P680
P680, or Photosystem II primary donor, refers to either of the two special chlorophyll dimers, PD1 or PD2. 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. The primary donor receives excitation energy either by absorbing a photon of suitable frequency 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 is subsequently reduced by an electron originating from water.
Oxidized P680 is the strongest biological oxidizing agent known. It has an estimated redox potential of ~1.3 V. This makes it possible to oxidize water during oxygenic photosynthesis.
