Spectral resolution of the primary electron acceptor A0 in Photosystem I

Energetic Diversity in the Electron-Transfer Pathways of Type I Photosynthetic Reaction Centers

Photosynthetic reaction centers from heliobacteria (HbRC) and green sulfur bacteria (GsbRC) are homodimeric proteins and share a common ancestor with photosystem I (PSI), classified as type I reaction centers. Using the HbRC crystal structure, we calculated the redox potential (Em) values in the electron-transfer branches, solving the linear Poisson-Boltzmann equation and considering the protonation states of all titratable sites in the entire protein-pigment complex. Em(A-1) for bacteriochlorophyll g at the secondary site in HbRC (-1157 mV) is as low as Em(A-1) for chlorophyll a in PSI (-1173 mV). Em(A0/HbRC) is at the same level as Em(A0/GsbRC) and is 200 mV higher than Em(A0/PSI) due to the replacement of PsaA-Trp697/PsaB-Trp677 in PSI with PshA-Arg554 in HbRC. In contrast, Em(FX) for the Fe4S4 cluster in HbRC (-420 mV) is significantly higher than Em(FX) in GsbRC (-719 mV) and PSI (-705 mV) due to the absence of acidic residues that correspond to PscA-Asp634 in GsbRC and PsaB-Asp575 in PSI. It seems likely that type I reaction centers have evolved, adopting (bacterio)chlorophylls suitable for their light environments while maintaining electron-transfer cascades.

Energetics of the Electron Transfer Pathways in the Homodimeric Photosynthetic Reaction Center

Photosynthetic reaction centers from a green sulfur bacterium (GsbRC), the PscA/PscA proteins, and photosystem I (PSI), PsaA/PsaB proteins, share structural similarities. Here, we report the redox potential (Em) values of GsbRC by solving the linear Poisson-Boltzmann equation and considering the protonation states of all titratable sites in the entire GsbRC protein and identify the factors that shift the Em values with respect to PSI. The Em values for one-electron reduction of the accessory (A-1) and adjacent (A0) chlorophylls in GsbRC are 100-250 mV higher than those in PSI, whereas the Em values for the Fe4S4 cluster (FX) are at the same level. The PsaA-Trp697/PsaB-Trp677 pair in PSI, which forms the A1-quinone binding site, is replaced with PscA-Arg638 in GsbRC. PsaB-Asp575 in PSI, which is responsible for the Em difference between A1A and A1B quinones in PSI, is absent in GsbRC. These discrepancies also contribute to the upshift in Em(A-1) and Em(A0) in GsbRC with respect to PSI. It seems likely that the upshifted Em for chlorophylls in GsbRC ultimately originates from the characteristics of the electrostatic environment that corresponds to the A1 site of PSI.

Current state of the primary charge separation mechanism in photosystem I of cyanobacteria

This review analyzes new data on the mechanism of ultrafast reactions of primary charge separation in photosystem I (PS I) of cyanobacteria obtained in the last decade by methods of femtosecond absorption spectroscopy. Cyanobacterial PS I from many species harbours 96 chlorophyll a (Chl a) molecules, including six specialized Chls denoted Chl1A/Chl1B (dimer P700, or PAPB), Chl2A/Chl2B, and Chl3A/Chl3B arranged in two branches, which participate in electron transfer reactions. The current data indicate that the primary charge separation occurs in a symmetric exciplex, where the special pair P700 is electronically coupled to the symmetrically located monomers Chl2A and Chl2B, which can be considered together as a symmetric exciplex Chl2APAPBChl2B with the mixed excited (Chl2APAPBChl2B)* and two charge-transfer states P700 +Chl2A - and P700 +Chl2B -. The redistribution of electrons between the branches in favor of the A-branch occurs after reduction of the Chl2A and Chl2B monomers. The formation of charge-transfer states and the symmetry breaking mechanisms were clarified by measuring the electrochromic Stark shift of β-carotene and the absorption dynamics of PS I complexes with the genetically altered Chl 2B or Chl 2A monomers. The review gives a brief description of the main methods for analyzing data obtained using femtosecond absorption spectroscopy. The energy levels of excited and charge-transfer intermediates arising in the cyanobacterial PS I are critically analyzed.

