what is the name of the enzyme found in thylakoid membranes that is used to produce atp

Thylakoid Membrane

Thylakoid membranes in the chloroplasts of higher plants are arranged in structured stacks of flat membrane disks ∼300–600nm in diameter (grana regions), which are linked past single sheets (stroma lamellae regions).

From: Encyclopedia of Interfacial Chemistry , 2018

Thylakoid Membrane Bioenergetics

Due east. Guaus , ... J. Hoyo , in Encyclopedia of Interfacial Chemistry, 2018

Structure of the Thylakoid Membrane and PET Reactions

The thylakoid membrane is constituted past a lipid matrix that maintains its fluidity, allows an electrochemical potential divergence across this membrane, and harbors all four main protein complexes of the OPh machinery ( Fig. 1 ): photosystem I (PSI), photosystem II (PSII), cytochrome b_sixf complex (Cyt b₆f), and ATP synthase. Moreover, this lipid matrix embeds low molecular-weight carriers, plastoquinone-9 (PQ) ( Scheme 1A ) and plastocyanin (PC), which are the electron and proton shuttles between PSII and PSI, respectively, via Cyt b₆f. ^{1 , iii , iv} Finally, an ATP synthase complex utilizes the energy stored in the proton gradient established across the thylakoid membrane past PET to synthesize ATP.

Scheme 1. Scheme of a molecule of (A) PQ-ix, (B) UQ-10, and (C) PhQ.

The core molecular machinery mediating PET is highly conserved from cyanobacteria to plants, although the light-collecting systems or photosynthetic pigments are more diverse, varying from soluble phycobilisomes in blue-green alga and cerise algae to membrane integrated lite-harvesting complexes (LHC) or pigment-protein antenna complexes in plants. Moreover, in algae and high plants, the photosynthetic complexes are unevenly distributed between the two subsystems stroma and grana, with PSII and the corresponding LHCII beingness concentrated in grana regions, whereas PSI and the ATP synthase are mainly institute in stroma lamellae, a status referred to every bit lateral heterogeneity. Thylakoids represent remarkably conserved energy-transducing membranes with a high protein complexes/lipid ratio that can reach upwardly to fourscore% in stacked grana regions of eukaryotic chloroplasts. The lipophilic matrix in which the protein/pigment complexes are embedded is unique, and has been conserved during the course of development. This suggests that a specific lipid environment is required to ensure the stability and activity of photosynthetic complexes. ³

The lipid content of the thylakoid matrix depends on the species and the external conditions. Yet, information technology can be agreed that the thylakoid membrane of a typical OPh organism is composed of the following lipids: monogalactosyldiacylglycerol (MGDG)   >   l% ( Scheme 2 A ), digalactosyldiacylglycerol (DGDG)   ≈   30% ( Scheme ii B), and minor amounts of other lipids, phosphatidylglycerol (PG) ( Scheme 2 C), and sulfoquinovosyldiacylglycerol (SQDG) ( Scheme 2 D). ^{3 , 5} These lipids have divers structural/functional roles which are dependent on their physicochemical properties. These properties include the presence of negatively charged head groups in only SQDG and PG, and the small-scale size of MGDG'south headgroup, which allows it to grade changed hexagonal structures. As a functional upshot, MGDG is essential for thylakoid germination and DGDG for stabilizing the bilayers constituting the membranes. Recent analyses support the disquisitional function of the MGDG/DGDG ratio for membrane phase transitions and highlight the function of DGDG for membrane stacking via the germination of hydrogen bonds between the head groups of adjacent bilayers. In addition to their matrix part, all glycerolipid classes have been shown to interact closely with photosynthetic protein complexes, in particular with PSII, as deduced from X-ray crystallographic analyses. This suggests that private lipids may also play disquisitional roles for the structure and function of the protein complexes. ³

Scheme 2. Scheme of a molecule of (A) MGDG, (B) DGDG, (C) PG, and (D) SQDG.

The protein complexes harbored in the thylakoid membrane (run across Fig. i ) catalyze the bulk of lite reactions of OPh. The PSII complex acts as a light-driven water:plastoquinone oxidoreductase. PSII utilizes the electrons taken from water to reduce PQ to plastoquinol (PQH₂), which diffuses into the membrane and is replaced by fresh PQ. The PQ and PQH₂ molecules accumulated in the thylakoid membrane are referred to every bit quinone pool. PQH₂ becomes reoxidized at the Cyt b₆f complex, which transfers the electrons to the water-soluble carrier PC, a copper-binding protein (or Cyt c₆, a hemeprotein) at the lumenal side of the membrane. PC delivers the electrons to PSI, a light-driven plastocyanin:ferredoxin oxidoreductase, which is composed of different polypeptides and binds, among other cofactors, three iron–sulfur clusters of the [4Fe–4S] blazon in its PET system. PSI reduces the soluble [2Fe–2S] protein ferredoxin (or flavodoxin) at the cytoplasmic/stroma side of the membrane, which in turn supplies the soluble flavoprotein ferredoxin:NADP⁺ reductase (FNR) with electrons for NADPH formation. The combined activeness of the membrane proteins involved in this reaction sequence results in a net transport of protons from the cytoplasmic/stroma to the lumenal side of the membrane ( Fig. 1 ), creating an electrochemical potential divergence, which is used by the ATP synthase for the formation of ATP. ^{6 , seven}

In blue-green alga, algae, and higher plants, the overall electron transfer of light reactions of OPh can be represented by a scheme know as Z-scheme ⁸ ( Fig. 2 ). The zig-zag nature of Z-scheme shows the two loci of photochemical reactions (1 in PSII and the other in PSI) needed to drive the electrons from water to NADP⁺. In purple bacteria, the PET is simpler and takes place cyclically in only ane middle. Moreover, in regal bacteria the photosynthetic complexes are incorporated in the plasmatic membrane and ubiquinone-10 (UQ) ( Scheme 1B ) is used as molecular electron carrier.

