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 (Chl ) molecules, including six specialized Chls denoted Chl/Chl (dimer P, or PP), Chl/Chl, and Chl/Chl 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 P is electronically coupled to the symmetrically located monomers Chl and Chl, which can be considered together as a symmetric exciplex ChlPPChl with the mixed excited (ChlPPChl) and two charge-transfer states P Chl and P Chl .

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.

Mutation H(M202)L does not lead to the formation of a heterodimer of the primary electron donor in reaction centers of Rhodobacter sphaeroides when combined with mutation I(M206)H.

In photosynthetic reaction centers (RCs) of purple bacteria, conserved histidine residues [His L173 and His M202 in Rhodobacter (Rba.) sphaeroides] are known to serve as fifth axial ligands to the central Mg atom of the bacteriochlorophyll (BChl) molecules (P and P, respectively) that constitute the homodimer (BChl/BChl) primary electron donor P. In a number of previous studies, it has been found that replacing these residues with leucine, which cannot serve as a ligand to the Mg ion of BChl, leads to the assembly of heterodimer RCs with P represented by the BChl/BPheo pair.

Coherent intradimer dynamics in reaction centers of photosynthetic green bacterium Chloroflexus aurantiacus.

Early-time dynamics of absorbance changes (light minus dark) in the long-wavelength Q absorption band of bacteriochlorophyll dimer P of isolated reaction centers (RCs) from thermophilic green bacterium Chloroflexus (Cfx.) aurantiacus was studied by difference pump-probe spectroscopy with 18-fs resolution at cryogenic temperature. It was found that the stimulated emission spectrum gradually moves to the red on the ~100-fs time scale and subsequently oscillates with a major frequency of ~140 cm.

Ultrafast excited-state dynamics in chlorosomes isolated from the photosynthetic filamentous green bacterium Chloroflexus aurantiacus.

Bacteriochlorophyll (BChl) c pigments in the aggregated state are responsible for efficient light harvesting in chlorosomes of the filamentous anoxygenic photosynthetic bacterium, Chloroflexus (Cfx.) aurantiacus. Absorption of light creates excited states in the BChl c aggregates.

Analysis of the transformation effect in cytochrome b559 of photosystem II in terms of the model of the heme-quinone redox interaction.

Transformation of three-component redox pattern of cytochrome (Cyt) b559 in PS II membrane fragments upon various treatments is manifested in decrease of the relative content (R) of the high potential (HP) redox form of Cyt b559 and concomitant increase in the fractions of the two lower potential forms. Redox titration of Cyt b559 in different types of PS II membrane preparations was performed and revealed that (1) alteration of redox titration curve of Cyt b559 upon treatment of a sample is not specific to the type of treatment; (2) each value of R defines the individual shape of the redox titration curve; (3) population of Cyt b559 may exist in several stable forms with multicomponent redox pattern: three types of three-component redox pattern and one type of two-component redox pattern as well as in the form with a single E; (4) transformation of Cyt b559 proceeds as successive conversion between the stable forms with multicomponent redox pattern; (5) upon harsh treatments, Cyt b559 abruptly converts into the state with a single E which value is intermediate between the E values of the two lower potential forms. Analysis of the data using the model of Cyt b559-quinone redox interaction revealed that diminution of R in a range from 80 to 10% reflects a shift in redox equilibrium between the heme group of Cyt b559 and the interacting quinone, due to a gradual decrease of 90 mV in E of the heme group at the virtually unchanged E of the quinone component.

Vyacheslav (Slava) Klimov (1945-2017): A scientist par excellence, a great human being, a friend, and a Renaissance man.

Vyacheslav Vasilevich (V.V.) Klimov (or Slava, as most of us called him) was born on January 12, 1945 and passed away on May 9, 2017.

Mechanism of adiabatic primary electron transfer in photosystem I: Femtosecond spectroscopy upon excitation of reaction center in the far-red edge of the Q band.

The ultrafast primary charge separation in Photosystem I (PS I) excited by femtosecond pulses centered at 720 and 760nm was studied by pump-to-probe laser spectroscopy. The absorbance in the red edge of PS I absorption spectrum has an unusual exponential dependence on wavelength. The cutoff of short wavelength components of 760nm pulse allows direct excitation of reaction center chlorophyll molecules without involvement of light-harvesting antenna.

