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]
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, excitedbylight energy as if they were a single molecule.[2]
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.
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+.
P680+ is the strongest biological oxidizing agent known, with an estimated redox potential of ~1.3 V.[3] 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.
^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.
