scholarly journals What is β–carotene doing in the photosystem II reaction centre?

2002 ◽  
Vol 357 (1426) ◽  
pp. 1431-1440 ◽  
Author(s):  
Alison Telfer
Keyword(s):  
Electron Transfer ◽  
Photosystem Ii ◽  
Excitation Energy ◽  
Triplet State ◽  
Singlet Oxygen ◽  
Spin Exchange ◽  
Primary Electron ◽  
Triplet States ◽  
Reaction Centre ◽  
Β Carotene

During photosynthesis carotenoids normally serve as antenna pigments, transferring singlet excitation energy to chlorophyll, and preventing singlet oxygen production from chlorophyll triplet states, by rapid spin exchange and decay of the carotenoid triplet to the ground state. The presence of two β–carotene molecules in the photosystem II reaction centre (RC) now seems well established, but they do not quench the triplet state of the primary electron–donor chlorophylls, which are known as P 680 . The β–carotenes cannot be close enough to P 680 for triplet quenching because that would also allow extremely fast electron transfer from β–carotene to P + 680 , preventing the oxidation of water. Their transfer of excitation energy to chlorophyll, though not very efficient, indicates close proximity to the chlorophylls ligated by histidine 118 towards the periphery of the two main RC polypeptides. The primary function of the β–carotenes is probably the quenching of singlet oxygen produced after charge recombination to the triplet state of P 680 . Only when electron donation from water is disturbed does β–carotene become oxidized. One β–carotene can mediate cyclic electron transfer via cytochrome b 559. The other is probably destroyed upon oxidation, which might trigger a breakdown of the polypeptide that binds the cofactors that carry out charge separation.

Molecular Basis of the Vulnerability of Photosystem II to Damage by Light

1995 ◽  
Vol 22 (2) ◽  
pp. 201 ◽  
Author(s):  
J Barber
Keyword(s):  
Triplet State ◽  
Singlet Oxygen ◽  
D1 Protein ◽  
Primary Electron ◽  
In Vivo Studies ◽  
Cytochrome B559 ◽  
Acceptor Side ◽  
Donor And Acceptor ◽  
Primary Cleavage

Using isolated reaction centres and cores of photosystem I1 (PSII) it has been possible to elucidate the details of two separate pathways which lead to photoinhibition. The acceptor side pathway involves charge recombination resulting in the formation of the triplet state of the primary electron donor, P680. This triplet state is harmless in the absence of oxygen but in its presence gives rise to highly reactive singlet oxygen. We have shown that this singlet oxygen specifically attacks the chlorophyll of P680 itself. This process, plus other possibilities, gives rise to degradation of Dl protein involving a primary cleavage in the stromal loop joining putative transmembrane regions four and five, to yield 23 kDa N-terminal and 10 kDa C-terminal fragments. In contrast a donor side pathway is oxygen independent and is due to detrimental secondary oxidations brought about by P680+. Oxidation of accessory chlorophyll (C670) and β-carotene are observed and D1 protein is degraded by a primary cleavage in the lumenal loop between the putative transmembrane segments one and two to yield 24 kDa C-terminal and 9 kDa N-terminal fragments. In vivo studies indicate that the acceptor pathway is more common. The reason for the inherent vulnerability of PSII to photoinduced damage is discussed in terms of the special nature of P68O and the implications of the role of cytochrome b559 as a versatile protectant against donor and acceptor side photoinactivation is also considered. The likely dimeric organisation of PSII in vivo adds an additional factor to the general discussion of the molecular processes which underlie the vulnerability of PSII to photoinduced damage.

scholarly journals Spin-Polarized Oxygen Evolution Reaction Under Magnetic Field

2021 ◽  
Author(s):  
Xiao Ren ◽  
Tianze Wu ◽  
Yuanmiao Sun ◽  
Yan Li ◽  
Guoyu Xian ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Electron Transfer ◽  
Triplet State ◽  
Oxygen Evolution ◽  
Oxygen Evolution Reaction ◽  
Spin Exchange ◽  
Momentum Conservation ◽  
Exclusion Principle ◽  
Fast Kinetics ◽  
Spin Polarized