Symmetry breaking in photosystem I: ultrafast optical studies of variants near the accessory chlorophylls in the A- and B-branches of electron transfer cofactors

Femtosecond absorption spectroscopy of Photosystem I (PS I) complexes from the cyanobacterium Synechocystis sp. PCC 6803 was carried out on three pairs of complementary amino acid substitutions located near the second pair of chlorophyll molecules Chl_2A and Chl_2B (also termed A_-1A and A_-1B). The absorption dynamics at delays of 0.1-500 ps were analyzed by decomposition into discrete decay-associated spectra and continuously distributed exponential components. The multi-exponential deconvolution of the absorption changes revealed that the electron transfer reactions in the PsaA-N600M, PsaA-N600H, and PsaA-N600L variants near the B-branch of cofactors are similar to those of the wild type, while the PsaB-N582M, PsaB-N582H, and PsaB-N582L variants near the A-branch of cofactors cause significant alterations of the photochemical processes, making them heterogeneous and poorly described by a discrete exponential kinetic model. A redistribution of the unpaired electron between the second and the third monomers Chl_2A/Chl_2B and Chl_3A/Chl_3B was identified in the time range of 9-20 ps, and the subsequent reduction of A₁ was identified in the time range of 24-70 ps. In the PsaA-N600L and PsaB-N582H/L variants, the reduction of A₁ occurred with a decreased quantum yield of charge separation. The decreased quantum yield correlates with a slowing of the phylloquinone A₀ → A₁ reduction, but not with the initial transient spectra measured at the shortest time delay. The results support a branch competition model, where the electron is sheared between Chl_2A-Chl_3A and Chl_2B-Chl_3B cofactors before its transfer to phylloquinone in either A_1A or A_1B sites.

Electron Acceptor-Donor Iron Sites in the Iron-Sulfur Cluster of Photosynthetic Electron-Transfer Pathways

In photosystem I, two electron-transfer pathways via quinones (A_1A and A_1B) are merged at the iron-sulfur Fe₄S₄ cluster F_X into a single pathway toward the other two Fe₄S₄ clusters F_A and F_B. Using a quantum mechanical/molecular mechanical approach, we identify the redox-active Fe sites in the clusters. In F_A and F_B, the Fe site, which does not belong to the CxxCxxCxxxCP motif, serves as an electron acceptor/donor. F_X has two independent electron acceptor Fe sites for A- and B-branch electron transfers, depending on the Asp-B575 protonation state, which causes the A_1A-to-F_X electron transfer to be uphill and the A_1B-to-F_X electron transfer to be downhill. The two asymmetric electron-transfer pathways from A₁ to F_X and the separation of the electron acceptor and donor Fe sites are likely associated with the specific role of F_X in merging the two electron transfer pathways into the single pathway.

A dimeric chlorophyll electron acceptor differentiates type I from type II photosynthetic reaction centers

This research addresses one of the most compelling issues in the field of photosynthesis, namely, the role of the accessory chlorophyll molecules in primary charge separation. Using a combination of empirical and computational methods, we demonstrate that the primary acceptor of photosystem (PS) I is a dimer of accessory and secondary chlorophyll molecules, Chl_2A and Chl_3A, with an asymmetric electron charge density distribution. The incorporation of highly coupled donors and acceptors in PS I allows for extensive delocalization that prolongs the lifetime of the charge-separated state, providing for high quantum efficiency. The discovery of this motif has widespread implications ranging from the evolution of naturally occurring reaction centers to the development of a new generation of highly efficient artificial photosynthetic systems.

