P680

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+•.

The structure of P680 consists of a heterodimer of two distinct chlorophyll molecules, referred to as P_D1 and P_D2. This “special pair” forms an excitonic dimer that functions as a single unit, excited by light energy as if they were a single molecule.

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 the strongest biological oxidizing agent known, with an estimated redox potential of ~1.3 V. This makes it possible to oxidize water during oxygenic photosynthesis. P680⁺ recovers its lost electron by oxidizing water via the oxygen-evolving complex, which regenerates P680.