Theoretical clues to the mechanism of dioxygen formation at the oxygen-evolving complex of photosystem II.
The mechanism of the generation of dioxygen at the oxygen-evolving complex (OEC) of photosystem II (PSII), a crucial step in photosynthesis, is still under debate. The simplest unit present in the OEC that can produce O2 is a dinuclear oxo-bridge manganese complex within the tetranuclear Mn4 cluster. In this paper we report a theoretical study of the model complexes [Mn2(mu-O)2(NH3)6(H2O)2]n+ (n = 2-5), for which density functional calculations have been carried out for several electronic configurations. The molecular orbital picture deduced from the calculations indicates that one-electron oxidation of the Mn2IV,IV/(O2-)2 complex (n = 4) mostly affects the oxygen atoms, thus ruling out the existence of a MnV oxidation state in this context, while the incipient formation of an O-O bond in the O2(3-) transient species evolves exothermally toward the dissociation of dioxygen and a Mn2II,III couple. These results identify the electronic features that could be needed to enable an intramolecular mechanism of oxygen-oxygen bond formation to exist at the OEC during photosynthesis.
Departament de Química Inorgànica, Centre de Recerca en Química Teòrica Universitat de Barcelona, Diagonal 647, 08028 Barcelona, Spain.
Gabriel
G
Aullón
Eliseo
E
Ruiz
Santiago
S
Alvarez
D004579:Electron Transport; D004583:Electrons; D008345:Manganese / Q000737:chemistry / Q000378:metabolism; D008667:Metalloproteins / Q000737:chemistry; D008958:Models, Molecular; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry / Q000378:metabolism*; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000378:metabolism; D045332:Photosystem II Protein Complex
D008667:Metalloproteins; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D008345:Manganese; D010100:Oxygen