Time-resolved visible and infrared difference spectroscopy for the study of photosystem I with different quinones incorporated into the A1 binding site

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.

Time-resolved FTIR difference spectroscopy for the study of photosystem I with high potential naphthoquinones incorporated into the A1 binding site

Time-resolved step-scan Fourier transform infrared difference spectroscopy has been used to study cyanobacterial photosystem I photosynthetic reaction centers from Synechocystis sp. PCC 6803 (S6803) with four high-potential, 1,4-naphthoquinones incorporated into the A₁ binding site. The high-potential naphthoquinones are 2-chloro-, 2-bromo-, 2,3-dichloro- and 2,3-dibromo-1,4-naphthoquinone. "Foreign minus native" double difference spectra (DDS) were constructed by subtracting difference spectra for native photosystem I (with phylloquinone in the A₁ binding site) from corresponding spectra obtained using photosystem I with the different quinones incorporated. To help assess and assign bands in the difference and double difference spectra, density functional theory based vibrational frequency calculations for the different quinones in solvent, or in the presence of a single asymmetric H- bond to either a water molecule or a peptide backbone NH group, were undertaken. Calculated and experimental spectra agree best for the peptide backbone asymmetrically H- bonded system. By comparing multiple sets of double difference spectra, several new bands for the native quinone (phylloquinone) are identified. By comparing calculated and experimental spectra we conclude that the mono-substituted halogenated NQs can occupy the binding site in either of two different orientations, with the chlorine or bromine atom being either ortho or meta to the H- bonded CO group.

Assessment of the orientation and conformation of pigments in protein binding sites from infrared difference spectra

Time resolved FTIR difference spectroscopy (DS) has been used to study photosystem I (PSI) with the disubstituted 1,4-naphthoquinones acequinocyl (AcQ) and lapachol (Lpc) incorporated into the A₁ binding site. AcQ is a 2-acetoxy-3-dodecyl-1,4-naphthoquinone, Lpc is a 2-hydroxy-3-(3-methyl-2-butenyl)-1,4-naphthoquinone. To assess whether the experimental spectra are specific to different orientations of the quinone and their substitutions ONIOM-type QM/MM vibrational frequency calculations were undertaken for various orientations of the pigments and side-chain conformations in the A₁ binding site. Comparison of calculated and experimental spectra for the reduced species (semiquinone anion) suggests that the orientation for the naphthoquinone ring in the binding site and specific side-chain conformations can be identified based on the spectra. In native PSI phylloquinone (PhQ) in the A₁ binding site binds with its phytyl chain ortho to the hydrogen bonded carbonyl group. This is not found to be the case for the hydrocarbon tail of AcQ, which is meta to the H-bonded carbonyl group. In contrast, Lpc in PSI binds with its hydrocarbon tail also ortho to the H-bonded carbonyl group. Furthermore, comparison of calculated and experimental spectra indicates which conformations the acetoxy group of AcQ and the hydroxy group of Lpc adopt in the A₁ binding site.

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.

Time-resolved FTIR difference spectroscopy for the study of quinones in the A1 binding site in photosystem I: Identification of neutral state quinone bands

Infrared absorption bands associated with the neutral state of quinones in the A₁ binding site in photosystem I (PSI) have been difficult to identify in the past. This problem is addressed here, where time-resolved step-scan FTIR difference spectroscopy at 77 K has been used to study PSI with six different quinones incorporated into the A₁ binding site. (P700⁺A₁^- - P700A₁) and (A₁^- - A₁) FTIR difference spectra (DS) were obtained for PSI with the different quinones incorporated, and several double-difference spectra (DDS) were constructed from the DS. From analysis of the DS and DDS, in combination with density functional theory based vibrational frequency calculations of the quinones, the neutral state bands of the incorporated quinones are identified and assigned. For neutral PhQ in the A₁ binding site, infrared absorption bands were identified near 1665 and 1635 cm^-1, that are due to the C₁O and C₄O stretching vibrations of the incorporated PhQ, respectively. These assignments indicate a 30 cm^-1 separation between the C₁O and C₄O modes, considerably less than the ~80 cm^-1 found for similar modes of PhQ^-. The C₄O mode downshifts due to hydrogen bonding, so the suggestion is that hydrogen bonding is weaker for the neutral state compared to the anion state, indicating radical-induced proton dynamics associated with the quinone in the A₁ binding site in PSI.

