Time-resolved electron paramagnetic resonance study of the higher plant photosystem I core complex

Electron paramagnetic resonance (EPR) spectroscopy, employing light modulation (LM) in the submillisecond range together with laser excitation-diode detection (DD) in the submicrosecond range has been carried out on the core complex of photosystem I (CC I) purified from Lemna gibba. The photophysical and photochemical processes that occur in CC I have been investigated as a function of the acceptor's redox state and of temperature, with use of different modes of EPR detection. Photoexcitation of the system in which all the acceptors are in their normal redox state P₇₀₀A₀A₁A₂A₃A₄ gives rise to triplet spectra mainly typical of Chla^T(triplet state chlorophyll a) in the internal antenna. When some of the acceptors are reduced (P₇₀₀A₀A₁[A₂A₃A ₄]^•-), the observed triplet species, by employing the LM method, are those of the reaction center P₇₀₀ and β-carotene. The latter is sensitized by Chla^T of the internal antenna. With the DD method, the triplet state of Chla is also observed and is accompanied by a decrease of the triplet yield of the other species. These differences in triplet yield are discussed in terms of triplet-triplet annihilation (Chla^T + β-carotene^T) and the cycling times of the partially reduced and nonreduced systems with respect to the two EPR methods employed.