Fig. 2. Z-Scheme of electron ship in the thylakoid membrane in cyanobacteria, algae, and higher plants.

Reprinted from Voet, D.; Voet, J. G.; Pratt, C. West. Fundamentals of Biochemistry, 2nd ed.; Jonh Wiley & Sons: New York, 2006; Chapter 18.

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Big-Scale QM/MM Calculations of Hydrogen Bonding Networks for Proton Transfer and Water Inlet Channels for Water Oxidation—Theoretical System Models of the Oxygen-Evolving Complex of Photosystem 2

M. Shoji , ... K. Yamaguchi , in Advances in Quantum Chemistry, 2015

A1.1 System Structure of Photosystem

Nature has adult a specialized intracellular membrane arrangement, the thylakoid membranes, where 4 large photosynthetic protein complexes, namely, photosystem II (PSII), cytochrome b6f (cytb6f), photosystem I (PSI), and ATP synthase, are embedded as shown in the text books ^1,two and in Fig. A1. In this paper, PSII is a main target for theoretical investigation. Past 13 years, XRD experiments ^fifteen–25 accept been performed to elucidate wholestructure of PSII. Nowadays, the iii-dimensional XRD structures of PSII refined to 1.ix   Å resolution ^24,25 are available, providing structural bases for large-scale QM/MM modeling of such a large complex molecular organization, where h2o oxidation by sunlight in Eq. (1) gain efficiently.

Effigy A1. The whole system construction of the photosynthetic arrangement (PS) for water oxidation by employ of sunlight in Eq. (1) in the text (Y. Umena, unpublished). Iv main units are schematically illustrated for lucid understanding of the arrangement structure of PS. Notations of the units are also shown. ^1,2 Solar energy captured at P680 of the photosystem II (PSII) is used for water oxidation, providing 4 electrons and 4 protons, together with molecular oxygen every bit the by-product. These electrons and protons are used for reduction of NADP into NADPH in the photosystem I (PSI) by using another solar energy captured at P700 and the conversion of ADP into ATP at ATP synthase. Thus, solar free energy (light energy) from Sun is converted into chemic energy at PS. Nature has adult a specialized intracellular membrane organisation, the thylakoid membrane, for this energy conversion, providing guiding principle for developments of bogus photosystems where CO_two is expected to be converted into H_ii, HCOOH, and CH₃OH, instead of the reduction of NADP into NADPH. The being of Cyt.b ₆f for proton pump in PS is particularly suggestive for developments of gating systems in artificial photosynthetic devices. Moreover, electrons and protons generated at PSII are transferred by using PQ, a closed-shell species, to avert utilize of open-shell species, suggesting a guiding principle for construction of a bioinspired molecular arrangement.

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Electronic and Spin Structures of the CaMn4O5(H2O)4 Cluster in OEC of PSII Refined to ane.9Å 10-ray Resolution

S. Yamanaka , ... Thou. Yamaguchi , in Advances in Quantum Chemical science, 2012

one Introduction

Oxygenic photosynthesis involves several protein–cofactor complexes embedded in the photosynthetic thylakoid membranes of plants, green algae, and cyanobacteria such as Thermosynechococcus vulcanus. Amid these complexes, photosystem II (PSII) has a prominent role because information technology catalyzes the oxidation of water as shown in Eq. (i) that is the prerequisite for all aerobic life ^i,ii :

(ane) $2{\text{H}}_{\mathrm{two}}\text{O}\to {\text{O}}_2+4{\text{H}}^{+}+\mathrm{iv}{\text{e}}^{-}.$

The main cyclic process to catalyze the water oxidation consists of four successive steps: that is, then-called Kok cycle every bit shown in Figure 5.i. During this process, the oxygen-evolving circuitous (OEC), which is the goad of the water-splitting reaction, takes five oxidation states (Due south₀–Southward₄). The OEC in PSII contains an inorganic cluster consisted of iv manganese ions and one calcium ion that are bridged by at least five oxygens: the active site is therefore expressed past CaMn₄O_five cluster. Past decades, molecular structures of the cluster have been investigated by the extended X-ray absorption fine construction (EXAFS), ^3–x X-ray diffraction (XRD), ^xi–19 and ENDOR ^20–23 studies of PSII. Despite these efforts, it has been yet not possible to derive an diminutive model of the CaMn_fourO_v cluster considering the 2.9   Å resolution ¹⁸ is not sufficient for a stardom between curt and long Mn–Mn distances (in the range of ii.vii–three.3   Å), and μ-oxo and di-μ-oxo bridges cannot be seen.

Figure v.one. The Kok bike proposed for h2o-splitting reactions in oxygen-evolution eye of photosystem II (see text).

