P680, or Photosystem II primary donor, (where P stands for pigment) refers to either of the two special chlorophyll dimers (also named special pairs), PD1 or PD2.[1] These 2 special pairs form an excitonic dimer, which means that they behave in function as a single entity; i.e., they are excited as if they were a single molecule. The 680 number is its absorption maximum in the red part of the visible spectrum (680 nm). The primary donor receives excitation energy either by absorbing a photon of suitable frequency (colour) or by excitation energy transfer from other chlorophylls within photosystem II. During excitation, an electron is excited to a higher energy level. This electron is subsequently captured by the primary electron acceptor, a pheophytin molecule located within photosystem II near P680. The oxidized P680 (P680+) is subsequently reduced by an electron originating from water (via Oxygen evolving complex).

Oxidized P680 (P680+) is the strongest biological oxidizing agent known. It has an estimated redox potential of ~1.3 V.[2] This makes it possible to oxidize water during oxygenic photosynthesis.


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

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