Excitonic interactions in wild-type and mutant PSI reaction centers

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.

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.

Uphill energy transfer in photosystem I from Chlamydomonas reinhardtii. Time-resolved fluorescence measurements at 77 K

Energetic properties of chlorophylls in photosynthetic complexes are strongly modulated by their interaction with the protein matrix and by inter-pigment coupling. This spectral tuning is especially striking in photosystem I (PSI) complexes that contain low-energy chlorophylls emitting above 700 nm. Such low-energy chlorophylls have been observed in cyanobacterial PSI, algal and plant PSI-LHCI complexes, and individual light-harvesting complex I (LHCI) proteins. However, there has been no direct evidence of their presence in algal PSI core complexes lacking LHCI. In order to determine the lowest-energy states of chlorophylls and their dynamics in algal PSI antenna systems, we performed time-resolved fluorescence measurements at 77 K for PSI core and PSI-LHCI complexes isolated from the green alga Chlamydomonas reinhardtii. The pool of low-energy chlorophylls observed in PSI cores is generally smaller and less red-shifted than that observed in PSI-LHCI complexes. Excitation energy equilibration between bulk and low-energy chlorophylls in the PSI-LHCI complexes at 77 K leads to population of excited states that are less red-shifted (by ~ 12 nm) than at room temperature. On the other hand, analysis of the detection wavelength dependence of the effective trapping time of bulk excitations in the PSI core at 77 K provided evidence for an energy threshold at ~ 675 nm, above which trapping slows down. Based on these observations, we postulate that excitation energy transfer from bulk to low-energy chlorophylls and from bulk to reaction center chlorophylls are thermally activated uphill processes that likely occur via higher excitonic states of energy accepting chlorophylls.

Excitation transfer and trapping kinetics in plant photosystem I probed by two-dimensional electronic spectroscopy

Photosystem I is a robust and highly efficient biological solar engine. Its capacity to utilize virtually every absorbed photon's energy in a photochemical reaction generates great interest in the kinetics and mechanisms of excitation energy transfer and charge separation. In this work, we have employed room-temperature coherent two-dimensional electronic spectroscopy and time-resolved fluorescence spectroscopy to follow exciton equilibration and excitation trapping in intact Photosystem I complexes as well as core complexes isolated from Pisum sativum. We performed two-dimensional electronic spectroscopy measurements with low excitation pulse energies to record excited-state kinetics free from singlet-singlet annihilation. Global lifetime analysis resolved energy transfer and trapping lifetimes closely matches the time-correlated single-photon counting data. Exciton energy equilibration in the core antenna occurred on a timescale of 0.5 ps. We further observed spectral equilibration component in the core complex with a 3-4 ps lifetime between the bulk Chl states and a state absorbing at 700 nm. Trapping in the core complex occurred with a 20 ps lifetime, which in the supercomplex split into two lifetimes, 16 ps and 67-75 ps. The experimental data could be modelled with two alternative models resulting in equally good fits-a transfer-to-trap-limited model and a trap-limited model. However, the former model is only possible if the 3-4 ps component is ascribed to equilibration with a "red" core antenna pool absorbing at 700 nm. Conversely, if these low-energy states are identified with the P700 reaction centre, the transfer-to-trap-model is ruled out in favour of a trap-limited model.

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.

Excitation dynamics in Photosystem I from Chlamydomonas reinhardtii. Comparative studies of isolated complexes and whole cells

Identical time-resolved fluorescence measurements with ~3.5-ps resolution were performed for three types of PSI preparations from the green alga, Chlamydomonas reinhardtii: isolated PSI cores, isolated PSI-LHCI complexes and PSI-LHCI complexes in whole living cells. Fluorescence decay in these types of PSI preparations has been previously investigated but never under the same experimental conditions. As a result we present consistent picture of excitation dynamics in algal PSI. Temporal evolution of fluorescence spectra can be generally described by three decay components with similar lifetimes in all samples (6-8ps, 25-30ps, 166-314ps). In the PSI cores, the fluorescence decay is dominated by the two fastest components (~90%), which can be assigned to excitation energy trapping in the reaction center by reversible primary charge separation. Excitation dynamics in the PSI-LHCI preparations is more complex because of the energy transfer between the LHCI antenna system and the core. The average trapping time of excitations created in the well coupled LHCI antenna system is about 12-15ps longer than excitations formed in the PSI core antenna. Excitation dynamics in PSI-LHCI complexes in whole living cells is very similar to that observed in isolated complexes. Our data support the view that chlorophylls responsible for the long-wavelength emission are located mostly in LHCI. We also compared in detail our results with the literature data obtained for plant PSI.