In the summer of 2010 yr, Umena et al. ²⁴ have reported the XRD structure of the OEC of PSII refined to the 1.9   Å resolution, which corresponds to the night-stable S_i land of the catalytic cycle: the S₀–S₄ states of the Kok bike ²⁵ are illustrated in Figure 5.1. Telescopic and limitation of previous EXAFS and XRD results are easily understood based on their new XRD result ²⁴ equally shown in Table 5.a1. Furthermore, their upshot is really a landmark contribution that elucidates positions of a lot of waters in the PSII as well as oxygen atoms in the OEC cluster. However, the XRD experiment even at this resolution ²⁴ does non reveal the protonated oxygen (or dehydrogenated water). Various spectroscopic experiments ²⁶ and breakthrough mechanical (QM) calculations such as DFT computations ²⁷ are considered as complementary and efficient procedures for the distinction between hydroxy anion (OH) and h2o (H₂O). On the footing of the 10-ray structure, ²⁴ nosotros causeless CaMn(Three)₂Mn(IV)₂O₅(H₂O)_iv for the S₁ land to avoid the structural ambivalence. Then, the next step is an investigation of electronic and spin structures of the CaMn₄O_five(H_iiO)_iv (ane) and its deprotonated species, for example CaMn₄O_four(OH)(H_iiO)₄ (two). In this chapter, we have summarized our DFT computational results for the parent cluster one as shown in Figure 5.two: a cursory discussion on 2 is given in Section five.four.

Effigy 5.2. (a) The X-ray structure of the active site for h2o-splitting reaction at oxygen-development complex (OEC) of photosynthesis II (PSII): CaMn_ivO_v(H_iiO)₄ (one) refined to the 1.9   Å resolution: green and red atoms denote, respectively, manganese ions and oxygen atoms. (B) The notations and numberings of Mn ions in 1 and effective substitution interactions (J _ab ) between Mn ions. (For interpretation of the references to color in this figure legend, the reader is referred to the online version of this affiliate.)

PSII arrangement involving the CaMn₄O₅(H_twoO)₄ (i) is huge for DFT computations at the present stage, and therefore, some simplification (modeling) is inevitable. The CaMn₄O₅(H₂O)₄ (one) cluster has the cubane-like structure CaMn_threeO₄ with an actress Mn ion linked with bis μ-oxo bridges. Each Mn ion of 1 has a distorted octahedral ligand field. Nosotros assume that four water molecules are non deprotonated as a start pace of theoretical investigation of one in Effigy 5.two: the ligand field of ane therefore mimics the starting S₀ state of the Kok bicycle (see Fig. v.a7). ²⁵ The protein ligands lining in binding sites of the cluster, mainly glutamates and asparates, have carboxyl anions that coordinate hard Mn ions. For surrounding proteins and ligands, we take all amino acid residues within the first coordination sphere of the X-ray structure refined to 1. nine   Å resolution by Umena et al. ²⁴ We hither fixed all heavy atoms in the XRD structure, but to reduce the computational complexity, Ala344, Asp342, Asp170, Glu333, Glu189, and Glu354 residues were modeled by acetate ions, and His332 residue past an imidazole that ligates to a manganese ion. Hydrogen atoms of these modeled amino acrid residues are generated by the UCSF Chimera version 1.v. Two oxygen atoms coordinated to the calcium ion and two oxygen atoms to the dangling manganese ion are assumed to be H₂O molecules, for which positions of hydrogen atoms are optimized using UB3LYP calculation with employing STO-3G basis prepare for the highest spin states of one with the Mn_1(d)(Three)Mn_2(c)(Three)Mn_iii(b)(IV)Mn_4(a)(4) mixed-valence (MV) structure that is abbreviated as (3344): the notations are given every bit Mn_one(d)(Iii)   =   No. 58, Mn_2(c)(3)   =   No. 59, Mn_3(b)(Four)   =   No. 60, and Mn_4(a)(IV)   =   No. 61 in Effigy 5.ii: the relation between the notation by Kamiya, Shen et al., and our previous i is 1234   = dcba. As a result, the h2o molecules, reasonably, coordinated to the calcium ion and the manganese ion via solitary pairs of electrons, and therefore, alignments of water molecules are fixed for farther calculations.

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Manganese: Water Splitting, Oxygen Cantlet Donor

Robert R. Crichton , in Biological Inorganic Chemistry, 2008

PHOTOSYNTHETIC OXIDATION OF Water: OXYGEN EVOLUTION

The water-plastoquinone photo-oxidoreductase, also known equally photosystem II (PSII), embedded in the thylakoid membrane of plants, algae and cyanobacteria, uses solar energy to power the oxidation of water to dioxygen past a special center containing four Mn ions. The overall reaction catalysed past PSII is outlined below:

The special pair of chlorophyll molecules in PSII, frequently called P680, absorbs light at 680 nm and transfers an electron to a nearby pheophytin (chlorophyll with two H⁺ in identify of the fundamental Mgⁱⁱ⁺), from where information technology is transferred through other electron carriers to an exchangeable plastoquinone pool (Figure xvi.half dozen). The positive charge, which is formed on the special pair, P680 ⁺, is a powerful oxidant. Each time a photon of light kicks an electron out of P680, P680 ⁺ extracts an electron from water molecules bound at the Mn centre, which are transferred through the redox-agile TyrZ to reduce P680 ⁺ back to P680 for yet another photosynthetic cycle. In classic experiments using an oxygen electrode and short flashes of light, it was established that four photochemical turnovers were required for every molecule of oxygen that was released, and the features of this were rationalized into a kinetic model, known as the Southward-state cycle (Figure 16.7). In this model, v states of the enzyme, designated S _{due north} , are proposed to exist, with n 0–4, where each state corresponds to a unlike level of oxidation of the tetra-Mn centre (Kok et al., 1970). When S₄ is generated, it reacts in less than a microsecond to release dioxygen and render to the reduced grade of the enzyme, S₀. The stable country of the enzyme in the dark is S₁, which corresponds to Mn(III)₂Mn(IV)₂, so that only three photochemical turnovers are required earlier O₂ is released.

