Mutations in algal and cyanobacterial Photosystem I that independently affect the yield of initial charge separation in the two electron transfer cofactor branches

Is the A-1 Pigment in Photosystem I Part of P700? A (P700+-P700) FTIR Difference Spectroscopy Study of A-1 Mutants

The involvement of the second pair of chlorophylls, termed A-1A and A-1B, in light-induced electron transfer in photosystem I (PSI) is currently debated. Asparagines at PsaA600 and PsaB582 are involved in coordinating the A-1B and A-1A pigments, respectively. Here we have mutated these asparagine residues to methionine in two single mutants and a double mutant in PSI from Synechocystis sp. PCC 6803, which we term NA600M, NB582M, and NA600M/NB582M mutants. (P700+-P700) FTIR difference spectra (DS) at 293 K were obtained for the wild-type and the three mutant PSI samples. The wild-type and mutant FTIR DS differ considerably. This difference indicates that the observed changes in the (P700+-P700) FTIR DS cannot be due to only the PA and PB pigments of P700. Comparison of the wild-type and mutant FTIR DS allows the assignment of different features to both A-1 pigments in the FTIR DS for wild-type PSI and assesses how these features shift upon cation formation and upon mutation. While the exact role the A-1 pigments play in the species we call P700 is unclear, we demonstrate that the vibrational modes of the A-1A and A-1B pigments are modified upon P700+ formation. Previously, we showed that the A-1 pigments contribute to P700 in green algae. In this manuscript, we demonstrate that this is also the case in cyanobacterial PSI. The nature of the mutation-induced changes in algal and cyanobacterial PSI is similar and can be considered within the same framework, suggesting a universality in the nature of P700 in different photosynthetic organisms.

Energy Transfer and Radical-Pair Dynamics in Photosystem I with Different Red Chlorophyll a Pigments

We establish a general kinetic scheme for the energy transfer and radical-pair dynamics in photosystem I (PSI) of Chlamydomonas reinhardtii, Synechocystis PCC6803, Thermosynechococcus elongatus and Spirulina platensis grown under white-light conditions. With the help of simultaneous target analysis of transient-absorption data sets measured with two selective excitations, we resolved the spectral and kinetic properties of the different species present in PSI. WL-PSI can be described as a Bulk Chl a in equilibrium with a higher-energy Chl a, one or two Red Chl a and a reaction-center compartment (WL-RC). Three radical pairs (RPs) have been resolved with very similar properties in the four model organisms. The charge separation is virtually irreversible with a rate of ≈900 ns-1. The second rate, of RP1 → RP2, ranges from 70-90 ns-1 and the third rate, of RP2 → RP3, is ≈30 ns-1. Since RP1 and the Red Chl a are simultaneously present, resolving the RP1 properties is challenging. In Chlamydomonas reinhardtii, the excited WL-RC and Bulk Chl a compartments equilibrate with a lifetime of ≈0.28 ps, whereas the Red and the Bulk Chl a compartments equilibrate with a lifetime of ≈2.65 ps. We present a description of the thermodynamic properties of the model organisms at room temperature.

Nanosecond time-resolved infrared spectroscopy for the study of electron transfer in photosystem I

Microsecond time-resolved step-scan FTIR difference spectroscopy was used to study photosystem I (PSI) from Thermosynechococcus vestitus BP-1 (T. vestitus, formerly known as T. elongatus) at 77 K. In addition, photoaccumulated (P700+-P700) FTIR difference spectra were obtained at both 77 and 293 K. The FTIR difference spectra are presented here for the first time. To extend upon these FTIR studies nanosecond time-resolved infrared difference spectroscopy was also used to study PSI from T. vestitus at 296 K. Nanosecond infrared spectroscopy has never been used to study PSI samples at physiological temperatures, and here it is shown that such an approach has great value as it allows a direct probe of electron transfer down both branches in PSI. In PSI at 296 K, the infrared flash-induced absorption changes indicate electron transfer down the B- and A-branches is characterized by time constants of 33 and 364 ns, respectively, in good agreement with visible spectroscopy studies. These time constants are associated with forward electron transfer from A1- to FX on the B- and A-branches, respectively. At several infrared wavelengths flash-induced absorption changes at 296 K recover in tens to hundreds of milliseconds. The dominant decay phase is characterized by a lifetime of 128 ms. These millisecond changes are assigned to radical pair recombination reactions, with the changes being associated primarily with P700+ rereduction. This conclusion follows from the observation that the millisecond infrared spectrum is very similar to the photoaccumulated (P700+-P700) FTIR difference spectrum.

Energy transfer and trapping in photosystem I with and without chlorophyll-f

We establish a general kinetic scheme for energy transfer and trapping in the photosystem I (PSI) of cyanobacteria grown under white light (WL) or far-red light (FRL) conditions. With the help of simultaneous target analysis of all emission and transient absorption datasets measured in five cyanobacterial strains, we resolved the spectral and kinetic properties of the different species present in PSI. WL-PSI can be described by Bulk Chl a, two Red Chl a, and a reaction center compartment (WL-RC). The FRL-PSI contains two additional Chl f compartments. The lowest excited state of the FRL-RC is downshifted by ≈ 29 nm. The rate of charge separation drops from ≈900 ns-1 in WL-RC to ≈300 ns-1 in FRL-RC. The delayed trapping in the FRL-PSI (≈130 ps) is explained by uphill energy transfer from the Chl f compartments with Gibbs free energies of ≈kBT below that of the FRL-RC.

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.

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^-.

Reversible inhibition and reactivation of electron transfer in photosystem I

In photosystem I (PSI) complexes at room temperature electron transfer from A₁^- to F_X is an order of magnitude faster on the B-branch compared to the A-branch. One factor that might contribute to this branch asymmetry in time constants is TrpB673 (Thermosynechococcus elongatus numbering), which is located between A_1B and F_X. The corresponding residue on the A-branch, between A_1A and F_X, is GlyA693. Here, microsecond time-resolved step-scan FTIR difference spectroscopy at 77 K has been used to study isolated PSI complexes from wild type and TrpB673Phe mutant (WB673F mutant) cells from Synechocystis sp. PCC 6803. WB673F mutant cells require glucose for growth and are light sensitive. Photoaccumulated FTIR difference spectra indicate changes in amide I and II protein vibrations upon mutation of TrpB673 to Phe, indicating the protein environment near F_X is altered upon mutation. In the WB673F mutant PSI samples, but not in WT PSI samples, the phylloquinone molecule that occupies the A₁ binding site is likely doubly protonated following long periods of repetitive flash illumination at room temperature. PSI with (doubly) protonated quinone in the A₁ binding site are not functional in electron transfer. However, electron transfer functionality can be restored by incubating the light-treated mutant PSI samples in the presence of added phylloquinone.

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.