Video abstract

Primary charge separation within the structurally symmetric tetrameric Chl2APAPBChl2B chlorophyll exciplex in photosystem I

In Photosystem I (PS I), the role of the accessory chlorophyll (Chl) molecules, Chl_2A and Chl_2B (also termed A_-1A and A_-1B), which are directly adjacent to the special pair P₇₀₀ and fork into the A- and B-branches of electron carriers, is incompletely understood. In this work, the Chl_2A and Chl_2B transient absorption ΔA₀(λ) at a time delay of 100 fs was identified by ultrafast pump-probe spectroscopy in three pairs of PS I complexes from Synechocystis sp. PCC 6803 with residues PsaA-N600 or PsaB-N582 (which ligate Chl_2B or Chl_2A through a H₂O molecule) substituted by Met, His, and Leu. The ΔA₀(λ) spectra were quantified using principal component analysis, the main component of which was interpreted as a mutation-induced shift of the equilibrium between the excited state of primary donor P₇₀₀^⁎ and the primary charge-separated state P₇₀₀⁺Chl₂^-. This equilibrium is shifted to the charge-separated state in wild-type PS I and to the excited P₇₀₀ in the PS I complexes with the substituted ligands to the Chl_2A and Chl_2B monomers. The results can be rationalized within the framework of an adiabatic model in which the P₇₀₀ is electronically coupled with the symmetrically arranged monomers Chl_2A and Chl_2B; such a structure can be considered a symmetric tetrameric exciplex Chl_2AP_AP_BChl_2B, in which the excited state (Chl_2AP_AP_BChl_2B)* is mixed with two charge-transfer states P₇₀₀⁺Chl_2A^- and P₇₀₀⁺Chl_2B^-. The electron redistribution between the two branches in favor of the A-branch apparently takes place in the picosecond time scale after reduction of the Chl_2A and Chl_2B monomers.

Generation of ion-radical chlorophyll states in the light-harvesting antenna and the reaction center of cyanobacterial photosystem I

The energy and charge-transfer processes in photosystem I (PS I) complexes isolated from cyanobacteria Thermosynechococcus elongatus and Synechocystis sp. PCC 6803 were investigated by pump-to-probe femtosecond spectroscopy. The formation of charge-transfer (CT) states in excitonically coupled chlorophyll a complexes (exciplexes) was monitored by measuring the electrochromic shift of β-carotene in the spectral range 500-510 nm. The excitation of high-energy chlorophyll in light-harvesting antenna of both species was not accompanied by immediate appearance of an electrochromic shift. In PS I from T. elongatus, the excitation of long-wavelength chlorophyll (LWC) caused a pronounced electrochromic effect at 502 nm assigned to the appearance of CT states of chlorophyll exciplexes. The formation of ion-radical pair P₇₀₀⁺A₁^- at 40 ps was limited by energy transfer from LWC to the primary donor P₇₀₀ and accompanied by carotenoid bleach at 498 nm. In PS I from Synechocystis 6803, the excitation at 720 nm produced an immediate bidentate bleach at 690/704 nm and synchronous carotenoid response at 508 nm. The bidentate bleach was assigned to the formation of primary ion-radical state P_B⁺Chl_2B^-, where negative charge is localized predominantly at the accessory chlorophyll molecule in the branch B, Chl_2B. The following decrease of carotenoid signal at ~ 5 ps was ascribed to electron transfer to the more distant molecule Chl_3B. The reduction of phylloquinone in the sites A_1A and A_1B was accompanied by a synchronous blue-shift of the carotenoid response to 498 nm, pointing to fast redistribution of unpaired electron between two branches in favor of the state P_B⁺A_1A^-.

Evidence that chlorophyll f functions solely as an antenna pigment in far-red-light photosystem I from Fischerella thermalis PCC 7521