Figure 16.6. Arrangement of cofactors in the dimeric (D1 and D2) subunits of PSII. The dashed line running through the non-haem Iron is the ii-fold axis of pseudosymmetry. Thin lines prove the eye-to-eye distances in Å between cofactors.

(From Voet and Voet, 2004. Reproduced with permission from John Wiley & Sons., Inc.) Copyright © 2004

Figure 16.seven. The S-state cycle model of O₂ generation.

(From Voet and Voet, 2004. Reproduced with permission from John Wiley & Sons., Inc.) Copyright © 2004

The structure of this middle, which contains the Mn_ivCa cluster and the redox-active Tyr_Z rest, has been recently solved (Figure 16.8) at three Å resolution, and both confirms and confounds earlier structures—one of the major bug seems to be reduction of Mn^three+ and Mn⁴⁺ to Mn²⁺ by 10-ray generated radicals in the course of information collection from the crystals. However, what this near recent structure shows conspicuously is that the fourth Mn(4) caps a distorted cubane CaMn_threeO₄ cluster, as suggested from the before studies. Combining crystallographic refinement with EXAFS data, the metal metallic distances are: Mn1–Mn2 and Mn2–Mn3, 2.7 Å apart (probably connected by di-p-oxo bridges), Mn1–Mn3 and Mn3–Mn4, iii.3 Å autonomously (suggestive of mono-µ-oxo bridges), and Ca²⁺ forming the vertex of a trigonal pyramid, equidistant (∼3.4 Å) from Mn1, Mn2 and Mn3. Combining spectroscopic studies (notably FTIR) with the assumption that the Due south₁ country involves an oxidation distribution of Mn(III)₂Mn(IV)₂, the authors suggest some of the possible oxidation states of the four Mn ions in the class of the catalytic bicycle. It is proposed that Mn1 and Mn3 can either be in oxidation state III or IV in the S₁ land, that Mn4, which is non oxidized during the transitions from Southward₀ to Due south_three is present every bit Mn(IV), while Mn2 probably changes from Mn(Three) to Mn(IV) in the S_one–South₂ transition.

Figure xvi.viii. Schematic view of the Mn_ivCa cluster: amino acids of the kickoff coordination sphere are in blackness, those of the 2d in grayness: distances are in A.

(From Loll et al., 2005. Reproduced past permission of Nature Publishing Group.) Copyright © 2005

Conspicuously, the S₀ state must have one Mn(2), one Mn(3) and two Mn(Iv) while the S₄ must have three Mn(IV) and one Mn(Five)—but where they are located in the tetra-Mn centre and how their iv-electron reduction is coupled with water splitting and dioxygen evolution remains to be established. (For recent reviews see Goussias et al., 2002; Ferreira et al., 2004; Rutherford and Boussac, 2004; Iverson, 2006.)

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Water Oxidation Catalysts

Sheng Ye , ... Can Li , in Advances in Inorganic Chemistry, 2019

5.1.1 Photosynthetic oxygen-evolving eye

A photosynthetic oxygen-evolving center, namely, a large protein complex photosystem 2 (PSII) is found in the thylakoid membranes of photosynthetic organisms (i.east., algae, cyanobacteria, and higher plants); information technology is the first enzyme for water oxidation to evolve O ₂. ^13,14,48,49 PSII utilizes photons to drive the h2o oxidation reaction at an oxygen evolving complex (OEC), where the light-adsorbing, charge separation and transfer accept place efficiently.

In general, the electron transfer chain in PSII tin exist divided into two parts, an acceptor side and a donor side, ^48,49 equally shown in Fig. 2. Firstly, the chlorophyll pigment P680 upon obtaining the calorie-free energy is photoexcited into a radical cation, P680 ⁺, with an oxidizing potential of ~   1.25 Five_NHE, which is the highest known in biology. ^l On the acceptor side of PSII, the electrons are rapidly transferred from the excited state P680 ⁺ to a pheophytin molecule (Phe⁻), and and then to the side by side acceptor, plastoquinone Q_A, and further to the electron acceptor plastoquinone Q_B. Thus, the charge separated country is stabilized by the electron transfer chain. Interestingly, after undergoing two-electron and 2-proton reductions, the acceptor Q_B becomes a mobile electron carrier, hydroquinone B (H_iiQ_B) and diffuses away from PSII to go on the adjacent electron send. On the donor side of PSII, the cationic radical P680 ⁺ is reduced by a redox-agile tyrosine residue of the D1 subunit, Tyr_Z (D1−Tyr161) to occupy the hole in P680. After, a neutral tyrosine radical Tyr_Z is generated as an oxidant for oxidizing the CaMn₄O₅ cluster via proton-coupled electron transfer (PCET) reactions. ⁵¹ Finally, the OEC couples a series of successive one-electron reductions to four-electron water oxidation for the liberation of O₂ and H⁺. ⁴⁴

Fig. ii. Redox potential scheme of cofactors concomitant with the pathways of accuse separation, ship and recombination.

