P680

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P680, or photosystem II primary donor, is the reaction-center chlorophyll a molecular dimer associated with Photosystem II in plants, algae, and cyanobacteria, and central to oxygenic photosynthesis.

Etymology[]

Its name is derived from the word “pigment” (P) and the presence of a major bleaching band centered around 680-685 nm in the flash-induced absorbance difference spectra of P680/ P680+•.[1]

Components[]

The structure of P680 consists of a heterodimer of two distinct chlorophyll molecules, referred to as PD1 and PD2. This “special pair” forms an excitonic dimer that functions as a single unit, excited by light energy as if they were a single molecule.[2]

Action and function[]

Excitation[]

P680 receives excitation energy either by directly absorbing a photon of suitable frequency or indirectly from other chlorophylls within Photosystem II, thereby exciting an electron to a higher energy level. The resulting P680 with a loosened electron is designated as P680*, which is a strong reducing agent.

Charge separation[]

Following excitation, the loosened electron of P680* is taken up by the primary electron acceptor, a pheophytin molecule located within Photosystem II near P680. During this transfer, P680* is ionized and oxidized, producing cationic P680+.

Recovery of P680[]

P680+ is a high-energy species[3] and the strongest biological oxidizing agent known, with a redox potential of ~1.3 V.[4] Its energy is used to split H2O into protons, electrons, and oxygen, producing high-energy O2 in the oxygen-evolving complex. In this process, P680+ recovers its lost electron, which regenerates P680.

See also[]

References[]

  1. ^ Shigeru Itoh, S; Iwaki, M; Tomo, T; Satoh, K (1996). Dibromothymoquinone (DBMIB) replaces the function of QA at 77 K in the isolated photosystem II reaction center (Dl-D2-cytochrome 6559) complex: Difference spectrum of the P680+ (DBMIB") state. Plant Cell Physiol. 37(6): 833-839.
  2. ^ Raszewski et al. (2008), pp. 105–119.
  3. ^ Schmidt-Rohr, K. (2021), "O2 and Other High-Energy Molecules in Photosynthesis: Why Plants Need Two Photosystems". Life 11, 1191. https://doi.org/10.3390/life11111191.
  4. ^ Rappaport et al. (2002), pp. 8518–8527.

Bibliography[]

  • Raszewski, Grzegorz; Diner, Bruce A.; Schlodder, Eberhard & Renger, Thomas (2008). "Spectroscopic properties of reaction center pigments in photosystem II core complexes: Revision of the multimer model". Biophys. J. 95 (1): 105–119. Bibcode:2008BpJ....95..105R. doi:10.1529/biophysj.107.123935. PMC 2426664. PMID 18339736.
  • Rappaport, F; Guergova-Kuras, M; Nixon, PJ; Diner, BA; Lavergne, J (2002). "Kinetics and pathways of charge recombination in photosystem II" (PDF). Biochemistry. 41 (26): 8518–8527. doi:10.1021/bi025725p. PMID 12081503.
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