Optically-detected magnetic field effects on the Dl-D2-cyt b-559 complex of Photosystem II. Temperature dependence of kinetics and structure

Abstract

The magnetic field effect (MFE) on photosynthetic reactions in the Dl-D2 reaction centre complex was studied using a new field modulation technique, and monitoring the absorbance from 1.2 to 200 K. The MFE recorded at low temperature (below 60 K) showed several features that cannot be due to the intrinsic properties of the primary donor triplet only. The temperature dependence of the amplitude of the MFE was simulated using a thermally activated charge-recombination rate from the radical-pair singlet state to the ground state and thermally activated charge recombination to the excited primary donor state. Due to repeated re-population of the excited state at high temperatures, triplet formation by intersystem crossing becomes important. The simulation yields upper limits for the activation energies of the two recombination processes of 26 meV and 70 meV for recombination to the singlet ground and excited state, respectively. From a Gaussian deconvolution of magnetic field-induced Triplet-minus-Singlet (T − S) spectra recorded at different temperatures in the range of 1.2 to 200 K, it is concluded that the conformation of one of the main components of the T − S spectra is transformed between 50 K and 70 K. Furthermore, going on to higher temperatures, an additional bleaching appears centered at 674.5 nm. The additional component could be due to delocalisation of the primary donor triplet. In the T − S spectra recorded above 50 K, also a bleaching at 545 nm was observed. The formation of the triplet giving rise to this bleaching (probably a pheophytin triplet) is related to triplet formation by the radical pair mechanism.