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Self-Assembly Processes at Interfaces

John Ricke , in Interface Science and Technology, 2018

vii.half dozen Bioinspired Energy Conversion

Accuse transport is not the exclusivity of metals and semiconductors: photosynthesis relies on long-range electron ship. Based on the structure of thylakoid membranes, a huge corporeality of research has been devoted to the self-assembly of electron donor–electron acceptor sparse films to design devices able to produce free energy from solar low-cal. More recently, natural biofilms accept been discovered, which are also able to send electrons over large distances. These biofilms are fabricated of leaner interconnected past conducting nanowires. The conductivity in those nanowires is ensured through electron hopping between redox proteins [110].

Inspired past those conductive biofilms, chimeric proteins fabricated from a self-assembling prion domain and an fe eye–containing rubredoxin protein grade spontaneous nanowires (in a pH-dependent manner) with an average diameter of 6   nm and an average length of 12   μm. Upon solvent casting on an amorphous carbon or indium tin can oxide electrode, those chimeric proteins class films up to 18   μm in thickness where the conductivity is of the order of a few μS   cm^−one [111].

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Ultrafast Spectroscopy and its Applications

M. Hayashi , ... Joseph L. Knee , in Encyclopedia of Physical Science and Engineering (3rd Edition), 2003

III.B.2 Energy Transfer in Antenna

The role of the antenna pigments is to collect light energy from the sun and transfer it to reaction centers. In dark-green plants and algae, these pigments are found in the thylakoid membranes of chloroplasts. In photosynthetic leaner, the pigments are establish in intracytoplasmic membranes or in special vesicles in the cell. The principle antenna pigments are chlorophyll a and b in plants, chlorophyll c in some algae, and bacteriochlorophyll a, b, or c in bacteria. Other pigments, called accessory pigments, are carotenoids and phycobiliproteins. Following light assimilation by a paint molecule, the electronic excitation is transferred until information technology is trapped by a reaction middle.

Cyanobacteria and cerise algae utilise antenna pigments called phycobilins packed into complexes called phycobilisomes, which are attached to the photosynthetic membranes. Phycobilisomes contain several hundred billion chromophores, linear tetrapyrroles fastened to the protein. These proteins are organized into disks that are themselves stacked into rods, with disks containing shorter wavelength pigments on 1 end, and longer wavelength pigments at the other terminate next to a primal core. Thus the shorter wavelength absorbers: phycoerythrins (PE), 570   nm; are on the outside, phycocyanins (PC), 630   nm; within them, allophycocyanins (APC), 650   nm; in the cadre, followed past chlorophyll a within the photosynthetic membrane.

The X-ray crystal structure of the trimeric aggregation state of APC isolated from the cyanobacterium Spirulina platensis is well-characterized: the APC trimer tin be described every bit a C_three-symmetric, ring-like homotrimer of α and β polypeptide monomers. The phycocyanobilin chromophore (PCB) is bundled as dimers formed beyond the polypeptide interfaces between next α, β polypeptide monomers.

The absorption spectra of the homogenous monomers for APC are most identical to that of the α-subunit, and it has an assimilation maximum at 614   nm while trimers accept a sharp maximum at 650   nm and a prominent shoulder at about 610–620   nm. The origin of electronic states involved in steady-land spectroscopy is needed for understanding of femtosecond time-resolved spectra, especially when one relates the mechanism of the kinetics appearing in the time-resolved spectra to nonradiative transitions among the electronic states. If electronic Hamiltonian is fully diagonalized, in other words, delocalized electronic wavefunction is used to describe the system, nonradiative transitions between the aforementioned spin multiplicity should exist due to the break-down of the adiabatic approximation, i.e., internal conversion (IC) process. In this case, to guess the electronic coupling constant, the Förster or Marcus blazon of rate constant cannot be used. Vibrational properties are also needed to estimate the electronic coupling constant. Vibrational frequencies involved in optical transition of APC trimer can be obtained by using a pump-probe technique with xx-fsec laser pulses. Effigy 5 shows femtosecond pump-probe signals of APC trimer at room temperature every bit a role of the probing wavenumber. Each point clearly shows oscillatory features called "quantum beats" accompanied by with ascent or decaying components. Insets of panel A exhibit oscillatory components extracted from the observed signals. Performing Fourier transform analysis to these oscillatory components provides possible vibrational frequencies involved in the optical transition. This information can be used to estimate the electronic coupling constant.

FIGURE 5. (a) Quantum beats observed in pump-probe signals of APC trimer at room temperature. Signals were obtained as a function of delay time between the pump and probe pulses as well every bit the probe wavelength. Insets are oscillatory components extracted from the observed signals. (b) Fourier transform analysis of the oscillatory components.

Femtosecond anisotropy measurements can provide useful information on the origin of rising and decay components actualization in the observed signals. Anisotropy can be obtained past monitoring the spectrally resolved probe transmission as a role of temporal delay and polarization relative to the pump. The anisotropy can be calculated from the pump-probe signals as

$\text{r}(\tau )=\frac{{\text{S}}_{\left|\right|}(\tau )-{\text{S}}_{\perp }(\tau )}{{\text{S}}_{\left|\right|}(\tau )-2{\text{S}}_{\perp }(\tau )},$

where S _∣(τ) and Southward _⊥(τ) are pump-probe transients obtained with parallel and perpendicular pump and probe polarizations, respectively. Figure half dozen shows obtained femtosecond anisotropy signals of APC trimer excited at 620   nm and probed at 660 and 620   nm. The obtained anisotropy indicates that there exist at least two electronic states between 660 and 620   nm. Fitting the anisotropy decay signal of probing at 660   nm to a single exponential decay role yields approximately 350   fsec. This issue implies that the electronically excited land via the 620   nm pump-pulse undergoes nonradiative transition to i of the lower electronic states within 350   fsec. The origin and nature of the electronic states of the APC trimer are notwithstanding nether extensive investigation.