Spectral resolution of the primary electron acceptor A0 in Photosystem I

The reduced state of the primary electron acceptor of Photosystem I, A(0), was resolved spectroscopically in its lowest energy Q(y) region for the first time without the addition of chemical reducing agents and without extensive data manipulation. To carry this out, we used the menB mutant of Synechocystis sp. PCC 6803 in which phylloquinone is replaced by plastoquinone-9 in the A(1) sites of Photosystem I. The presence of plastoquinone-9 slows electron transfer from A(0) to A(1), leading to a long-lived A(0)(-) state. This allows its spectral signature to be readily detected in a time-resolved optical pump-probe experiment. The maximum bleaching (A(0)(-) - A(0)) was found to occur at 684 nm with a corresponding extinction coefficient of 43 mM(-1) cm(-1). The data show evidence for an electrochromic shift of an accessory chlorophyll pigment, suggesting that the latter Q(y) absorption band is centered around 682 nm.

Femtosecond primary charge separation in Synechocystis sp. PCC 6803 photosystem I

The ultrafast (<100 fs) conversion of delocalized exciton into charge-separated state between the primary donor P700 (bleaching at 705 nm) and the primary acceptor A0 (bleaching at 690 nm) in photosystem I (PS I) complexes from Synechocystis sp. PCC 6803 was observed. The data were obtained by application of pump-probe technique with 20-fs low-energy pump pulses centered at 720 nm. The earliest absorbance changes (close to zero delay) with a bleaching at 690 nm are similar to the product of the absorption spectrum of PS I complex and the laser pulse spectrum, which represents the efficiency spectrum of the light absorption by PS I upon femtosecond excitation centered at 720 nm. During the first approximately 60 fs the energy transfer from the chlorophyll (Chl) species bleaching at 690 nm to the Chl bleaching at 705 nm occurs, resulting in almost equal bleaching of the two forms with the formation of delocalized exciton between 690-nm and 705-nm Chls. Within the next approximately 40 fs the formation of a new broad band centered at approximately 660 nm (attributed to the appearance of Chl anion radical) is observed. This band decays with time constant simultaneously with an electron transfer to A1 (phylloquinone). The subtraction of kinetic difference absorption spectra of the closed (state P700+A0A1) PS I reaction center (RC) from that of the open (state P700A0A1) RC reveals the pure spectrum of the P700+A0- ion-radical pair. The experimental data were analyzed using a simple kinetic scheme: An*-->k1[(PA0)*A1--><100 fs P+A0-A1]-->k2P+A0A1-, and a global fitting procedure based on the singular value decomposition analysis. The calculated kinetics of transitions between intermediate states and their spectra were similar to the kinetics recorded at 694 and 705 nm and the experimental spectra obtained by subtraction of the spectra of closed RCs from the spectra of open RCs. As a result, we found that the main events in RCs of PS I under our experimental conditions include very fast (<100 fs) charge separation with the formation of the P700+A0-A1 state in approximately one half of the RCs, the approximately 5-ps energy transfer from antenna Chl* to P700A0A1 in the remaining RCs, and approximately 25-ps formation of the secondary radical pair P700+A0A1-.

Effect of the P700 pre-oxidation and point mutations near A(0) on the reversibility of the primary charge separation in Photosystem I from Chlamydomonas reinhardtii

Time-resolved fluorescence studies with a 3-ps temporal resolution were performed in order to: (1) test the recent model of the reversible primary charge separation in Photosystem I (Müller et al., 2003; Holwzwarth et al., 2005, 2006), and (2) to reconcile this model with a mechanism of excitation energy quenching by closed Photosystem I (with P700 pre-oxidized to P700+). For these purposes, we performed experiments using Photosystem I core samples isolated from Chlamydomonas reinhardtii wild type, and two mutants in which the methionine axial ligand to primary electron acceptor, A(0), has been change to either histidine or serine. The temporal evolution of fluorescence spectra was recorded for each preparation under conditions where the "primary electron donor," P700, was either neutral or chemically pre-oxidized to P700+. For all the preparations under study, and under neutral and oxidizing conditions, we observed multiexponential fluorescence decay with the major phases of approximately 7 ps and approximately 25 ps. The relative amplitudes and, to a minor extent the lifetimes, of these two phases were modulated by the redox state of P700 and by the mutations near A(0): both pre-oxidation of P700 and mutations caused slight deceleration of the excited state decay. These results are consistent with a model in which P700 is not the primary electron donor, but rather a secondary electron donor, with the primary charge separation event occurring between the accessory chlorophyll, A, and A(0). We assign the faster phase to the equilibration process between the excited state of the antenna/reaction center ensemble and the primary radical pair, and the slower phase to the secondary electron transfer reaction. The pre-oxidation of P700 shifts the equilibrium between the excited state and the primary radical pair towards the excited state. This shift is proposed to be induced by the presence of the positive charge on P700+. The same charge is proposed to be responsible for the fast A+A(0)(-)-->AA(0) charge recombination to the ground state and, in consequence, excitation quenching in closed reaction centers. Mutations of the A(0) axial ligand shift the equilibrium in the same direction as pre-oxidation of P700 due to the up-shift of the free energy level of the state A+A(0)(-).

