Triplet states in photosystem I complexes from Synechococcus elongatus

Abstract

Using Laser-flash induced transient absorption spectroscopy the dynamics of chlorophyll a (Chl) and carotenoid triplet states have been investigated in PS I complexes from Synechococcus elongatus. In the antenna, Chl triplet states arise from the lowest excited singlet state by intersystem crossing. They are efficiently quenched by triplet energy transfer to the carotenoids (Car). The triplet-triplet energy transfer from Chl to Car occurs with half-lives of » 10 ns (» 60%) and » 85 ns (» 40%). The kinetics and the efficiency of the triplet transfer have been analysed as a function of temperature. The Car triplet state decays with half-lives of » 3 µs at room temperature and » 7 µs at 80 K. In addition, triplet-minus-singlet absorption difference spectra of the carotenoids have been measured at 295 K and 78 K. In photosystem I, the maximum triplet absorption for the ß-carotenes is observed at 528 nm. The triplet state of the primary electron donor, P700, is formed by charge recombination of the radical pair P700+A0-, when the electron transfer to A1 is blocked. The decay kinetics and the yield of 3P700 have been studied as a function of temperature. The decay rate increases with increasing temperature indicating an activation energy of about 40 meV. The results will be discussed in terms of the recent 2.5 Å X-ray structure of photosystem I