Effigy half-dozen. Anisotropy calculated from pump-probe signals of APC trimer.

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Bioenergetics

Richard E. McCarty , Eric A. Johnson , in Encyclopedia of Physical Science and Technology (Third Edition), 2003

Two.C ATP Synthesis

ATP synthesis in chloroplasts is called photophosphorylation and is similar to oxidative phosphorylation in mitochondria. The low-cal-driven transport of electrons from water to NADP⁺ is coupled to the translocation of protons from the stroma across the thylakoid membrane (the green, energy-converting membrane) into the lumen. Electron send from Q ⁻ to P700⁺ is exergonic. Part of the energy released by electron send is conserved by the germination of an electrochemical proton gradient. The cytochrome b _vi f complex of chloroplasts functions not merely in electron transport, but also in proton translocation.

The active site of the oxygen-evolving enzyme is bundled then that the protons formed during water oxidation are released into the thylakoid lumen. These protons contribute to the electrochemical proton potential. The thylakoid membrane contains a protein that functions to transport Cl⁻ beyond the membrane. Proton accumulation in the thylakoid lumen is electrically balanced in large part by Cl⁻ uptake. As a outcome, thylakoids accumulate HCl and the membrane potential beyond the membrane is depression. The pH inside the lumen during steady-state photosynthesis is about 5.0.

One of the earliest experiments that supported the hypothesis that ATP synthesis and electron send were linked by the electrochemical proton potential was carried out with isolated thylakoid membranes. Thylakoid membranes were placed in a buffer at pH iv.0 and after a few seconds the pH was rapidly increased to viii.0, which resulted in the germination of a proton action gradient. This artificially formed gradient was shown to drive the synthesis of ATP from ADP and P_i. The experiments were carried out in the dark so that the possibility that electron send contributed to the ATP synthesis was excluded. Thus, a proton activity gradient was proven capable of driving ATP synthesis.

The thylakoid membrane enzyme that couples ATP synthesis to the menses of protons down their electrochemical gradient is called the chloroplast ATP synthase (see Fig. 10). This enzyme has remarkable similarities to ATP synthases in mitochondria and certain leaner. For instance, the β subunits of the chloroplast ATP synthase have 76% amino acid sequence identity with the β subunits of the ATP synthase of the bacterium E. coli.

The reaction catalyzed by ATP synthases is

(11) $\text{due north}{\text{H}}_{\text{a}}^{+}+\text{ADP}+{\text{P}}_{\text{i}}+{\text{H}}^{+}\to {\text{nH}}_{\text{b}}^{+}+\text{ATP}+{\text{H}}_{\mathrm{two}}\text{O},$

where n is the number of protons translocated per ATP synthesized, probably three or four, and a and b refer to the reverse sides of the coupling membrane. Provided the electrochemical proton potential is high, the reaction is poised in the direction of ATP synthesis. In principle, when the proton potential is depression, ATP synthases should hydrolyze ATP and cause the pumping of protons beyond the membrane in the management opposite that which occurs during ATP synthesis. ATP-dependent proton transport past the ATP synthase is of physiological significance in E. coli under anaerobic weather in that it generates the electrochemical proton potential across the plasma membrane of the bacterium. This potential is used for the active uptake of some carbohydrates and amino acids.

In contrast, ATP hydrolysis past the chloroplast ATP synthase in the dark has no physiological role and would be wasteful. In fact, the rate of ATP hydrolysis by the ATP synthase in thylakoids in the dark is less than one% of the rate of ATP synthesis in the lite. Remarkably, within 10–20   msec afterward the initiation of illumination, ATP synthesis reaches its steady-land charge per unit. Thus, the activity of the chloroplast ATP synthase is switched on in the light and off in the dark. In addition to being the driving force for ATP synthesis, the electrochemical proton potential is involved in switching the enzyme on. Structural perturbations of the enzyme induced past the proton potential overcome inhibitory interactions with bound ADP too equally with a polypeptide subunit of the synthase. An additional regulatory mechanism that is unique to the chloroplast ATP synthase is reductive activation. Reduction of a disulfide bond in a subunit of the chloroplast ATP synthase to a dithiol enhances the rate of ATP synthesis, especially at physiological values of the proton potential. The electrons for this reduction are derived from the chloroplast electron transport chain.