Kinetics and Energetics of Phylloquinone Reduction in Photosystem I: Insight From Modeling of the Site Directed Mutants

Two phylloquinone molecules (A ₁), one being predominantly coordinated by PsaA subunit residues (A _1A) the other by those of PsaB (A _1B), act as intermediates in the two parallel electron transfer chains of Photosystem I. The oxidation kinetics of the two phyllosemiquinones by the iron-sulfur cluster F_X differ by approximately one order of magnitude, with A 1 A - being oxidized in about 200 ns and A 1 B - in about 20 ns. These differences are generally explained in terms of asymmetries in the driving force for F_X reduction on the two electron transfer chains. Site directed mutations of conserved amino acids composing the A ₁ binding site have been engineered on both reaction center subunits, and proved to affect selectively the oxidation lifetime of either A 1 A - , for PsaA mutants, or A 1 B - , for PsaB mutants. The mutation effects are here critically reviewed, also by novel modeling simulations employing the tunneling formalism to estimate the electron transfer rates. Three main classes of mutation effects are in particular addressed: (i) those leading to an acceleration, (ii) those leading to a moderated slowing (~5-folds), and (iii) those leading to a severe slowing (>20-folds) of the kinetics. The effect of specific amino acid perturbations contributing to the poising of the phylloquinones redox potential and, in turn, to PSI functionality, is discussed.

Modelling electron transfer in photosystem I: limits and perspectives

Uncovering the parameters underlying the electron transfer (ET) in photosynthetic reaction centres is of importance for understanding the molecular mechanisms underpinning their functionality. The reductive nature of most cofactors involved in photosynthetic ET makes the direct estimation of their properties difficult. Photosystem I (PSI) operates in a highly reducing regime, making the assessment of cofactor properties even more difficult. Kinetic modelling coupled to a non-adiabatic description of ET is a useful approach in overcoming this hindrance. Here we review the theory and modelling approaches that have been used in assessing parameters associated with ET reactions in PSI, with particular attention to ET reactions involving the phylloquinones and the iron-sulphur clusters. In most modelling studies, the goal is to estimate the driving force of ET, which is usually associated with the cofactor midpoint potentials. The driving force is sensitive to many factors, which define the ET rate, i.e. the reorganisation energy, the coupling with nuclear modes and the electronic matrix elements, which are explored and discussed here. The importance of an inclusive modelling of both forward and reverse ET processes is discussed and highlighted. It is shown that although estimates are indeed sensitive to the exact parameter sets employed in the modelling, a general consensus is still attained, pointing to a scenario where Δ G A 1 A → F X 0 / Δ G A 1 B → F X 0 is weakly endergonic/exergonic, respectively. It is emphasised that to further refine those estimates, it will require a joint effort between computational modelling and more wide-ranging experimental studies.

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.

Photosystem I with benzoquinone analogues incorporated into the A1 binding site

Time-resolved FTIR difference spectroscopy has been used to study photosystem I (PSI) particles with three different benzoquinones [plastoquinone-9 (PQ), 2,6-dimethyl-1,4-benzoquinone (DMBQ), 2,3,5,6-tetrachloro-1,4-benzoquinone (Cl₄BQ)] incorporated into the A₁ binding site. If PSI samples are cooled in the dark to 77 K, the incorporated benzoquinones are shown to be functional, allowing the production of time-resolved (P700⁺A₁^--P700A₁) FTIR difference spectra. If samples are subjected to repetitive flash illumination at room temperature prior to cooling, however, the time-resolved FTIR difference spectra at 77 K display contributions typical of the P700 triplet state (³P700), indicating a loss of functionality of the incorporated benzoquinones, that occurs because of double protonation of the incorporated benzoquinones. The benzoquinone protonation mechanism likely involves nearby water molecules but does not involve the terminal iron-sulfur clusters F_A and F_B. These results and conclusions resolve discrepancies between results from previous low-temperature FTIR and EPR studies on similar PSI samples with PQ incorporated.

