Structural Analysis of High-Spin States of S0-S4 at OEC Complex: A Theoretical Approach of Small Models

Abstract

S-states at oxygen evolving complex (OEC) are widely studied due to its large importance in photo-oxidation water process. The structural aspects involving S0, S1, S2, S3 and S4 states are still not solved theoretically. Particularly, spin states have been analyzed as an important aspect in S-state models. Seeking to obtain a relevant and simple model to cover high-spin (HS) S0-S4 states we develop a 55-57 atoms model. Through quantum chemical calculations we figured out that our interatomic distance parameters are in agreement with experimental and other theoretical reference values by ca. 10.0 and 3.5%, respectively, being also in good agreement with other theoretical models containing a large number of atoms. Our HS models presented expected oxidation states according to other data on literature for small theoretical models. Keywords: OEC complex, photo-oxidation, S-states, structural analysis, theoretical model IntroductionWater and dioxygen are fundamental substances for the maintenance of life as it appears on Earth. These substances provide the minimum conditions for a large part of living organisms maintain their vital functions in the environmental conditions of our planet. The most part of oxygen gas available on nature comes from the photooxidative process of water molecules developed by green plants, algae and cyanobacteria. [1][2][3][4][5][6][7][8][9][10][11] (1) As a result of this photochemical process (reaction 1), based on the oxidation of water molecules, we have the evolving process of oxygen gas and the energy supply to the maintenance of organisms that carry the Photosystem II (PSII) into their cells. 12 The reaction 1 is developed in several steps which are catalyzed by PSII-a protein complex found, mainly, in thylakoid membrane from plant chloroplasts or in inner cyanobacteria membrane. 4 The PSII monomer consists of a cluster of 19 protein subunits, 35 chlorophyll molecules, 2 pheophytin units, 11 β-carotene molecules, more than 20 lipids, 2 plastoquinone, 2 heme irons, 1 non-heme iron, 4 manganese atoms, 3 or 4 calcium atoms, 3 chloride ions, bicarbonate and more than 15 detergents. 1,7,9,13,14 The PSII is found in photo-dependent organisms in its dimeric form ( Figure 1a). Within this protein system we can find an oxygen evolving complex (OEC- Figure 1b) composed of 4 manganese atoms, 4 oxygen atoms and 1 calcium atom (Mn 4 CaO 5 ). [1][2][3][4][5][6][7][8][9][10][11][13][14][15][16][17] This complex constitutes the Castilho-Almeida et al. 243 Vol. 28, No. 2, 2017 target of recent research involving the water oxidation processes in PSII.The oxygen evolving complex (OEC) has a structure similar to a cubic box with a lid (Figure 1b). In this complex, three manganese atoms are linked to four oxygen atoms (O1, O2, O3 e O5), calcium atom is linked directly to three oxygen atoms (O1, O2 and O5) and another manganese atom outside the cubic box (MnA4) binds to two oxygen atoms (O4 and O5). The structure highlighted in Figure 1b is derived from crystallographic data for PSII at 1.9 Å resol...