OPTICALLY-DETECTED MAGNETIC-FIELD EFFECTS ON THE D1-D2-CYT-B-559 COMPLEX OF PHOTOSYSTEM-II - TEMPERATURE-DEPENDENCE OF KINETICS AND STRUCTURE

The magnetic field effect (MFE) on photosynthetic reactions in the D1- D2 reaction centre complex was studied using a new field modulation te chnique, and monitoring the absorbance from 1.2 to 200 K. The MFE reco rded at low temperature (below 60 K) showed several features that cann ot be due to the intrinsic properties of the primary donor triplet onl y. The temperature dependence of the amplitude of the MFE was simulate d using a thermally activated charge-recombination rate from the radic al-pair singlet state to the ground state and thermally activated char ge recombination to the excited primary donor state. Due to repeated r e-population of the excited state at high temperatures, triplet format ion by intersystem crossing becomes important. The simulation yields u pper limits for the activation energies of the two recombination proce sses 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 diffe rent temperatures in the range of 1.2 to 200 K, it is concluded that t he 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 tem peratures, an additional bleaching appears centered at 674.5 nm. The a dditional component could be due to delocalisation of the primary dono r 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 th is bleaching (probably a pheophytin triplet) is related to triplet for mation by the radical pair mechanism.