The photosynthetic reaction centre from the purple bacterium Rhodopseudomonas viridis

Electron transfer through the complex: The RC is composed of four subunits Light energy is used to excite the 'special pair' to its lowest excited singlet state and within 3 ps the electron is transferred to the photoactive Bpheo molecule possibly after reduction of the accessary Bchl molecule on the active branch. The tyrosine residue at M208 is important for optimal primary electron transfer. The reduced Bpheo molecule reduces QA within 200 ps possibly aided by superexchange mechanism involving M-W250 . Reduction of QB by QA- occurs in xx. QA is a single electron carrier whereas after two turnovers of the RC QB is doubly reduced and becomes protonated to form the neutral quinol which leaves the RC to be replaced by a fresh quinone molecule. Protonation of reduced QB is thought to occur by a proton relay system involving amino acid side chains such as L-S223 .