The Photosystem I (PSI) reaction center in cyanobacteria is comprised of ~96 chlorophyll (Chl) molecules, including six specialized Chl molecules denoted Chl1A/Chl1B (P₇₀₀), Chl2A/Chl2B, and Chl3A/Chl3B that are arranged in two branches and function in primary charge separation. It has recently been proposed that PSI from Chroococcidiopsis thermalis (Nürnberg et al. (2018) Science 360, 1210-1213) and Fischerella thermalis PCC 7521 (Hastings et al. (2019) Biochim. Biophys. Acta 1860, 452-460) contain Chl f in the positions Chl2A/Chl2B. We tested this proposal by exciting RCs from white-light grown (WL-PSI) and far-red light grown (FRL-PSI) F. thermalis PCC 7521 with femtosecond pulses and analyzing the optical dynamics. If Chl f were in the position Chl2A/Chl2B in FRL-PSI, excitation at 740 nm should have produced the charge-separated state P₇₀₀⁺A₀^- followed by electron transfer to A₁ with a τ of ≤25 ps. Instead, it takes ~230 ps for the charge-separated state to develop because the excitation migrates uphill from Chl f in the antenna to the trapping center. Further, we observe a strong electrochromic shift at 685 nm in the final P₇₀₀⁺A₁^- spectrum that can only be explained if Chl a is in the positions Chl2A/Chl2B. Similar arguments rule out the presence of Chl f in the positions Chl3A/Chl3B; hence, Chl f is likely to function solely as an antenna pigment in FRL-PSI. We additionally report the presence of an excitonically coupled homo- or heterodimer of Chl f absorbing around 790 nm that is kinetically independent of the Chl f population that absorbs around 740 nm.

PSI-SMALP, a Detergent-free Cyanobacterial Photosystem I, Reveals Faster Femtosecond Photochemistry

Cyanobacterial photosystem I (PSI) functions as a light-driven cyt c6-ferredoxin/oxidoreductase located in the thylakoid membrane. In this work, the energy and charge transfer processes in PSI complexes isolated from Thermosynechococcus elongatus via conventional n-dodecyl-β-D-maltoside solubilization (DM-PSI) and a, to our knowledge, new detergent-free method using styrene-maleic acid copolymers (SMA-PSI) have been investigated by pump-to-probe femtosecond laser spectroscopy. In DM-PSI preparations excited at 740 nm, the excitation remained localized on the long-wavelength chlorophyll forms within 0.1-20 ps and revealed little or no charge separation and oxidation of the special pair, P700. The formation of ion-radical pair P700+A1- occurred with a characteristic time of 36 ps, being kinetically controlled by energy transfer from the long-wavelength chlorophyll to P700. Quite surprisingly, the detergent-free SMA-PSI complexes upon excitation by these long-wave pulses undergo an ultrafast (<100 fs) charge separation in ∼45% of particles. In the remaining complexes (∼55%), the energy transfer to P700 occurred at ∼36 ps, similar to the DM-PSI. Both isolation methods result in a trimeric form of PSI, yet the SMA-PSI complexes display a heterogenous kinetic behavior. The much faster rate of charge separation suggests the existence of an ultrafast pathway for charge separation in the SMA-PSI that may be disrupted during detergent isolation.

Photochemistry beyond the red limit in chlorophyll f-containing photosystems

Photosystems I and II convert solar energy into the chemical energy that powers life. Chlorophyll a photochemistry, using red light (680 to 700 nm), is near universal and is considered to define the energy "red limit" of oxygenic photosynthesis. We present biophysical studies on the photosystems from a cyanobacterium grown in far-red light (750 nm). The few long-wavelength chlorophylls present are well resolved from each other and from the majority pigment, chlorophyll a. Charge separation in photosystem I and II uses chlorophyll f at 745 nm and chlorophyll f (or d) at 727 nm, respectively. Each photosystem has a few even longer-wavelength chlorophylls f that collect light and pass excitation energy uphill to the photochemically active pigments. These photosystems function beyond the red limit using far-red pigments in only a few key positions.

Ultrafast dynamics of the photo-excited hemes b and cn in the cytochrome b6f complex

The dynamics of hemes b and c_n within the cytochrome b₆f complex are investigated by means of ultrafast broad-band transient absorption spectroscopy. On the one hand, the data reveal that, subsequent to visible light excitation, part of the b hemes undergoes pulse-limited photo-oxidation, with the liberated electron supposedly being transferred to one of the adjacent aromatic amino acids. Photo-oxidation is followed by charge recombination in about 8.2 ps. Subsequent to charge recombination, heme b is promoted to a vibrationally excited ground state that relaxes in about 4.6 ps. On the other hand, heme c_n undergoes ultrafast ground state recovery in about 140 fs. Interestingly, the data also show that, in contrast to previous beliefs, Chl a is involved in the photochemistry of hemes. Indeed, subsequent to heme excitation, Chl a bleaches and recovers to its ground state in 90 fs and 650 fs, respectively. Chl a bleaching allegedly corresponds to the formation of a short lived Chl a anion. Beyond the previously suggested structural role, this study provides unique evidence that Chl a is directly involved in the photochemistry of the hemes.

