Engineering hydrogen bonding at tyrosine-201 in the orange carotenoid protein using halogenated analogues.

The Orange Carotenoid Protein (OCP) is a unique water-soluble photoactive protein that plays a critical role in regulating the balance between light harvesting and photoprotective responses in cyanobacteria. The challenge in understanding OCP´s photoactivation mechanism stems from the heterogeneity of the initial configurations of its embedded ketocarotenoid, which in the dark-adapted state can form up to two hydrogen bonds to critical amino acids in the protein's C-terminal domain, and the extremely low quantum yield of primary photoproduct formation. While a series of experiments involving point mutations within these contacts helped us to identify these challenges, they did not resolve them.

Specificity of Photochemical Energy Conversion in Photosystem I from the Green Microalga .

Primary excitation energy transfer and charge separation in photosystem I (PSI) from the extremophile desert green alga grown in low light were studied using broadband femtosecond pump-probe spectroscopy in the spectral range from 400 to 850 nm and in the time range from 50 fs to 500 ps. Photochemical reactions were induced by the excitation into the blue and red edges of the chlorophyll Qy absorption band and compared with similar processes in PSI from the cyanobacterium  sp. PCC 6803.

Femtosecond optical studies of the primary charge separation reactions in far-red photosystem II from Synechococcus sp. PCC 7335.

Primary processes of light energy conversion by Photosystem II (PSII) were studied using femtosecond broadband pump-probe absorption difference spectroscopy. Transient absorption changes of core complexes isolated from the cyanobacterium Synechococcus sp. PCC 7335 grown under far-red light (FRL-PSII) were compared with the canonical Chl a containing spinach PSII core complexes upon excitation into the red edge of the Q band.

Femtosecond Dynamics of Excited States of Chlorophyll Tetramer in Water-Soluble Chlorophyll-Binding Protein BoWSCP.

The paper reports on the absorption dynamics of chlorophyll a in a symmetric tetrameric complex of the water-soluble chlorophyll-binding protein BoWSCP. It was measured by a broadband femtosecond laser pump-probe spectroscopy within the range from 400 to 750 nm and with a time resolution of 20 fs-200 ps. When BoWSCP was excited in the region of the Soret band at a wavelength of 430 nm, nonradiative intramolecular conversion S→S was observed with a characteristic time of 83 ± 9 fs.

Role of pheophytin a in the primary charge separation of photosystem I from Acaryochloris marina: Femtosecond optical studies of excitation energy and electron transfer reactions.

Photosystem I (PSI) of the cyanobacterium Acaryochloris marina is capable of performing an efficient photoelectrochemical conversion of far-red light due to its unique suite of cofactors. Chlorophyll d (Chl-d) has been long known as the major antenna pigment in the PSI from A. marina, while the exact cofactor composition of the reaction centre (RC) was established only recently by cryo-electron microscopy.

Inverted region in the reaction of the quinone reduction in the A-site of photosystem I from cyanobacteria.

Photosystem I from the menB strain of Synechocystis sp. PCC 6803 containing foreign quinones in the A sites was used for studying the primary steps of electron transfer by pump-probe femtosecond laser spectroscopy. The free energy gap (- ΔG) of electron transfer between the reduced primary acceptor A and the quinones bound in the A site varied from 0.

Donor-Acceptor Complexes of (5,10,15,20-Tetra(4-methylphenyl)porphyrinato)cobalt(II) with Fullerenes C: Self-Assembly, Spectral, Electrochemical and Photophysical Properties.

The noncovalent interactions of (5,10,15,20-tetra(4-methylphenyl)porphinato)cobalt(II) (CoTTP) with C and 1-N-methyl-2-(pyridin-4-yl)-3,4-fullero[60]pyrrolidine (PyC) were studied in toluene using absorption and fluorescence titration methods. The self-assembly in the 2:1 complexes (the triads) (C)CoTTP and (PyC)CoTTP was established. The bonding constants for (C)CoTTP and (PyC)CoTTP are defined to be (3.

Comparative Absorption Dynamics of the Singlet Excited States of Chlorophylls a and d.

Transient absorption dynamics of chlorophylls a and d dissolved in tetrahydrofuran was measured by the broadband femtosecond laser pump-probe spectroscopy in a spectral range from 400 to 870 nm. The absorption spectra of the excited S singlet states of chlorophylls a and d were recorded, and the dynamics of the of the Q band shift of the stimulated emission (Stokes shift of fluorescence) was determined in a time range from 60 fs to 4 ps. The kinetics of the intramolecular conversion Q→Q (electronic transition S→S) was measured; the characteristic relaxation time was 54 ± 3 and 45 ± 9 fs for chlorophylls a and d, respectively.

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 .

Donor-acceptor interactions of gold(III) porphyrins with cobalt(II) phthalocyanine: chemical structure of products, their spectral characterization and DFT study.

In the context of the development of coordination energy-harvesting systems, the axial bonding of cobalt(II) octakis(3,5-di--butylphenoxy)phthalocyanine (1) with gold(III) 2,3,7,8,12,18-hexamethyl,13,17-diethyl,5-(pyridin-4-yl)- and (2,3,7,8,12,18-hexamethyl,13,17-diethyl,5-(pyridin-3-yl)porphin (2 and 3), the structure, the spectral/electrochemical properties of the resulting donor-acceptor complexes and photoinduced electron transfer in them are studied. The process of the dyad formation passing as self-assembly in the donor-acceptor phthalocyanine-porphyrin systems was explored using UV-Visible, IR, and H NMR spectroscopy and mass spectrometry. The geometric and electronic structures of the dyads were identified using density functional theory (DFT) and time-dependent DFT calculations.

