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

Abstract

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 Mn(4)Ca clusters modeling the successive S(0) 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)(3)Mn(IV) for S(0) to Mn(III)(2)Mn(IV)(2) for S(1) to Mn(III)Mn(IV)(3) for S(2) to Mn(IV)(4) for S(3), 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 S(0) → S(1) transition, by 1.43 eV for the S(1) → S(2) transition, and by 0.63 eV for the S(2) → S(3) 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 S(2) → S(3) transition obtained by both the half-height and second-derivative methods, thus giving strong support to the previously suggested structures and assignments.