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Biophotovoltaic Systems Based on Photosynthetic Complexes

J. Kargul , ... G. Andryianau , in Encyclopedia of Interfacial Chemistry, 2018

Photosystem I equally the Model Component for Structure of the BPVDs

The PSI complex functions as the light-activated plastocyanin:ferredoxin oxidoreductase which acts in tandem with another lite-harvesting macromolecular machine, photosystem II (PSII) whose catalytic activity of photon-induced water splitting triggers proton-coupled vectorial electron transfer in the photosynthetic membranes (thylakoids) of oxygenic phototrophs (come across Fig. 2 ). 2 photons absorbed by PSII and PSI drive each h2o-derived electron through the system, providing sufficient energy to oxidize water and to reduce atmospheric CO_two at ambient temperature. Photocatalytic water oxidation conducted by PSII in conjunction with photoreductive chemistry of PSI produces not only the reducing equivalents used for production of biomass only also most all the atmospheric molecular dioxygen that sustains the aerobic life on our planet. ^v

Natural PSI complex is i of the most promising LH macromolecular components for BPVDs. This large pigment–protein complex is a highly efficient light energy converter plant in thylakoid membranes of blue-green alga, algae, and college plants. It operates with a quantum yield close to unity (for λ  <   680   nm) for generation of the primary charge separation land. The PSI complex is not but the light-harvesting system but too a charge separator, generating long-lived e⁻/hole pairs upon light activation. ^{10 , eleven} These features make PSI an splendid model system for improving efficiency of artificial solar-converting devices. To this end, PSI has been applied in a plethora of model systems to study highly efficient photochemical conversion into current and in some cases, likewise solar fuels such equally hydrogen. ^{11 , 12}

The X-ray structures of prokaryotic and eukaryotic PSI complexes were determined to two.5   Å and ii.8   Å resolution, respectively. ^13–15 They provide an insight not only into the detailed system of protein subunits and inbound pigments but likewise the precise organization of electron transfer pathways (see Fig. 3 ). In higher plants, the PSI complex consist of the cadre domain and asymmetrically located LH complex I (LHCI), forming a PSI-LHCI supercomplex. ^xiv–18 The cyanobacterial PSI core forms a 1068-kDa homotrimer, with each semispherical monomer comprising 12 poly peptide subunits encompassing 128 cofactors (96 chlorophylls (Chls), 2 phylloquinones, iii [4Fe   −   4S] clusters, 22 carotenoids, iv lipids, and a Ca^{2   +} ion). ¹³

Fig. 3. X-ray construction of the PSI complex and its association with the mobile electron transfer cofactors. Left: The membrane view of the two.8   Å X-ray structure of Pisum sativum PSI-LHCI supercomplex shown from the LHCI side; (PDB: 4XK8; reproduced from Qin, X.; Suga, M.; Kuang, T.; Shen, J.-R. Structural Footing for Energy Transfer Pathways in the Plant PSI-LHCI Supercomplex. Scientific discipline 2015, 348, 989–995. doi:x.1126/science.aab0214). Colour coding: PSI core subunits: pinkish, PsaA; gray, PsaB; light blue, PsaC; khaki, PsaD; light green, PsaE; orange, PsaF; bluish, PsaG and PsaK; cerise, PsaH; purple, PsaI and PsaJ; green, PsaL. Lhca proteins: green, Lhca1; cyan, Lhca2; magenta, Lhca3; yellow, Lhca4. Cofactors: dark-green, Chls a of PSI core complex; yellow, Chls a of LHCI; magenta, Chls b of LHCI; blue, carotenoids; black, lipids; cerise, cofactors of the electron transfer chain (Chls a, phylloquinones, and [4Fe-4S] clusters). Annotation that 2 of the core subunits (PsaO and PsaN) were not resolved in this construction. Middle: Arrangement of the electron transfer chain obtained from a two.5   Å X-ray structure of cyanobacterial PSI reaction heart (PDB: 1JB0; reproduced from Jordan, P.; Fromme, P.; Witt, H.T.; Klukas, O.; Saenger, W.; Krauss, N. Three-Dimensional Structure of Cyanobacterial Photosystem I at 2.5 A Resolution. Nature. 2001, 411, 909–917. doi:ten.1038/35082000). Photoinduced electron transfer pathways are indicated with light and nighttime blue arrows. Right: Electrostatic and hydrophobic interaction of PSI with the mobile redox cofactors (cytochrome c ₆ (cyt c ₆) or plastocyanin (PC) on the donor side and ferredoxin on the acceptor side). Subunits PsaA and PsaB of the reaction eye heterodimer are indicated with yellow and blueish, respectively. For clarity, only ii of 11-transmembrane helices of the PsaA/PsaB heterodimer are displayed. Shown are Trp658 of PsaA and Trp625 of PsaB (both in blackness) forming the lumenal hydrophobic patch for interaction with the mobile cofactors on the donor side. All the interacting amino acid residues are indicated, as is the P700 reaction center and atomic number 26–sulfur clusters of the electron transfer chain. Positively and negatively charged surface patches involved in poly peptide–protein interaction are indicated with a blueish and carmine mesh, respectively.

Reproduced from Kargul, J.; Janna Olmos, J.D.; Krupnik, T. Structure and Role of Photosystem I and Its Application in Biomimetic Solar-to-Fuel Systems. J. Establish Physiol. 2012, 169, 1639–1653. doi:10.1016/j.jplph.2012.05.018.

