P680
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+•. Components. The structure of P680 consists of a heterodimer of two distinct chlorophyll molecules, referred to as P and P. This “special pair” forms an excitonic dimer that functions as a single unit, excited by light energy as  if they were a single molecule. 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 and the strongest biological oxidizing agent known, with a redox potential of ~1.3 V. Its energy is used to split HO into protons, electrons, and oxygen, producing high-energy O in the oxygen-evolving complex. In this process, P680 recovers its lost electron, which regenerates P680.