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

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-trehalosematrix and no alteration of cofactors' distance and relative orientation under temperature andmatrix variation. In dry trehalose glasses at room temperature (RT), PS Iwas stable for months. Flash-induced charge recombination kinetics 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 P700 + by the reduced terminal iron-sulfur clusters. Upon dehydration, the P700 + decay accelerated and became more distributed. Continuous distributions of lifetimes π 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 π ∼300 ms and ∼60 ms), attributed to P700 + [FA/FB]•- recombination, decreased in parallel with the increase of the fastest component (π ∼ 150 μs), and of additional distributed phaseswith intermediate lifetimes. Dehydration at RTmimicked the effects of freezingwater-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.