The higher institute PSI-LHCI supercomplex exists as a monomer of 600   kDa and contains iv unique to plants subunits (PsaG, PsaH, PsaN, and PsaO) in the core domain over and higher up the cyanobacterial counterparts, as well equally and four Lhca subunits (Lhca1 to Lhca4) in the LHCI complex which assemble every bit heterodimers. This big circuitous comprises a total of 18 protein subunits (14 cadre subunits and 4 Lhcas) and 205 cofactors (155 Chls, 35 carotenoids, 2 phylloquinones, 3 [4Fe   −   4S] clusters, 10 lipids, and several water molecules). ^xiv

At the heart of PSI is the special pair of Chl a molecules, termed P700 reaction centre, which forms the master electron donor and is located on the donor (oxidizing) side of the complex, close to thylakoid lumen. Photogenerated electrons are transferred via highly conserved and precisely positioned redox cofactors which are organized forth ii pseudosymmetrical branches that diverge at P700 and connect at the F₀ atomic number 26–sulfur cluster located at the interface of the PsaA/PsaB heterodimer, close to the stromal side of the complex (run into Fig. 3 ). The terminal electron acceptor is formed by the F_B iron–sulfur cluster coordinated by Cys residues of the extrinsic PsaC subunit located on the acceptor (reducing) side of the complex. Exposed to visible lite, P700 reaction center is exited to P700*excited state: P700   +   hν  →   P700* (E _M  =   + 0.43   eV   → E ₁₀₀₀  =−   1.3   eV). The highly reducing P700* rapidly oxidizes to P700⁺ supplying costless electron from the primary charge separation state P700⁺A₀, with the quantum yield for this reaction close to unity. The electron is then transferred along the conserved chain of redox cofactors down the energy gradient to the F_B cluster (Due east _M−   0.58   eV) that terminates the secondary electron transfer pathway ¹⁹ (see Fig. 3 , eye). The accuse separated state P700⁺F_B ⁻ is generated with an internal quantum efficiency (IQE) of   ∼   60% and has a lifetime of 65   ms. The redox potential of the F_B ⁻ cluster is 0.166   mV more reducing than the potential for the H⁺/H_two half-prison cell at pH 7.0. Thus, the reaction 2H⁺ + 2e⁻ →   H_ii is thermodynamically favorable to exist driven to completion. ^{19 , 20}

Subsequently the photoinduced accuse separation, P700⁺ reaction center is rereduced past a mobile low-molecular-weight electron transfer mediators, cytochrome c ₆, or plastocyanin, whilst terminal F_B ⁻ cluster is oxidized with another mobile electron transport mediator, ferredoxin. ¹¹ Through 3.five billion years of evolution the electron transfer and intermolecular interaction between PSI, and these freely diffusing electron ship cofactors take been optimized (reviewed in Ref.eleven). Therefore, awarding of these natural electron transport mediators every bit the biological conductive interfaces tin potentially lead to the well-defined PSI architectures within the biophotoelectrodes, and as such can be utilized for the improvement of direct electron transfer within the BPVDs.

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Algal Biocathodes

C. Nagendranatha Reddy , ... Booki Min , in Microbial Electrochemical Applied science, 2019

3.7.two.ii.i Photosynthesis Mechanism

Algae human action equally the efficient sink for CO_ii past conveying out oxygenic photosynthesis and play a significant role in global carbon and nitrogen cycles every bit primary producers. Photosynthesis is the process of CO₂ sequestration using water and low-cal energy to generate a variety of products viz., fuels, polyunsaturated fatty acids, nutraceuticals, chemicals pigments, dyes etc. The algal metabolic activity tin uptake inorganic carbon in the presence of carbonic anhydrase. From the puddle of bicarbonate entering the plastid of the cell, carbonic anhydrase provides CO₂ to ribulose one,v bisphosphate carboxylase-oxygenase (RuBisCO) [5] . The light-harvesting centre and chlorophyll harness the energy from electron transport chain on the thylakoid membranes of chloroplasts and shop it in the grade of nicotinamide adenine dinucleotide phosphate hydrogen (NADPH) and adenosine triphosphate (ATP) to be further consumed for carbohydrates synthesis in the light-contained phase. The two reaction centers, viz., photosystem II (PSII) and photosystem I (PSI), on thylakoid are specialized to absorb the light of wavelength 680 and 700  nm, respectively. They are connected to several electron carriers such as plastocyanin, cytochrome b6f, plastoquinone etc. that are bundled in the order of increasing redox potential to permit the flow of electron from negative to positive redox potential. With the lite absorbed at 680   nm, the h2o is split into H⁺, east⁻, and O₂ molecules at oxygen evolution complex in PSII (Eq. 3.7.three). The O₂ formed will escape, while H⁺ accumulates in the lumen of the thylakoid membrane thereby creating proton gradient to form ATP. The excited electrons at PSII travel to PSI and information technology further gets excited to reduce ferredoxin (Fd) [101]. Reduced Fd transfers its electrons to the ferredoxin nicotinamide adenine dinucleotide phosphate (NADP) reductase, which catalyzes the formation of NADPH using NADP⁺ and H⁺ coming from the lumen. The production of 2   mol of NAD(P)H requires eight photons and two   mol of water equally shown in Eq. 3.7.4 [102]. The overall photosynthetic reaction of CO₂ fixation to generate O₂ molecules is shown in Eq. three.7.v.

(3.7.3) $2{\text{H}}_2\text{O}\to \mathrm{four}{\text{H}}^{+}+4{\text{e}}^{-}+{\text{O}}_2$

(3.7.4) $\mathrm{ii}{\text{H}}_2\text{O}+2\text{NADP}++8\text{Photons}\to {\text{O}}_{\mathrm{ii}}+2\text{NAD}\left(\text{P}\right)\text{H}+\mathrm{two}{\text{H}}^{+}$

(3.7.5) $6{\text{CO}}_2+6{\text{H}}_2\text{O}\left(\text{in}\text{presence}\text{of}\text{lite}\text{and}\text{chlorophyll}\right)\to {\text{C}}_6{\text{H}}_{12}{\text{O}}_6+6{\text{O}}_2$

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