Ultrafast Quenching of Excitons in the ZnCdS/ZnS Quantum Dots Doped with Mn through Charge Transfer Intermediates Results in Manganese Luminescence.

For the first time, a specific time-delayed peak was registered in the femtosecond transient absorption (TA) spectra of ZnCdS/ZnS (x~0.5) alloy quantum dots (QDs) doped with Mn, which was interpreted as the electrochromic Stark shift of the band-edge exciton. The time-delayed rise and decay kinetics of the Stark peak in the manganese-doped QDs significantly distinguish it from the kinetics of the Stark peak caused by exciton-exciton interaction in the undoped QDs.

Intramolecular photo-driven charge transfer in a series of pyridyl substituted phenyloxazoles. Structural relaxation in meta-substituted ethylpyridinium derivative of phenyloxazole.

A series of pyridyl (pyridinium) substituted benzoxazoles were studied by steady state absorption, fluorescence spectroscopy, time-resolved fluorescence spectroscopy, fs pulse absorption and polarization spectroscopy, and quantum-chemical calculations. The spectral and kinetic parameters of the fluorophores in MeCN and EtOAc were obtained experimentally and were calculated by means of DFT and TDDFT methods. A scheme including four transient excited states was proposed for the interpretation of differential absorption kinetics of the charged fluorophores.

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 and Chl (also termed A and A). The absorption dynamics at delays of 0.

The donor-acceptor dyad based on high substituted fullero[70]pyrrolidine-coordinated manganese (III) phthalocyanine for photoinduced electron transfer.

Donor-acceptor dyads based on manganese porphyrins/phthalocyanines and fullerene derivatives with N-basicity centers have proved as promising photoinduced electron-transfer systems for photovoltaic devices, biologically active compounds, and molecular magnetic materials. The macroheterocyclic chromophore characterized by rich UV-visible-near IR absorption is the basis for the applications above. The problem of the synthesis and the characterization of new effective dyads was solved in this work on the example of the self-organizing system consisting of (octakis-3,5-di-tert-butylphenoxy)phthalocyaninato)manganese(III) acetate, (AcO)MnPc(3,5-di-BuPhO), 2',5-di(pyridin-2'-yl)-3,4-fullero[70]pyrrolidine, PyC, and toluene.

Role of hydrogen bond alternation and charge transfer states in photoactivation of the Orange Carotenoid Protein.

Here, we propose a possible photoactivation mechanism of a 35-kDa blue light-triggered photoreceptor, the Orange Carotenoid Protein (OCP), suggesting that the reaction involves the transient formation of a protonated ketocarotenoid (oxocarbenium cation) state. Taking advantage of engineering an OCP variant carrying the Y201W mutation, which shows superior spectroscopic and structural properties, it is shown that the presence of Trp201 augments the impact of one critical H-bond between the ketocarotenoid and the protein. This confers an unprecedented homogeneity of the dark-adapted OCP state and substantially increases the yield of the excited photoproduct S*, which is important for the productive photocycle to proceed.

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

In Photosystem I (PS I), the role of the accessory chlorophyll (Chl) molecules, Chl and Chl (also termed A and A), which are directly adjacent to the special pair P and fork into the A- and B-branches of electron carriers, is incompletely understood. In this work, the Chl and Chl transient absorption ΔA(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 or Chl through a HO molecule) substituted by Met, His, and Leu.

Comparative Femtosecond Spectroscopy of Primary Photoreactions of Rhodopsin and Bacteriorhodopsin.

The primary stages of the rhodopsin (ESR) photocycle were investigated by femtosecond absorption laser spectroscopy in the spectral range of 400-900 nm with a time resolution of 25 fs. The dynamics of the ESR photoreaction were compared with the reactions of bacteriorhodopsin (bR) in purple membranes (bR) and in recombinant form (bR). The primary intermediates of the ESR photocycle were similar to intermediates , , and in bacteriorhodopsin photoconversion.

Femtosecond excited state dynamics of stilbene-viologen complexes with a weakly pronounced charge transfer.

The femtosecond dynamics of photoinduced electron transfers in supramolecular donor-acceptor complexes between (E)-bis(18-crown-6)stilbene (D) and tetraperchlorates of 2,7-di(2-ammonioethyl)(2,7-diazapyrenium) (A1), 3,3'-(E)-ethene-1,2-diylbis[1-(3-ammoniopropyl)pyridinium] (A2) and 4,4'-ethane-1,2-diylbis[1-(3-ammoniopropyl)pyridinium] (A3) was studied. The acceptors A2 and A3 are weak electron acceptors whose first reduction potentials are equal to -1.0 and -1.

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 (P), 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.

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.

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

Cyanobacterial photosystem I (PSI) functions as a light-driven cyt c-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.

Monitoring the electric field in CdSe quantum dots under ultrafast interfacial electron transfer via coherent phonon dynamics.

Coherent phonon dynamics in CdSe quantum dots (QD) under an ultrafast electron transfer (ET) reaction of the (1S-1S) exciton quenched by methyl viologen (MV) adsorbed onto the QD surface was studied by ultrafast pump-probe spectroscopy. Frequency and amplitude modulations (FM, AM) of the transient absorption ΔA(ω,t) in the pure CdSe and coupled CdSe/MV QDs were identified in the bleach band dynamics of the red-edge exciton. The fast Fourier transform (FFT) and continuous wavelet transform analysis of the FM and AM oscillations revealed peaks at 0.

Comparisons of Electron Transfer Reactions in a Cyanobacterial Tetrameric and Trimeric Photosystem I Complexes.

Photosystem I (PSI) is a Type-I reaction center and is the largest photosynthetic complex to be characterized. In cyanobacteria, PSI is organized as a trimer with a three-fold axis of symmetry. Recently, a tetrameric form of PSI has been identified in cyanobacteria.