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 potential 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 , and to * To whom correspondence should be addressed † Uppsala University ‡ Stockholm University ¶ Royal Institute of Technology 1 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. Using the half-height method, the theoretical IPE's are shifted by 0.93 eV for the S 0 to S 1 transition, by 1.43 eV for the S 1 to S 2 transition, and by 0.63 eV for the S 2 to S 3 transition. The 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 to S 3 transition obtained both with the half-height and with the second derivative methods, thus giving strong support to the previously suggested structures and assignments.

Simulation of the isotropic EXAFS spectra for the S 2 and S 3 structures of the oxygen evolving complex in photosystem II

“…This change, supported by results of earlier and later XANES spectra (33), led to the conclusion that an oxygen rather than a manganese is oxidized in this transition, which strongly affected the suggested O-O bond formation mechanism (34,35). The conclusion from the XANES spectra has been questioned experimentally (36), and also recently on the basis of DFT model calculations (37). The Dau and coworkers results, however, gave one change from a distance larger than 3.0 Å down to 2.77 Å; this led to a suggested structural change in this transition with a formation of an additional Mn-Mn bis-μ-oxo bridge in S 3 (38).…”

Metal oxidation states in biological water splitting

“…This defines the pre-edge region that extends over approximately 7 eV and contains information about the local electronic structure and ligand environment of the Mn ions. Computational studies of the Mn K edge (1s–4p) for diverse OEC models have been interpreted in favor of both the LV 106 and the HV 187 schemes, albeit using different theoretical approaches. Given the ambiguities in calculating the Mn K edge, we here focus on the more well-defined and computationally accessible 1s–3d transitions that give rise to the pre-edge spectral region.…”

Characterization of gas phase iron phthalocyanine with X‐ray photoelectron and absorption spectroscopies

“…The film of FePc on Si(100) shows molecules standing up with respect to the surface, whereas the gas phase measurements show an average of all the possible orientation for the molecules. Moreover, the possibility that the molecules could be distorted in the film with respect to the vapor case, as for example due to an effect of adsorption, to a change of the ionic state of the metal centre or a charge transfer to some unoccupied molecular states could also result in a shift of the features at such photon energy. However, this latter reason should be considered quite carefully since we don't have any other evidence that the metallic centre for the molecule have changed the ionic state and we think we can exclude any charge transfer in the multilayer.…”