Amorphous Manganese-Calcium Oxides as a Possible Evolutionary Origin for the CaMn4 Cluster in Photosystem II

Abstract

In this paper a few calcium-manganese oxides and calcium-manganese minerals are studied as catalysts for water oxidation. The natural mineral marokite is also studied as a catalyst for water oxidation for the first time. Marokite is made up of edge-sharing Mn(3+) in a distorted octahedral environment and eight-coordinate Ca(2+) centered polyhedral layers. The structure is similar to recent models of the oxygen evolving complex in photosystem II. Thus, the oxygen evolving complex in photosystem II does not have an unusual structure and could be synthesized hydrothermally. Also in this paper, oxygen evolution is studied with marokite (CaMn₂O₄), pyrolusite (MnO₂) and compared with hollandite (Ba(0.2)Ca(0.15)K(0.3)Mn(6.9)Al(0.2)Si(0.3)O(16)), hausmannite (Mn₃O₄), Mn₂O₃.H₂O, Ca Mn₃O₆.H₂O, CaMn₄O₈.H₂O, CaMn₂O₄.H₂O and synthetic marokite (CaMn₂O₄). I propose that the origin of the oxygen evolving complex in photosystem II resulted from absorption of calcium and manganese ions that were precipitated together in the archean oceans by protocyanobacteria because of changing pH from ~5 to ~8-10. As reported in this paper, amorphous calcium-manganese oxides with different ratios of manganese and calcium are effective catalysts for water oxidation. The bond types and lengths of the calcium and manganese ions in the calcium-manganese oxides are directly comparable to those in the OEC. This primitive structure of these amorphous calcium-manganese compounds could be changed and modified by environmental groups (amino acids) to form the oxygen evolving complex in photosystem II.

Photo-Electrochemical Oxygen Evolution Reaction by Biomimetic CaMn2O4 Catalyst

“…This natural process is the most efficient known, for the photo-generation of H 2 from water. Among many calcium manganese oxides, CaMn 2 O 4 contains structural sub-units of (Mn 4 CaO x ), similar to the structure of natural CaMn 4 O 5 cluster in OEC [5,6]. This has led to extensive investigation of catalytic water oxidation properties of CaMn 2 O 4 [7][8][9].…”

A Chemical Evolution‐Like Method to Synthesize a Water‐Oxidizing Catalyst

“…From a simple view on the chemical evolution, [23–32] which could be extended to a useful conception, it can be imagined that among many metal ions around the apo‐enzyme of photosystem II and under the OER conditions, Ca and Mn ions were selected and used to finally construct the OEC [28–32] . Recently, Dau's group showed that photosystem II can efficiently form layered Mn oxide particles under photochemical reaction, even in the absence of specific protein subunits [32] …”

The effect of different metal ions between nanolayers of manganese oxide on water oxidation

“…Mn oxides were reported as water-oxidizing catalysts for artificial photosynthetic systems [23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40]. Among Mn oxides, nanolayered Mn oxides with different organic or inorganic ions between layers are efficient and stable in water oxidation [41].…”