Structure of Photosystems I and II

Photosynthesis is the major process that converts solar energy into chemical energy on Earth. Two and a half billion years ago, the ancestors of cyanobacteria were able to use water as electron source for the photosynthetic process, thereby evolving oxygen and changing the atmosphere of our planet Earth. Two large membrane protein complexes, Photosystems I and II, catalyze the primary step in this energy conversion, the light-induced charge separation across the photosynthetic membrane. This chapter describes and compares the structure of two Photosystems and discusses their function in respect to the mechanism of light harvesting, electron transfer and water splitting.

Trap-limited charge separation kinetics in higher plant photosystem I complexes

Time-resolved fluorescence measurements were performed on isolated core and intact Photosystem I (PS I) particles and stroma membranes from Arabidopsis thaliana to characterize the type of energy-trapping kinetics in higher plant PS I. Target analysis confirms the previously proposed "charge recombination" model. No bottleneck in the energy flow from the bulk antenna compartments to the reaction center has been found. For both particles a trap-limited kinetics is realized, with an apparent charge separation lifetime of approximately 6 ps. No red chlorophylls (Chls) are found in the PS I-core complex from A. thaliana. Rather, the observed red-shifted fluorescence (700-710 nm range) originates from the reaction center. In contrast, two red Chl compartments, located in the peripheral light-harvesting complexes, are resolved in the intact PS I particles (decay lifetimes 33 and 95 ps, respectively). These two red states have been attributed to the two red states found in Lhca 3 and Lhca 4, respectively. The influence of the red Chls on the slowing of the overall trapping kinetics in the intact PS I complex is estimated to be approximately four times larger than the effect of the bulk antenna enlargement.

Replacement of the methionine axial ligand to the primary electron acceptor A0 slows the A0− reoxidation dynamics in Photosystem I

The recent crystal structure of photosystem I (PSI) from Thermosynechococcus elongatus shows two nearly symmetric branches of electron transfer cofactors including the primary electron donor, P(700), and a sequence of electron acceptors, A, A(0) and A(1), bound to the PsaA and PsaB heterodimer. The central magnesium atoms of each of the putative primary electron acceptor chlorophylls, A(0), are unusually coordinated by the sulfur atom of methionine 688 of PsaA and 668 of PsaB, respectively. We [Ramesh et al. (2004a) Biochemistry 43:1369-1375] have shown that the replacement of either methionine with histidine in the PSI of the unicellular green alga Chlamydomonas reinhardtii resulted in accumulation of A(0)(-) (in 300-ps time scale), suggesting that both the PsaA and PsaB branches are active. This is in contrast to cyanobacterial PSI where studies with methionine-to-leucine mutants show that electron transfer occurs predominantly along the PsaA branch. In this contribution we report that the change of methionine to either leucine or serine leads to a similar accumulation of A(0)(-) on both the PsaA and the PsaB branch of PSI from C. reinhardtii, as we reported earlier for histidine mutants. More importantly, we further demonstrate that for all the mutants under study, accumulation of A(0)(-) is transient, and that reoxidation of A(0)(-) occurs within 1-2 ns, two orders of magnitude slower than in wild type PSI, most likely via slow electron transfer to A(1). This illustrates an indispensable role of methionine as an axial ligand to the primary acceptor A(0) in optimizing the rate of charge stabilization in PSI. A simple energetic model for this reaction is proposed. Our findings support the model of equivalent electron transfer along both cofactor branches in Photosystem I.

