X‑ray Free Electron Laser Radiation Damage through the S‑State Cycle of the Oxygen-Evolving Complex of Photosystem II

The oxygen-evolving complex (OEC) catalyzes water-splitting through a reaction mechanism that cycles the OEC through the “S-state” intermediates. Understanding structure/function relationsships of the S-states is crucial for elucidating the water-oxidation mechanism. Serial femtosecond X-ray crystallography has been used to obtain radiation damage-free structures. However, it remains to be established whether “diffraction-before-destruction” is actually accomplished or if significant changes are produced by the high-intensity X-ray pulses during the femtosecond scattering measurement. Here, we use <i>ab initio</i> molecular dynamics simulations to estimate the extent of structural changes induced on the femtosecond time scale. We found that the radiation damage is dependent on the bonding and charge of each atom in the OEC, in a manner that may provide lessons for XFEL studies of other metalloproteins. The maximum displacement of Mn and oxygen centers is 0.25 and 0.39 Å, respectively, during the 50 fs pulse, which is significantly smaller than the uncertainty given the 1.9 Å resolution of the current PSII crystal structures. However, these structural changes might be detectable when comparing isomorphous Fourier differences of electron density maps of the different S-states. One conclusion is that pulses shorter than 15 fs should be used to avoid significant radiation damage

Relative stability of the S2 isomers of the oxygen evolving complex of photosystem II

The oxidation of water to O-2 is catalyzed by the Oxygen Evolving Complex (OEC), a Mn4CaO5 complex in Photosystem II (PSII). The OEC is sequentially oxidized from state S-0 to S-4. The S-2 state, (Mn-III)(Mn-IV)(3), coexists in two redox isomers: S-2,S-g=2, where Mn4 is Mn-IV and S-2,S-g=4.1, where Mn1 is Mn-IV. Mn4 has two terminal water ligands, whose proton affinity is affected by the Mn oxidation state. The relative energy of the two S-2 redox isomers and the protonation state of the terminal water ligands are analyzed using classical multi-conformer continuum electrostatics (MCCE). The Monte Carlo simulations are done on QM/MM optimized S-1 and S-2 structures docked back into the complete PSII, keeping the protonation state of the protein at equilibrium with the OEC redox and protonation states. Wild-type PSII, chloride-depleted PSII, PSII in the presence of oxidized Y-Z/protonated D1-H190, and the PSII mutants D2-K317A, D1-D61A, and D1-S169A are studied at pH 6. The wild-type PSII at pH 8 is also described. In qualitative agreement with experiment, in wild-type PSII, the S-2,S-g=2 redox isomer is the lower energy state; while chloride depletion or pH 8 stabilizes the S-2,S-g=4.1 state and the mutants D2-K317A, D1-D61A, and D1-S169A favor the S-2,S-g=2 state. The protonation states of D1-E329, D1-E65, D1-H337, D1-D61, and the terminal waters on Mn4 (W1 and W2) are affected by the OEC oxidation state. The terminal W2 on Mn4 is a mixture of water and hydroxyl in the S-2,S-g=2 state, indicating the two water protonation states have similar energy, while it remains neutral in the S-1 and S-2,S-g=4.1 states. In wild-type PSII, advancement to S-2 leads to negligible proton loss and so there is an accumulation of positive charge. In the analyzed mutations and Cl- depleted PSII, additional deprotonation is found upon formation of S-2 state

Color-stable highly luminescent sky-blue perovskite light-emitting diodes

10.1038/s41467-018-05909-8Nature Communications91354