) of oxygenic photosynthetic organisms. Here, we examine the currently available structural and functional information from an engineer's point of view with the long-term goal of reproducing the key features of natural photosystems in de novo designed and custom-built molecular solar energy conversion devices. We find that the basic physics of the transfer processes, namely, the time constraints imposed by the rates of incoming photon flux and the various decay processes allow for a large degree of tolerance in the engineering parameters. Moreover, we find that the requirements to guarantee energy and electron transfer rates that yield high efficiency in natural photosystems are largely met by control of distance between chromophores and redox cofactors. Thus, for projected de novo designed constructions, the control of spatial organization of cofactor molecules within a dense array is initially given priority. Nevertheless, constructions accommodating dense arrays of different cofactors, some well within 1 nm from each other, still presents a significant challenge for protein design.

Replacement or exclusion of the B-branch bacteriopheophytin in the purple bacterial reaction centre: The HB cofactor is not required for assembly or core function of the Rhodobacter sphaeroides complex

All of the membrane-embedded cofactors of the purple bacterial reaction centre have well-defined functional or structural roles, with the exception of the bacteriopheophytin (H(B)) located approximately half-way across the membrane on the so-called inactive- or B-branch of cofactors. Sequence alignments indicate that this bacteriochlorin cofactor is a conserved feature of purple bacterial reaction centres, and a pheophytin is also found at this position in the Photosystem-II reaction centre. Possible structural or functional consequences of replacing the H(B) bacteriopheophytin by bacteriochlorophyll were investigated in the Rhodobacter sphaeroides reaction centre through mutagenesis of residue Leu L185 to His (LL185H). Results from absorbance spectroscopy indicated that the LL185H mutant assembled with a bacteriochlorophyll at the H(B) position, but this did not affect the capacity of the reaction centre to support photosynthetic growth, or change the kinetics of charge separation along the A-branch of cofactors. It was also found that mutation of residue Ala M149 to Trp (AM149W) caused the reaction centre to assemble without an H(B) bacteriochlorin, demonstrating that this cofactor is not required for correct assembly of the reaction centre. The absence of a cofactor at this position did not affect the capacity of the reaction centre to support photosynthetic growth, or the kinetics of A-branch electron transfer. A combination of X-ray crystallography and FTIR difference spectroscopy confirmed that the H(B) cofactor was absent in the AM149W mutant, and that this had not produced any significant disturbance of the adjacent ubiquinol reductase (Q(B)) site. The data are discussed with respect to possible functional roles of the H(B) bacteriopheophytin, and we conclude that the reason(s) for conservation of a bacteriopheophytin cofactor at this position in purple bacterial reaction centres are likely to be different from those underlying conservation of a pheophytin at the analogous position in Photosystem-II.

Local Electrostatic Field Induced by the Carotenoid Bound to the Reaction Center of the Purple Photosynthetic Bacterium Rhodobacter Sphaeroides

Electroabsorption (EA) spectra were recorded in the region of the reaction center (RC) Qy absorption bands of bacteriochlorophyll (Bchl) and bacteriopheophytin, to investigate the effect of carotenoid (Car) on the electrostatic environment of the RCs of the purple bacterium Rhodobacter (Rb.) sphaeroides. Two different RCs were prepared from Rb. sphaeroides strain R26.1 (R26.1-RC); R26.1 RC lacking Car and a reconstituted RC (R26.1-RC+ Car) prepared by incorporating a synthetic Car (3,4-dihydrospheroidene). Although there were no detectable differences between these two RCs in their near infrared (NIR) absorption spectra at 79 and 293 K, or in their EA spectra at 79 K, significant differences were detected in their EA spectra at 293 K. Three nonlinear optical parameters of each RC were determined in order to evaluate quantitatively these differences; transition dipole-moment polarizability and hyperpolarizability (D factor), the change in polarizability upon photoexcitation (Deltaalpha), and the change in dipole-moment upon photoexcitation (Deltamu). The value of D or Deltaalpha determined for each absorption band of the two RC samples showed similar values at 77 or 293 K. However, the Deltamu values of the special pair Bchls (P) and the monomer Bchls absorption bands showed significant differences between the two RCs at 293 K. X-ray crystallography of the two RCs has revealed that a single molecule of the solubilizing detergent LDAO occupies part of the carotenoid binding site in the absence of a carotenoid. The difference in the value of Deltamu therefore represents the differential effect of the detergent LDAO and the carotenoid on P. The change of electrostatic field around P induced by the presence of Car was determined to be 1.7 x 10(5) [V/cm], corresponding to a approximately 10% change in the electrostatic field around P.