P680

P680 is the primary electron donor of photosystem II (PSII), a specialized pair of chlorophyll a molecules located in the PSII reaction center in cyanobacteria, algae, and plants. The name derives from its absorption maximum near 680 nm. When PSII absorbs a photon, P680 forms an excited state (P680*), which rapidly transfers an electron to the first electron acceptor, pheophytin (Pheo), yielding P680+ and Pheo−. The electron then travels through a series of acceptors, notably QA and QB plastoquinones, and into the plastoquinone pool, continuing down the photosynthetic electron transport chain via the cytochrome b6f complex and plastocyanin to photosystem I.

The oxidized P680+ is a very strong oxidant and acts as the primary driver for water oxidation. It extracts electrons from water bound at the oxygen-evolving complex Mn4CaO5, initiating the splitting of water and release of molecular oxygen. The energy stored in the charge-separated state is thus converted into a chemical gradient across the thylakoid membrane, ultimately leading to ATP and NADPH synthesis.

P680 is a component of the PSII reaction center with its properties defined by the protein-pigment complex. The term P680 can refer to the neutral excited state P680*, the reduced donor P680, or the oxidized form P680+ depending on context.