Electron Transfer through the Acceptor Side of Photosystem I: Interaction with Exogenous Acceptors and Molecular Oxygen

This review considers the state-of-the-art on mechanisms and alternative pathways of electron transfer in photosynthetic electron transport chains of chloroplasts and cyanobacteria. The mechanisms of electron transport control between photosystems (PS) I and II and the Calvin-Benson cycle are considered. The redistribution of electron fluxes between the noncyclic, cyclic, and pseudocyclic pathways plays an important role in the regulation of photosynthesis. Mathematical modeling of light-induced electron transport processes is considered. Particular attention is given to the electron transfer reactions on the acceptor side of PS I and to interactions of PS I with exogenous acceptors, including molecular oxygen. A kinetic model of PS I and its interaction with exogenous electron acceptors has been developed. This model is based on experimental kinetics of charge recombination in isolated PS I. Kinetic and thermodynamic parameters of the electron transfer reactions in PS I are scrutinized. The free energies of electron transfer between quinone acceptors A_1A/A_1B in the symmetric redox cofactor branches of PS I and iron-sulfur clusters F_X, F_A, and F_B have been estimated. The second-order rate constants of electron transfer from PS I to external acceptors have been determined. The data suggest that byproduct formation of superoxide radical in PS I due to the reduction of molecular oxygen in the A₁ site (Mehler reaction) can exceed 0.3% of the total electron flux in PS I.

Kinetic modeling of electron transfer reactions in photosystem I complexes of various structures with substituted quinone acceptors

The reduction kinetics of the photo-oxidized primary electron donor P₇₀₀ in photosystem I (PS I) complexes from cyanobacteria Synechocystis sp. PCC 6803 were analyzed within the kinetic model, which considers electron transfer (ET) reactions between P₇₀₀, secondary quinone acceptor A₁, iron-sulfur clusters and external electron donor and acceptors - methylviologen (MV), 2,3-dichloro-naphthoquinone (Cl₂NQ) and oxygen. PS I complexes containing various quinones in the A₁-binding site (phylloquinone PhQ, plastoquinone-9 PQ and Cl₂NQ) as well as F _X-core complexes, depleted of terminal iron-sulfur F _A/F _B clusters, were studied. The acceleration of charge recombination in F _X-core complexes by PhQ/PQ substitution indicates that backward ET from the iron-sulfur clusters involves quinone in the A₁-binding site. The kinetic parameters of ET reactions were obtained by global fitting of the P₇₀₀⁺ reduction with the kinetic model. The free energy gap ΔG ₀ between F _X and F _A/F _B clusters was estimated as -130 meV. The driving force of ET from A₁ to F _X was determined as -50 and -220 meV for PhQ in the A and B cofactor branches, respectively. For PQ in A_1A-site, this reaction was found to be endergonic (ΔG ₀ = +75 meV). The interaction of PS I with external acceptors was quantitatively described in terms of Michaelis-Menten kinetics. The second-order rate constants of ET from F _A/F _B, F _X and Cl₂NQ in the A₁-site of PS I to external acceptors were estimated. The side production of superoxide radical in the A₁-site by oxygen reduction via the Mehler reaction might comprise ≥0.3% of the total electron flow in PS I.