<p><a></a><a>The oxygen evolution reaction (OER) is the bottleneck that limits the energy efficiency of water-splitting. The process involves four electrons’ transfer and the generation of triplet state O<sub>2</sub> from singlet state species (OH<sup>- </sup>or H<sub>2</sub>O). Recently, explicit spin selection was described as a possible way to promote OER in alkaline conditions, but the specific spin-polarized kinetics remains unclear. </a><a></a><a>Here, we report that </a><a>by using ferromagnetic ordered catalysts as the spin polarizer for spin selection under </a><a></a><a>a constant magnetic field</a>, <a>the OER can be enhanced.</a> However, it does not applicable to non-ferromagnetic catalysts. We found that the spin <a>polarization occurs at the first electron transfer step in OER</a>, where <a></a><a>coherent spin exchange happens </a>between the <a></a><a>ferromagnetic</a> catalyst and the adsorbed oxygen species <a>with fast kinetics</a>, under the principle of spin angular momentum conservation. In the next three electron transfer steps, as the adsorbed O species adopt fixed spin direction, the OER electrons need to follow the Hund rule and Pauling exclusion principle, thus to carry out spin polarization spontaneously and finally lead to the generation of triplet state O<sub>2</sub>. Here, we showcase spin-polarized kinetics of oxygen evolution reaction, which gives references in the understanding and design of spin-dependent catalysts.</p>

scholarly journals An examination of how structural changes can affect the rate of electron transfer in a mutated bacterial photoreaction centre

2000 ◽  
Vol 351 (3) ◽  
pp. 567-578 ◽  
Author(s):  
Justin P. RIDGE ◽  
Paul K. FYFE ◽  
Katherine E. McAULEY ◽  
Marion E. VAN BREDERODE ◽  
Bruno ROBERT ◽  
...  
Keyword(s):  
Electron Transfer ◽  
Structural Changes ◽  
Structural Model ◽  
Transient Absorption ◽  
Photosynthetic Bacterium ◽  
Primary Electron ◽  
Interface Region ◽  
Reaction Centre ◽  
Primary Donor ◽  
Out Of Plane

A series of reaction centres bearing mutations at the (Phe) M197 position were constructed in the photosynthetic bacterium Rhodobacter sphaeroides. This residue is adjacent to the pair of bacteriochlorophyll molecules (PL and PM) that is the primary donor of electrons (P) in photosynthetic light-energy transduction. All of the mutations affected the optical and electrochemical properties of the P bacteriochlorophylls. A mutant reaction centre with the change Phe M197 to Arg (FM197R) was crystallized, and a structural model constructed at 2.3 Å (1Å = 0.1nm) resolution. The mutation resulted in a change in the structure of the protein at the interface region between the P bacteriochlorophylls and the monomeric bacteriochlorophyll that is the first electron acceptor (BL). The new Arg residue at the M197 position undergoes a significant reorientation, creating a cavity at the interface region between P and BL. The acetyl carbonyl substituent group of the PM bacteriochlorophyll undergoes an out-of-plane rotation, which decreases the edge-to-edge distance between the macrocycles of PM and BL. In addition, two new buried water molecules partially filled the cavity that is created by the reorientation of the Arg residue. These waters are in a suitable position to connect the macrocycles of P and BL via three hydrogen bonds. Transient absorption measurements show that, despite an inferred decrease in the driving force for primary electron transfer in the FM197R reaction centre, there is little effect on the overall rate of the primary reaction in the bulk of the reaction-centre population. Examination of the X-ray crystal structure reveals a number of small changes in the structure of the reaction centre in the interface region between the P and BL bacteriochlorophylls that could account for this faster-than-predicted rate of primary electron transfer.

scholarly journals Spin-polarized oxygen evolution reaction under magnetic field

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Xiao Ren ◽  
Tianze Wu ◽  
Yuanmiao Sun ◽  
Yan Li ◽  
Guoyu Xian ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Electron Transfer ◽  
Triplet State ◽  
Oxygen Evolution ◽  
Spin Polarization ◽  
Oxygen Evolution Reaction ◽  
Spin Exchange ◽  
Exclusion Principle ◽  
Fast Kinetics ◽  
Spin Polarized

AbstractThe oxygen evolution reaction (OER) is the bottleneck that limits the energy efficiency of water-splitting. The process involves four electrons’ transfer and the generation of triplet state O2 from singlet state species (OH- or H2O). Recently, explicit spin selection was described as a possible way to promote OER in alkaline conditions, but the specific spin-polarized kinetics remains unclear. Here, we report that by using ferromagnetic ordered catalysts as the spin polarizer for spin selection under a constant magnetic field, the OER can be enhanced. However, it does not applicable to non-ferromagnetic catalysts. We found that the spin polarization occurs at the first electron transfer step in OER, where coherent spin exchange happens between the ferromagnetic catalyst and the adsorbed oxygen species with fast kinetics, under the principle of spin angular momentum conservation. In the next three electron transfer steps, as the adsorbed O species adopt fixed spin direction, the OER electrons need to follow the Hund rule and Pauling exclusion principle, thus to carry out spin polarization spontaneously and finally lead to the generation of triplet state O2. Here, we showcase spin-polarized kinetics of oxygen evolution reaction, which gives references in the understanding and design of spin-dependent catalysts.

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