Molecular orbital study of photosynthetic water decomposition

Abstract

Despite its importance in understanding the conversion of sunlight into useful chemical energy in green plants, the molecular mechanism of oxygenevolving reaction (1) remains to be elucidated [ 1,2]. (HOW')* -e----t (HO)* + H'*where H' stands for a releasing proton. Our principal assumption is that some valence state of manganese works as a catalytic metal cation, which directly decomposes Hz0 into H' and OH-(or OH radical). In vivo systems must have a proton-accepting site which forms a hydrogen bond with a water molecule to be oxidized by the manganese protein.Whether the proton-accepting site is a water molecule in the Elsevier~North-Hoi~nd Biomedical Press second sdvation shell 1121 or any another kind of proton acceptor in the water-splitting enzyme (WSE) system is entirely unknown. Here we assume the former possibility; as will be shown later, we found that no proton acceptor stronger than neutral water is necessary. Here, using the MO investigation, we aim to answer the following questions: fig.2).

Role of quantum chemical calculations in molecular biophysics with a historical perspective

“…The developed model for the water splitting system of thylakoid membranes using the molecular orbital study system was presented in [206]. The authors of this work have made the molecular orbital calculations of the process of water oxidation.…”

“…The water-oxidizing complex is so efficient that does not need help from additional defense systems such as antioxidants, superoxide dismutases, and catalases. In this work the new model of the oxygen evolving complex (OEC) based on the cubane-like Mn3CaO4 model [206] was proposed. In doing so, the authors used the ligand field theory and the rules of 18 and 10 electrons for stable shells of Mn and Ca ions respectively.…”

“…In doing so, the authors used the ligand field theory and the rules of 18 and 10 electrons for stable shells of Mn and Ca ions respectively. By the same procedure they added model [206] by chlorine and water molecules. The authors proposed the mechanism of water oxidation in which calcium ion and remote manganese Mn4 play the main role.…”

“…To simplify the calculations, they used HCOOˉ groups instead of aspartic and glutamic acids, and the NH4 + group instead of histidine. To optimize the system the authors changed the length of the bonds with those molecules that were not obtained from the structure [206], i.e., four water molecules and the chlorine ion. In doing so, it was obtained that the bond energy in each of S-states is negative.…”