Modeling near-edge fine structure x-ray spectra of the manganese catalytic site for water oxidation in photosystem II

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 S ₃ 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 → S ₁ 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.