Directing electron transfer within Photosystem I by breaking H-bonds in the cofactor branches

Photosystem I has two branches of cofactors down which light-driven electron transfer (ET) could potentially proceed, each consisting of a pair of chlorophylls (Chls) and a phylloquinone (PhQ). Forward ET from PhQ to the next ET cofactor (FX) is described by two kinetic components with decay times of approximately 20 and approximately 200 ns, which have been proposed to represent ET from PhQB and PhQA, respectively. Immediately preceding each quinone is a Chl (ec3), which receives a H-bond from a nearby tyrosine. To decrease the reduction potential of each of these Chls, and thus modify the relative yield of ET within the targeted branch, this H-bond was removed by conversion of each Tyr to Phe in the green alga Chlamydomonas reinhardtii. Together, transient optical absorption spectroscopy performed in vivo and transient electron paramagnetic resonance data from thylakoid membranes showed that the mutations affect the relative amplitudes, but not the lifetimes, of the two kinetic components representing ET from PhQ to F(X). The mutation near ec3A increases the fraction of the faster component at the expense of the slower component, with the opposite effect seen in the ec3B mutant. We interpret this result as a decrease in the relative use of the targeted branch. This finding suggests that in Photosystem I, unlike type II reaction centers, the relative efficiency of the two branches is extremely sensitive to the energetics of the embedded redox cofactors.

Ultrafast transient absorption studies on photosystem I reaction centers from Chlamydomonas reinhardtii. 2: mutations near the P700 reaction center chlorophylls provide new insight into the nature of the primary electron donor

The energy transfer and charge separation kinetics in several core Photosystem I particles of Chlamydomonas reinhardtii with point mutations around the PA and PB reaction center chlorophylls (Chls) have been studied using ultrafast transient absorption spectroscopy in the femtosecond to nanosecond time range to characterize the influence on the early electron transfer processes. The data have been analyzed in terms of kinetic compartment models. The adequate description of the transient absorption kinetics requires three different radical pairs in the time range up to approximately 100 ps. Also a charge recombination process from the first radical pair back to the excited state is present in all the mutants, as already shown previously for the wild-type (Müller, M. G., J. Niklas, W. Lubitz, and A. R. Holzwarth. 2003. Biophys. J. 85:3899-3922; and Holzwarth, A. R., M. G. Müller, J. Niklas, and W. Lubitz. 2005. J. Phys. Chem. B. 109:5903-59115). In all mutants, the primary charge separation occurs with the same effective rate constant within the error limits as in the wild-type (>>350 ns(-1)), which implies an intrinsic rate constant of charge separation of <1 ps(-1). The rate constant of the secondary electron transfer process is slowed down by a factor of approximately 2 in the mutant B-H656C, which lacks the ligand to the central metal of Chl PB. For the mutant A-T739V, which breaks the hydrogen bond to the keto carbonyl of Chl PA, only a slight slowing down of the secondary electron transfer is observed. Finally for mutant A-W679A, which has the Trp near the PA Chl replaced, either no pronounced effect or, at best, a slight increase on the secondary electron transfer rate constants is observed. The effective charge recombination rate constant is modified in all mutants to some extent, with the strongest effect observed in mutant B-H656C. Our data strongly suggest that the Chls of the PA and PB pair, constituting what is traditionally called the "primary electron donor P700", are not oxidized in the first electron transfer process, but rather only in the secondary electron transfer step. We thus propose a new electron transfer mechanism for Photosystem I where the accessory Chl(s) function as the primary electron donor(s) and the A0 Chl(s) are the primary electron acceptor(s). This new mechanism also resolves in a straightforward manner the difficulty with the previous mechanism, where an electron would have to overcome a distance of approximately 14 A in <1 ps in a single step. If interpreted within a scheme of single-sided electron transfer, our data suggest that the B-branch is the active branch, although parallel A-branch activity cannot be excluded. All the mutations do affect to a varying extent the energy difference between the reaction center excited state RC* and the first radical pair and thus affect the rate constant of charge recombination. It is interesting to note that the new mechanism proposed is in fact analogous to the electron transfer mechanism in Photosystem II, where the accessory Chl also plays the role of the primary electron donor, rather than the special Chl pair P680 (Prokhorenko, V. and A. R. Holzwarth. 2000. J. Phys. Chem. B. 104:11563-11578).

Characterization of Low-Energy Chlorophylls in the PSI-LHCI Supercomplex from Chlamydomonas reinhardtii. A Site-Selective Fluorescence Study

Almost all photosystem I (PSI) complexes from oxygenic photosynthetic organisms contain chlorophylls that absorb at longer wavelength than that of the primary electron donor P700. We demonstrate here that the low-energy pool of chlorophylls in the PSI-LHCI complex from the green alga Chlamydomonas reinhardtii, containing five to six pigments, is significantly blue-shifted (A(max) at 700 nm at 4 K) compared to that in the PSI core preparations from several species of cyanobacteria and in PSI-LHCI particles from higher plants. This makes them almost isoenergetic with the primary donor. However, they keep the other characteristic features of "red" chlorophylls: clear spectral separation from the bulk chlorophylls, big Stokes shift revealing pronounced electron-phonon coupling, and large homogeneous and inhomogeneous broadening of approximately 170 and approximately 310 cm(-1), respectively.

