Trehalose matrix effects on charge-recombination kinetics in Photosystem I of oxygenic photosynthesis at different dehydration levels

摘要

Matrix effects on long-range electron transfer were studied in cyanobacterial Photosystem I (PS I) complexes, embedded into trehalose glasses at different hydration levels. W-band EPR studies demonstrated, via nitroxide spin probes, structural homogeneity of the dry PS I-trehalose matrix and no alteration of cofactors' distance and relative orientation under temperature and matrix variation. In dry trehalose glasses at room temperature (RT), PS I was stable for months. Flash-induced charge recombination ldnetics were examined by high-field time-resolved EPR and optical spectroscopies. The kinetics in hydrated PS I-trehalose glasses mostly reflected the reduction of the photooxidized primary donor P-700(center dot+) by the reduced terminal iron-sulfur clusters. Upon dehydration, the P-700(center dot+) decay accelerated and became more distributed. Continuous distributions of lifetimes tau were extracted from the kinetics by two numerical approaches: a maximum entropy method (MemExp program) and a constrained regularization method (CONTIN program). Both analyses revealed that upon dehydration the contribution of the two slowest components (lifetimes tau similar to 300 ms and similar to 60 ms), attributed to P-700(center dot+)[FA/FB]center dot-recombination, decreased in parallel with the increase of the fastest component (tau similar to 150 mu s), and of additional distributed phases with intermediate lifetimes. Dehydration at RT mimicked the effects of freezing water-glycerol PS I systems, suggesting an impairment of PS I protein dynamics in the dry trehalose glass. Similar effects were observed previously in bacterial reaction centers. The work presented for PS I provides new insights into the crucial issue of protein-matrix interactions for protein functionality as controlled by hydrogen-bond networks of the hydration shell. (C) 2016 Elsevier B.V. All rights reserved.