Inverted-region electron transfer as a mechanism for enhancing photosynthetic solar energy conversion efficiency

In all photosynthetic organisms, light energy is used to drive electrons from a donor chlorophyll species via a series of acceptors across a biological membrane. These light-induced electron-transfer processes display a remarkably high quantum efficiency, indicating a near-complete inhibition of unproductive charge recombination reactions. It has been suggested that unproductive charge recombination could be inhibited if the reaction occurs in the so-called inverted region. However, inverted-region electron transfer has never been demonstrated in any native photosynthetic system. Here we demonstrate that the unproductive charge recombination in native photosystem I photosynthetic reaction centers does occur in the inverted region, at both room and cryogenic temperatures. Computational modeling of light-induced electron-transfer processes in photosystem I demonstrate a marked decrease in photosynthetic quantum efficiency, from 98% to below 72%, if the unproductive charge recombination process does not occur in the inverted region. Inverted-region electron transfer is therefore demonstrated to be an important mechanism contributing to efficient solar energy conversion in photosystem I. Inverted-region electron transfer does not appear to be an important mechanism in other photosystems; it is likely because of the highly reducing nature of photosystem I, and the energetic requirements placed on the pigments to operate in such a regime, that the inverted-region electron transfer mechanism becomes important.

Time-resolved infrared spectroscopy in the study of photosynthetic systems

Time-resolved (TR) infrared (IR) spectroscopy in the nanosecond to second timescale has been extensively used, in the last 30 years, in the study of photosynthetic systems. Interesting results have also been obtained at lower time resolution (minutes or even hours). In this review, we first describe the used techniques-dispersive IR, laser diode IR, rapid-scan Fourier transform (FT)IR, step-scan FTIR-underlying the advantages and disadvantages of each of them. Then, the main TR-IR results obtained so far in the investigation of photosynthetic reactions (in reaction centers, in light-harvesting systems, but also in entire membranes or even in living organisms) are presented. Finally, after the general conclusions, the perspectives in the field of TR-IR applied to photosynthesis are described.

Time-resolved visible and infrared absorption spectroscopy data obtained using photosystem I particles with non-native quinones incorporated into the A1 binding site

Time-resolved visible and infrared absorption difference spectroscopy data at both 298 and 77 K were obtained using cyanobacterial menB⁻ mutant photosystem I particles with several non-native quinones incorporated into the A₁ binding site. Data was obtained for photosystem I particles with phylloquinone (2-methyl-3-phytyl-1,4-naphthoquinone), 2-bromo-1,4-naphthoquinone, 2-chloro-1,4-naphthoquinone, 2-methyl-1,4-naphthoquinone, 2,3-dibromo-1,4-naphthoquinone, 2,3-dichloro-1,4-naphthoquinone, and 9,10-anthraquinone incorporated. Transient absorption data were obtained at 487 and 703 nm in the visible spectral range, and 1950–1100 cm⁻¹ in the infrared region. Time constants obtained from fitting the time-resolved infrared and visible data are in good agreement. The measured time constants are crucial for the development of appropriate kinetic models that can describe electron transfer processes in photosystem I, “Modeling Electron Transfer in Photosystem I” Makita and Hastings (2016) [1].

Comparative kinetic and energetic modelling of phyllosemiquinone oxidation in Photosystem I

The oxidation kinetics of phyllo(semi)quinone (PhQ), which acts as an electron transfer (ET) intermediate in the Photosystem I reaction centre, are described by a minimum of two exponential phases, characterised by lifetimes in the 10-30 ns and 150-300 ns ranges. The fastest phase is considered to be dominated by the oxidation of the PhQ molecule coordinated by the PsaB reaction centre subunit (PhQB), and the slowest phase is dominated by the oxidation of the PsaA coordinated PhQ (PhQA). Testing different energetic schemes within a unified theory-based kinetic modelling approach provides reliable limit-values for some of the physical-chemical parameters controlling these ET reactions: (i) the value of ΔG(0) associated with PhQA oxidation is smaller than ∼+30 meV; (ii) the value of the total reorganisation energy (λt) likely exceeds 0.7 eV; (iii) different mean nuclear modes are coupled to PhQB and PhQA oxidation, the former being larger, and both being ≥100 cm(-1).