Calcium manganese oxides as oxygen evolution catalysts: O2 formation pathways indicated by 18O-labelling studies.

Oxygen evolution catalysed by calcium manganese and manganese-only oxides was studied in (18)O-enriched water. Using membrane-inlet mass spectrometry, we monitored the formation of the different O(2) isotopologues (16)O(2), (16)O(18)O and (18)O(2) in such reactions simultaneously with good time resolution. From the analysis of the data, we conclude that entirely different pathways of dioxygen formation catalysis exist for reactions involving hydrogen peroxide (H(2)O(2)), hydrogen persulfate (HSO(5)(-)) or single-electron oxidants such as Ce(IV) and [Ru(III) (bipy)(3)](3+) . Like the studied oxide catalysts, the active sites of manganese catalase and the oxygen-evolving complex (OEC) of photosystem II (PSII) consist of µ-oxido manganese or µ-oxido calcium manganese sites. The studied processes show very similar (18)O-labelling behaviour to the natural enzymes and are therefore interesting model systems for in vivo oxygen formation by manganese metalloenzymes such as PSII.

A possible evolutionary origin for the Mn4 cluster in photosystem II: from manganese superoxide dismutase to oxygen evolving complex.

The recently published X-ray absorption fine structure of photosystem II provides a more detailed architecture of the oxygen-evolving complex (OEC) and the surrounding amino acids. In this paper, a comparison between manganese superoxide dismutase, dinuclear manganese catalase enzymes and the oxygen evolving complex in photosystem II is reported. The author suggests that the development of oxygenic photosynthesis occurred in steps, the first of which involved only one manganese ion (Mn(II)) that oxidized two water molecules to hydrogen peroxide and then oxygen.

4,4'-Bipyridine-butane-1,2,3,4-tetra-carboxylic acid (1/1).

The title compound, C(10)H(8)N(2)·C(8)H(10)O(8), is an example of a system with a short O⋯H⋯N hydrogen bond [O⋯N = 2.565 (3) Å]. The crystal structure comprises a 1:1 adduct between 4,4'-bipyridine and butane-1,2,3,4-tetra-carboxylic acid, where both components are centrosymmetric. The component mol-ecules are linked through strong O⋯H⋯N hydrogen bonds, forming chains extending approximately along [11]. The chains are inter-connected by O⋯H⋯O hydrogen bonds and weak stacking inter-actions involving the pyridyl rings of the 4,4'-bipyridine mol-ecules [centroid-centroid distance = 3.73 (2) Šand inter-planar distance = 3.35 (1) Å]. The H atom of the short O⋯H⋯N hydrogen bond is disordered over two positions with site occupancy factors of ca 0.6 and 0.4. One methylene group is disordered over two positions; the site occupancy factors are ca 0.9 and 0.1.

Bis(guanidinium) trans-diaqua-bis(malonato-κO,O')cobaltate(II).

In the title compound, (CH(6)N(3))(2)[Co(C(3)H(2)O(4))(2)(H(2)O)(2)], the anions lie on crystallographic centres of inversion. The crystal structure adopts a layered structure, stabilized by an extensive network of N-H⋯O and O-H⋯O hydrogen bonds. One H atom of the guanidinium cation does not participate in any strong hydrogen bonds.