Femtosecond Visible Transient Absorption Spectroscopy of Chlorophyll f-Containing Photosystem I

Photosystem I (PSI) from Chroococcidiopsis thermalis PCC 7203 grown under far-red light (FRL; >725 nm) contains both chlorophyll a and a small proportion of chlorophyll f. Here, we investigated excitation energy transfer and charge separation using this FRL-grown form of PSI (FRL-PSI). We compared femtosecond transient visible absorption changes of normal, white-light (WL)-grown PSI (WL-PSI) with those of FRL-PSI using excitation at 670 nm, 700 nm, and (in the case of FRL-PSI) 740 nm. The possibility that chlorophyll f participates in energy transfer or charge separation is discussed on the basis of spectral assignments. With selective pumping of chlorophyll f at 740 nm, we observe a final ∼150 ps decay assigned to trapping by charge separation, and the amplitude of the resulting P700^+•A₁^-• charge-separated state indicates that the yield is directly comparable to that of WL-PSI. The kinetics shows a rapid 2 ps time constant for almost complete transfer to chlorophyll f if chlorophyll a is pumped with a wavelength of 670 nm or 700 nm. Although the physical role of chlorophyll f is best supported as a low-energy radiative trap, the physical location should be close to or potentially within the charge-separating pigments to allow efficient transfer for charge separation on the 150 ps timescale. Target models can be developed that include a branching in the formation of the charge separation for either WL-PSI or FRL-PSI.

Photo-induced dynamics of the heme centers in cytochrome bc1

The ultrafast response of cytochromebc₁is investigated for the first time,viatransient absorption spectroscopy.

A microfluidic flow-cell for the study of the ultrafast dynamics of biological systems

The study of biochemical dynamics by ultrafast spectroscopic methods is often restricted by the limited amount of liquid sample available, while the high repetition rate of light sources can induce photodamage. In order to overcome these limitations, we designed a high flux, sub-ml, capillary flow-cell. While the 0.1 mm thin window of the 0.5 mm cross-section capillary ensures an optimal temporal resolution and a steady beam deviation, the cell-pump generates flows up to ∼0.35 ml/s that are suitable to pump laser repetition rates up to ∼14 kHz, assuming a focal spot-diameter of 100 μm. In addition, a decantation chamber efficiently removes bubbles and allows, via septum, for the addition of chemicals while preserving the closed atmosphere. The minimal useable amount of sample is ∼250 μl.

Temporal and spectral characterization of the photosynthetic reaction center from Heliobacterium modesticaldum

A time-resolved spectroscopic study of the isolated photosynthetic reaction center (RC) from Heliobacterium modesticaldum reveals that thermal equilibration of light excitation among the antenna pigments followed by trapping of excitation and the formation of the charge-separated state P800 (+)A0 (-) occurs within ~25 ps. This time scale is similar to that reported for plant and cyanobacterial photosystem I (PS I) complexes. Subsequent electron transfer from the primary electron acceptor A0 occurs with a lifetime of ~600 ps, suggesting that the RC of H. modesticaldum is functionally similar to that of Heliobacillus mobilis and Heliobacterium chlorum. The (A0 (-) - A0) and (P800 (+) - P800) absorption difference spectra imply that an 8(1)-OH-Chl a F molecule serves as the primary electron acceptor and occupies the position analogous to ec3 (A0) in PS I, while a monomeric BChl g pigment occupies the position analogous to ec2 (accessory Chl). The presence of an intense photobleaching band at 790 nm in the (A0 (-) - A0) spectrum suggests that the excitonic coupling between the monomeric accessory BChl g and the 8(1)-OH-Chl a F in the heliobacterial RC is significantly stronger than the excitonic coupling between the equivalent pigments in PS I.