Photooxidation and triplet formation of the primary electron donor of the green photosynthetic bacterium Prosthecochloris aestuarii observed with ESR spectroscopy

Biological and Soft Matter Physic

Evidence for photoreduction of monomeric bacteriochlorophyll a as an electron acceptor in the reaction center of the green photosynthetic bacterium Prosthecochloris aestuarii

Biological and Soft Matter Physic

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