Functional Models for the Oxygen-Evolving Complex of Photosystem II

Abstract

In the last ten years, a number of advances have been made in the study of the oxygen-evolving complex (OEC) of photosystem II (PSII). Along with this new understanding of the natural system has come rapid advance in chemical models of this system. The advance of PSII model chemistry is seen most strikingly in the area of functional models where the few known systems available when this topic was last reviewed has grown into two families of model systems. In concert with this work, numerous mechanistic proposals for photosynthetic water oxidation have been proposed. Here, we review the recent efforts in functional model chemistry of the oxygen-evolving complex of photosystem II.

Figure 1

Four proposed structures for the OEC of PSII. A) 3.5 Å crystal structure model of the OEC with anomalous diffraction electron densities of manganese and calcium in purple and cyan, respectively (figure was reproduced from ref. [1], with permission of the copyright holders). B) 3.0 Å crystal structure model of the OEC (figure was reproduced from ref. [2], with permission of the copyright holders). C) One of three proposed structures for the OEC determined by oriented EXAFS spectroscopy (figure was reproduced from ref. [3], with permission of the copyright holders). D) OEC structure calculated using QM/MM methods and basic structural parameters from the 3.5 Å structure (figure was reproduced from ref. [32], with permission of the copyright holders).

Figure 2

Schematic diagram of Kok’s S-state cycle showing four oxidations necessary for water oxidation (figure was reproduced from ref. [18], with permission of the copyright holders).

Figure 3

Top: The proposed mechanism for water oxidation by the OEC known as the “double pivot”or “butterfly” mechanism. This mechanism is based on a cubane-type structure for the OEC (figure was reproduced from ref. [39], with permission of the copyright holders). Bottom: Schematic representation of the structure of the cubane-like complex synthesized by Dismukes et al. as well as its proposed mechanism of oxygen evolution (Figure was reproduced from ref. [40], with permission of the copyright holders).

Figure 4

Top: bis-porphyrin functional model for oxygen evolution (figure was reproduced from ref. [80], with permission of the copyright holders). Bottom: a proposed mechanism for biological oxygen evolution that closely resembles the proposed reactions of the bis-porphyrin complex (figure was reproduced from ref. [44], with permission of the copyright holders).

Figure 5

Top: schematic diagram of the proposed mechanism of water oxidation by complex 7 (figure was reproduced from ref. [93], with permission of the copyright holders). Bottom: schematic representation of the proposed O-O bond forming step in the OEC based on the nucleophilic water mechanism (figure was reproduced from ref. [19], with permission of the copyright holders).

Figure 6

Schematic diagram showing the four main pathways proposed for water oxidation by 1 (figure was reproduced from ref. [59], with permission of the copyright holders).

Figure 7

ORTEP diagrams of [Ru₂^II(μ-OAc)(bpp)(trpy)₂]²⁺ (2) (top left) (figure was reproduced from ref. [73], with permission of the copyright holders) and a catalytic ruthenium monomer system 4 (top right) (figure was reproduced from ref. [74], with permission of the copyright holders). Chemdraw diagrams of the bridging ligand in complex 2 (L1) as well as the multi-dentate ligand from complex 4 (L2) are shown (bottom).

Figure 8

Proposed mechanism of water oxidation by the [Mn(3,5-Cl-salen)(H₂O)₂]₂ complex (figure was reproduced from ref. [88], with permission of the copyright holders).

Figure 9

Proposed mechanism of water oxidation by [Mn^II₂(mcbpen)₂(H₂O)₂]²⁺ (6) using tert-butyl hydrogen peroxide as an oxidant (figure was reproduced from ref. [82], with permission of the copyright holders).

Figure 10

Structure of 7. Hydrogen atoms, nitrate counter ions and waters of crystallization are omitted for clarity.