Charge Recombination Fluorescence in Photosystem I Reaction Centers from Chlamydomonas reinhardtii

The fluorescence kinetics of photosystem I core particles from Chlamydomonas reinhardtii have been measured with picosecond resolution in order to test a previous hypothesis suggesting a charge recombination mechanism for the early electron-transfer steps and the fluorescence kinetics (Müller et al. Biophys. J. 2003, 85, 3899-3922). Performing global target analyses for various kinetic models on the original fluorescence data confirms the "charge recombination" model as the only acceptable one of the models tested while all of the other models can be excluded. The analysis allowed a precise determination of (i) the effective charge separation rate constant from the equilibrated reaction center excited state (438 ns(-1)) confirming our previous assignment based on transient absorption data (Müller et al. Biophys. J. 2003, 85, 3899-3922), (ii) the effective charge recombination rate constant back to the excited state (52 ns(-1)), and (iii) the intrinsic secondary electron-transfer rate constant (80 ns(-1)). The average energy equilibration lifetime core antenna/RC is about 1 ps in the "charge recombination" model, in agreement with previous transient absorption data, vs the 18-20 ps energy transfer lifetime from antenna to RC within "transfer-to-the-trap-limited" models. The apparent charge separation lifetime in the recombination model is about three times faster than in the "transfer-to-the-trap-limited" model. We conclude that the charge separation kinetics is trap-limited in PS I cores devoid of red antenna states such as in C. reinhardtii.

Structure of cyanobacterial photosystem I

Photosystem I is one of the most fascinating membrane protein complexes for which a structure has been determined. It functions as a bio-solar energy converter, catalyzing one of the first steps of oxygenic photosynthesis. It captures the light of the sun by means of a large antenna system, consisting of chlorophylls and carotenoids, and transfers the energy to the center of the complex, driving the transmembrane electron transfer from plastoquinone to ferredoxin. Cyanobacterial Photosystem I is a trimer consisting of 36 proteins to which 381 cofactors are non-covalently attached. This review discusses the complex function of Photosystem I based on the structure of the complex at 2.5 A resolution as well as spectroscopic and biochemical data.

Bidirectional Electron Transfer in Photosystem I: Accumulation of A0-in A-Side or B-Side Mutants of the Axial Ligand to Chlorophyll A0†

Photosystem I contains two potential electron transfer pathways between P(700) and F(X). These branches are made up of the electron transfer chain components A, A(0), and A(1). The primary electron acceptor A(0) is a chlorophyll a monomer that could be one or both of the two chlorophyll molecules, eC-A(3)/eC-B(3), identified in the 2.5 A resolution structure. The eC-A(3)/eC-B(3) chlorophylls are both coordinated by the sulfur atom of a methionine. This coordination is highly unusual, as interactions between the acid Mg(2+) and the soft base sulfur are weak. The eC-A(3)/eC-B(3) chlorophylls also are located close to one of the connecting chlorophylls that may link the antenna and the electron transfer chain chlorophylls. Due to their location in the structure, the eC-A(3)/eC-B(3) chlorophylls may play a role in both excitation energy transfer and electron transfer. To test the role of the eC-A(3)/eC-B(3) chlorophylls in electron transfer, Met-684 of PsaA and Met-664 of PsaB have been changed to His, Ser, and Leu. Replacement of either M(A684) or M(B664) results in a significant alteration in growth phenotype. The His and Leu mutants are very light sensitive in the presence of oxygen. Growth is impaired to a greater extent in the B-side mutants. However, all of the mutants are able to grow anaerobically at comparable rates. The His and Ser mutants all accumulate PSI at a level similar to that of wild type, whereas the Leu mutants have reduced amounts of PSI. Ultrafast transient absorbance measurements show that the (A(0)(-) - A(0)) difference signal accumulates in the MH(A684) and MH(B664) mutants under neutral conditions, demonstrating that electron transfer between A(0)(-) and A(1) is blocked or significantly slowed. The results show that both the A-branch and the B-branch of the ETC are active in PSI from Chlamydomonas reinhardtii.