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Modeling Near-Edge Fine Structure X-ray Spectra of the Manganese Catalytic Site for Water Oxidation in Photosystem II

Brena, Barbara, Siegbahn, Per E. M., Ågren, Hans
Journal of the American Chemical Society 2012 v.134 no.41 pp. 17157-17167
X-radiation, X-ray absorption spectroscopy, absorption, active sites, energy, manganese, molecular models, oxidation, oxygen evolving complex, oxygen production, simulation models, spectral analysis
The Mn 1s near-edge absorption fine structure (NEXAFS) has been computed by means of transition-state gradient-corrected density functional theory (DFT) on four Mn₄Ca clusters modeling the successive S₀ to S₃ steps of the oxygen-evolving complex (OEC) in photosystem II (PSII). The model clusters were obtained from a previous theoretical study where they were determined by energy minimization. They are composed of Mn(III) and Mn(IV) atoms, progressing from Mn(III)₃Mn(IV) for S₀ to Mn(III)₂Mn(IV)₂ for S₁ to Mn(III)Mn(IV)₃ for S₂ to Mn(IV)₄ for S₃, implying an Mn-centered oxidation during each step of the photosynthetic oxygen evolution. The DFT simulations of the Mn 1s absorption edge reproduce the experimentally measured curves quite well. By the half-height method, the theoretical IPEs are shifted by 0.93 eV for the S₀ → S₁ transition, by 1.43 eV for the S₁ → S₂ transition, and by 0.63 eV for the S₂ → S₃ transition. The inflection point energy (IPE) shifts depend strongly on the method used to determine them, and the most interesting result is that the present clusters reproduce the shift in the S₂ → S₃ transition obtained by both the half-height and second-derivative methods, thus giving strong support to the previously suggested structures and assignments.