Variability of aggregation extent of light-harvesting pigments in peripheral antenna of Chloroflexus aurantiacus.

The stationary ground state and femtosecond time-resolved absorption spectra as well as spectra of circular dichroism were measured at room temperature using freshly prepared samples of chlorosomes isolated from fresh cultures of the green bacterium Chloroflexus aurantiacus. Cultures were grown by using as inoculum the same seed culture but under different light conditions. All measured spectra clearly showed the red shift of BChl c Q bands (up to 5 nm) for low-light chlorosomes as compared to high-light ones, together with concomitant narrowing of these bands and increasing of their amplitudes.

Characterization of the low-temperature triplet state of chlorophyll in photosystem II core complexes: Application of phosphorescence measurements and Fourier transform infrared spectroscopy.

Phosphorescence measurements at 77 K and light-induced FTIR difference spectroscopy at 95 K were applied to study of the triplet state of chlorophyll a ((3)Chl) in photosystem II (PSII) core complexes isolated from spinach. Using both methods, (3)Chl was observed in the core preparations with doubly reduced primary quinone acceptor QA. The spectral parameters of Chl phosphorescence resemble those in the isolated PSII reaction centers (RCs).

Towards an understanding of redox heterogeneity of the photosystem II cytochrome b559 in the native membrane.

A complex redox titration pattern of cytochrome (Cyt) b559 in preparations of thylakoid membranes and photosystem (PS) II membrane fragments is commonly attributed to the presence of three conformational forms differing by a structure of the heme microenvironment. However, despite decades of research, structural determinants underlying differences between the redox forms of Cyt b559 have not been defined. In this work, we propose a different interpretation of redox heterogeneity in the native population of Cyt b559 assuming redox interaction between the Cyt b559 heme group and a nearby bound quinone (Q).

Elastic Vibrations in the Photosynthetic Bacterial Reaction Center Coupled to the Primary Charge Separation: Implications from Molecular Dynamics Simulations and Stochastic Langevin Approach.

Primary electron transfer reactions in the bacterial reaction center are difficult for theoretical explication: the reaction kinetics, almost unalterable over a wide range of temperature and free energy changes, revealed oscillatory features observed initially by Shuvalov and coauthors (1997, 2002). Here the reaction mechanism was studied by molecular dynamics and analyzed within a phenomenological Langevin approach. The spectral function of polarization around the bacteriochlorophyll special pair PLPM and the dielectric response upon the formation of PL(+)PM(-) dipole within the special pair were calculated.

Low-temperature (77 K) phosphorescence of triplet chlorophyll in isolated reaction centers of photosystem II.

Phosphorescence characterized by the main emission band at 952 ± 1 nm (1.30 eV), the lifetime of 1.5 ± 0.

My journey in photosynthesis research.

At the invitation of Suleyman I. Allakhverdiev, I provide here a brief autobiography for this special issue that recognizes my service and research for the larger international community of photosynthesis research.

Orientation of B798 BChl a Q y transition dipoles in Chloroflexus aurantiacus chlorosomes: polarized transient absorption spectroscopy studies.

Isotropic and anisotropic pump-probe spectra of Cfx. aurantiacus chlorosomes were measured on the fs-through ps-time scales for the B798 BChl a Q y band upon direct excitation of the B798 band at T = 293 K and T = 90 K. Upon direct excitation of the B798 band, the anisotropy parameter value r(λ) was constant within the whole BChl a Q y band at any delay time at both temperatures.

The L(M196)H mutation in Rhodobacter sphaeroides reaction center results in new electrostatic interactions.

New histidine residue was introduced in M196 position in the reaction center of Rhodobacter sphaeroides in order to alter polarity of the BChl dimer's protein environment and to study how it affects properties and structure of the primary electron donor P. It was shown that in the absorption spectrum of the mutant RC the 6 nm red shift of the Q Y P band was observed together with considerable decrease of its amplitude. The mid-point potential of P/P (+) in the mutant RC was increased by +65 (±15) mV as compared to the E m P/P (+) value in the wild-type RC suggesting that the mutation resulted in new pigment-protein interactions.

Spectral exhibition of electron-vibrational relaxation in P* state of Rhodobacter sphaeroides reaction centers.

