ИСТИНА

The forward and backward electron transfer (ET) kinetics were measured in the intact P700-FA/FB Photosystem I (PS I) complexes, P700-FX cores, lacking terminal FA/FB iron-sulfur clusters and P700-A1 cores, lacking FX, FA and FB iron-sulfur clusters, both in liquid and in trehalose glass at 11% humidity. By comparing kinetic events in increasingly simplified versions of PS I at 480 nm, a wavelength that reports primarily oxidation of phylloquinone molecules A1A–/A1B– in the A and B symmetrical branches of cofactors, and at 830 nm, a wavelength that reports the chlorophyll dimer P700+ reduction, assignments could be made for nearly all of the resolved kinetic phases. The recombination between the primary chlorophyll acceptor A0– and P700+ occurs in ~40% of trehalose-embedded P700–FA/FB complexes, P700–FX cores, and P700–A1 cores, indicating inefficiency in forward ET beyond A0 in a fraction of PS I. The forward ET from A1A– to FX in trehalose-embedded PS I is slowed from 200 ns to 13 μs. The ~10 µs and ~150 µs components were assigned to recombination between A1B– and P700+ and between A1A– and P700+, respectively. The ~1.5 ms recombination between FX– and P700+ in trehalose-embedded PS I becomes heterogeneous and splits into lifetimes of ~550 µs/4.5 ms, whereas the ~100 ms recombination reaction between [FA/FB]– and P700+ becomes heterogeneously distributed between 39 ms and ~2 s. The kinetics and amplitudes of all ET reactions can be well-fitted by a kinetic model that allowed calculation of the asymmetrical contribution of the A1A– and A1B– quinones to the electrochromic signal at 480 nm. The relative contributions of trehalose-embedded PS I with partially arrested forward ET were determined to be 0.53, 0.16, 0.22 and 0.10 for P700–A0 cores, P700–A1 cores, P700–FX-cores, and P700–FA/FB complexes, respectively. The amplitude of the signal at 480 nm attributed to A1A– should be multiplied by a factor of 2.33 to obtain the true relative contribution of ET from P700 to A1A at this wavelength. Low temperature X-band EPR studies of the PS I complexes in dry trehalose matrix showed that the redox potential of the terminal FB cluster may be altered by dehydration to become more oxidizing than in the control PS I.