Topic
Oxygen
About: Oxygen is a research topic. Over the lifetime, 48149 publications have been published within this topic receiving 1113788 citations. The topic is also known as: O & Oxygen.
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TL;DR: Evidence is reported that the S2 to S3 transition occurring in Ca2+-depleted PS-II corresponds to the oxidation of histidine, one of the few examples of amino-acid oxidation in enzyme chemistry.
Abstract: THE evolution of oxygen as a result of light-driven water oxidation occurs in plants and is catalysed by photosystem-II (PS-II). A manganese-cluster probably acts both as the active site and as a charge-accumulating device (for a review, see ref. 1). The enzyme cycle involves five redox states which are denoted as S0–S4, depend-ing on the number of positive equivalents stored2. Oxygen is released after formation of the transient S4 state. Ca2+ is an obligatory cofactor in this process and its depletion inhibits the enzyme cycle at the step before water oxidation, that is, after formation of the S3 state3. In chelator-treated, Ca2+-depleted PS-II, a new electron paramagnetic resonance (EPR) signal arising from a formal S3 state has been reported4. It was suggested that the S3 EPR signal could originate from the oxidation of an amino acid, interacting magnetically with the manganese cluster4. There are only a few examples of amino-acid oxidation in enzyme chemistry and these are limited to tyrosine5 and tryptophan6. Here we report evidence that the S2 to S3 transition occurring in Ca2+-depleted PS-II corresponds to the oxidation of histidine.
213 citations
TL;DR: In this article, the authors measured the thermal expansion coefficients of the ceramic materials, calculated from dilatometric data, are in the range (10.5-13.2) × 10============ −6============ -1======
Abstract: The solid solutions La
2
Ni
1–x
Fe
x
O
4+δ
(x = 0.02 and 0.10), La
1.9
Sr
0.1
Ni
1 – x
Fe
x
O
4 + δ
(x = 0.02 and 0.10) and La
2
Ni
0.88
Fe
0.02
Cu
0.10
O
4+δ
with the tetragonal K
2
NiF
4
-type structure were prepared by a standard ceramic technique. The thermal expansion coefficients of the ceramic materials, calculated from dilatometric data, are in the range (10.5–13.2) × 10
–6
K
–1
at 300–1100 K. Oxygen permeation fluxes through dense La
2
NiO
4 + δ
-based membranes at 970–1170 K were found to be limited by both surface exchange and bulk ionic transport, whereas the limiting effect of the oxygen interphase exchange increases with decreasing oxygen pressure at the membrane permeate side and with decreasing temperature. Applying porous cermet layers of dispersed platinum and praseodymium oxide onto the membrane surface results in enhanced permeation fluxes. The maximum oxygen permeability was found for the La
2
Ni
0.98
Fe
0.02
O
4+δ
and La
2
Ni
0.88
Fe
0.02
Cu
0.10
O
4+δ
solid solutions; oxygen permeability data demonstrated that a vacancy diffusion mechanism and oxygen interstitial migration make significant contributions to the total ionic conductivity.
213 citations
TL;DR: In this article, the surface and near-surface regions of an active catalyst and the adjacent gas-phase reactants were investigated simultaneously under reaction conditions using in situ X-ray photoelectron spectroscopy (XPS).
Abstract: The surface and near-surface regions of an active catalyst and the adjacent gas-phase reactants were investigated simultaneously under reaction conditions using in situ X-ray photoelectron spectroscopy (XPS). This investigation of methanol oxidation on a copper catalyst showed that there was a linear correlation between the catalytic activity of the sample and the presence of a subsurface oxygen species that can only be observed in situ. The concentration profile of the subsurface oxygen species within the first few nanometers below the surface was determined using photon-energy-dependent depth-profiling. The chemical composition of the surface and the near-surface regions varied strongly with the oxygen-to-methanol ratio in the reactant stream. The experiments show that the pure metal is not an active catalyst for the methanol oxidation reaction, but that a certain amount of oxygen has to be present in the subsurface region to activate the catalytic reaction. Oxide formation was found to be detrimental t...
213 citations
TL;DR: In this article, the oxidation of ultra-thin Mg films supported on a Mo(100) surface has been studied using X-ray photoelectron spectroscopy (XPS) in the 90-1300 K sample temperature range.
212 citations
TL;DR: In this paper, it was shown that at pressures of around 100 GPa, solid molecular oxygen becomes superconducting, with a transition temperature of 6 K. The transition is revealed by both resistivity measurements and a Meissner demagnetization signal.
Abstract: Among the simple diatomic molecules, oxygen is of particular interest because it shows magnetism at low temperatures Moreover, at pressures exceeding 95 GPa (∼095 Mbar), solid molecular oxygen becomes metallic, accompanied by a structural transition1 The metallization process is characterized by an increase in optical reflectivity2, and a change in the slope of the resistance–temperature curve3 Here we report that at pressures of around 100 GPa, solid oxygen becomes superconducting, with a transition temperature of 06 K The transition is revealed by both resistivity measurements and a Meissner demagnetization signal
212 citations