Electron acceptors of photosynthetic bacterial reaction centers. Direct observation of oscillatory behaviour suggesting two closely equivalent ubiquinones.

When reaction centers are illuminated by a series of single turnover flashes ubisemiquinone is formed and destroyed on alternate flashes. This oscillatory behaviour can be observed with both optical and electron spin resonance techniques. The oscillations are dependent upon the presence of excess ubiquinone in a manner which suggests that two molecules may act almost equivalently as metastable primary acceptors forming a two-electron gate between the one-electron primary photoact and a two-electron secondary acceptor pool.