Introduction of a hydrogen bond between phylloquinone PhQ_A and a threonine side-chain OH group in photosystem i

The phylloquinone acceptor PhQ_A in photosystem I binds to the protein through a single H-bond to the backbone nitrogen of PsaA-L722. Here, we investigate the effect of this H-bond on the electron transfer (ET) kinetics by substituting threonine for PsaA-L722. Room temperature spin-polarized transient EPR measurements show that in the PsaA-L722T mutant, the rate of PhQ _A^- to F_X ET increases and the hyperfine coupling to the 2-methyl group of PhQ_A is much larger than in the wild type. Molecular dynamics simulations and ONIOM type electronic structure calculations indicate that it is possible for the OH group of the Thr side chain to form an H-bond to the carbonyl oxygen atom, O₄ of the phylloquinone, and that this results in an increase in the 2-methyl hyperfine couplings as observed in the transient EPR data. The Arrhenius plot of the PhQ_A^- to F_X ET in the PsaA-L722T mutant suggests that the increased rate is probably the result of a slight change in the electronic coupling between PhQ_A^- and F_X. The strong deviation from Arrhenius behavior observed at ∼200 K can be reproduced using a semiclassical model, which takes the zero-point energy of the mode coupled to the ET into account. However, since the change in slope of the Arrhenius plot occurs at the protein glass transition temperature, it is argued that it could be the result of a change in the protein relaxation dynamics at this temperature rather than quantum mechanical effects.