Electron-vibrational relaxation in the excited state of the primary electron donor, bacteriochlorophyll dimer P, in the reaction centers (RCs) of purple photosynthetic bacteria Rhodobacter sphaeroides is modeled. A multimode model of three states (i.e.

Chemically modified reaction centers of photosystem II: Exchange of pheophytin a with 7-deformyl-7-hydroxymethyl-pheophytin b.

The native pheophytin a (Pheo a) in isolated reaction centers of photosystem II (PSII RCs) has been chemically exchanged with extraneous 7-deformyl-7-hydroxymethyl-Pheo b (7(1)-OH-Pheo b) which differs from Pheo a by the C-7 substituent (hydroxymethyl instead of methyl). The two pigments have similar reduction potentials in vitro [M. Meyer, Dissertation, Universität München, 1997], while their absorption spectra show small but distinct differences in the visible region.

Evidence that histidine forms a coordination bond to the A(0A) and A(0B) chlorophylls and a second H-bond to the A(1A) and A(1B) phylloquinones in M688H(PsaA) and M668H(PsaB) variants of Synechocystis sp. PCC 6803.

The axial ligands of the acceptor chlorophylls, A(0A) and A(0B), in Photosystem I are the Met sulfur atoms of M688(PsaA) and M668(PsaB). To determine the role of the Met, His variants were generated in Synechocystis sp. PCC 6803.

Biphasic reduction of cytochrome b559 by plastoquinol in photosystem II membrane fragments: evidence for two types of cytochrome b559/plastoquinone redox equilibria.

In photosystem II membrane fragments with oxidized cytochrome (Cyt) b559 reduction of Cyt b559 by plastoquinol formed in the membrane pool under illumination and by exogenous decylplastoquinol added in the dark was studied. Reduction of oxidized Cyt b559 by plastoquinols proceeds biphasically comprising a fast component with a rate constant higher than (10s)(-1), named phase I, followed by a slower dark reaction with a rate constant of (2.7min)(-1) at pH6.

FTIR spectroscopy of the reaction center of Chloroflexus aurantiacus: photoreduction of the bacteriopheophytin electron acceptor.

Mid-infrared spectral changes associated with the photoreduction of the bacteriopheophytin electron acceptor H(A) in reaction centers (RCs) of the filamentous anoxygenic phototrophic bacterium Chloroflexus (Cfl.) aurantiacus are examined by light-induced Fourier transform infrared (FTIR) spectroscopy. The light-induced H(A)(-)/H(A) FTIR (1800-1200cm(-1)) difference spectrum of Cfl.

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.

Primary light-energy conversion in tetrameric chlorophyll structure of photosystem II and bacterial reaction centers: I. A review.

The purpose of the review is to show that the tetrameric (bacterio)chlorophyll ((B)Chl) structures in reaction centers of photosystem II (PSII) of green plants and in bacterial reaction centers (BRCs) are similar and play a key role in the primary charge separation. The Stark effect measurements on PSII reaction centers have revealed an increased dipole moment for the transition at approximately 730 nm (Frese et al., Biochemistry 42:9205-9213, 2003).

Wave packet motions coupled to electron transfer in reaction centers of Chloroflexus aurantiacus.

Transient absorption difference spectroscopy with approximately 20 femtosecond (fs) resolution was applied to study the time and spectral evolution of low-temperature (90 K) absorbance changes in isolated reaction centers (RCs) of Chloroflexus (C.) aurantiacus. In RCs, the composition of the B-branch chromophores is different with respect to that of purple bacterial RCs by occupying the B(B) binding site of accessory bacteriochlorophyll by bacteriopheophytin molecule (Phi(B)).

Substitution of isoleucine L177 by histidine in Rhodobacter sphaeroides reaction center results in the covalent binding of PA bacteriochlorophyll to the L subunit.

In this work, we report the unique case of bacteriochlorophyll (BChl) - protein covalent attachment in a photosynthetic membrane complex caused by a single mutation. The isoleucine L177 was substituted by histidine in the photosynthetic reaction center (RC) of Rhodobacter sphaeroides. Pigment analysis revealed that one BChl molecule was missing in the acetone-methanol extract of the I(L177)H RCs.