Evidence for a new early acceptor in Photosystem I of plants. An ESR investigation of reaction center triplet yield and of the reduced intermediary acceptors

The yield of the triplet state of the primary electron donor of Photosystem I of photosynthesis (PT-700) and the characteristic parameters (g value, line shape, saturation behavior) of the ESR signal of the photoaccumulated intermediary acceptor A have been measured for two types of Photosystem I subchloroplast particles: Triton particles (TSF 1, about 100 chlorophyll molecules per P-700) that contain the iron-sulfur acceptors FX, FB and FA, and lithium dodecyl sulfate (LDS) particles (about 40 chlorophyll molecules per P-700) that lack these iron-sulfur acceptors. The results are: (i) In Triton particles the yield of PT-700 upon illumination is independent of the redox state of A and of FX,B,A and is maximally about 5% of the active reaction centers at 5 K. The molecular sublevel decay rates are kx = 1100 s−1 ± 10%, ky = 1300 s−1 ± 10% and kz = 83 s−1 ± 20%. In LDS particles the triplet yield decreases linearly with concentration of reduced intermediary acceptors, the maximal yield being about 4% at 5 K assuming full P-700 activity. (ii) In Triton particles the acceptor complex A consists of two acceptors A0 and A1, with A0 preceding A1. In LDS particles at temperatures below −30°C only A0 is photoactive. (iii) The spin-polarized ESR signal found in the time-resolved ESR experiments with Triton particles is attributed to a polarized P-700-A−1 spectrum. The decay kinetics are complex and are influenced by transient nutation effects, even at low microwave power. It is concluded that the lifetime at 5 K of P-700A0A−1 must exceed 5 ms. We conclude that PT-700 originates from charge recombination of P-700A−0, and that in Triton particles A0 and A1 are both photoaccumulated upon cooling at low redox potential in the light. Since the state P-700AF−X does not give rise to triplet formation the 5% triplet yield in Triton particles is probably due to centers with damaged electron transport.Biological and Soft Matter Physic