Oxygen evolution

Oxygen evolution is the process of generating molecular oxygen through chemical reaction. Mechanisms of oxygen evolution include the photolysis of water during oxygenic photosynthesis, electrolysis of water into oxygen and hydrogen, and electrocatalytic oxygen evolution from oxides and oxoacids.

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Oxygen evolution in nature

Photolytic oxygen evolution is the fundamental process by which breathable oxygen is generated in earth's biosphere. The reaction is part of the light-dependent reactions of photosynthesis in cyanobacteria and the chloroplasts of green algae and plants. It utilizes the energy of light to split a water molecule into its protons and electrons for photosynthesis. Free oxygen is generated as a waste product of this reaction, and is released into the atmosphere.

Biochemical reaction

Photolytic oxygen evolution occurs via the light-dependent oxidation of water to molecular oxygen and can be written as the following simplified chemical reaction:

2H₂O 4e^- + 4H⁺ + O₂

The reaction requires the energy of four photons. The electrons from the oxidized water molecules replace electrons in the P₆₈₀ component of photosystem II which have been removed into an electron transport chain via light-dependent excitation and resonance energy transfer onto plastoquinone.^[1] Photosytem II therefore has also been referred to as water-plastoquinone oxido-reductase.^[2] The protons are released into the thylakoid lumen, thus contributing to the generation of a proton gradient across the thylakoid membrane. This proton gradient is the driving force for ATP synthesis via photophosphorylation and coupling the absorption of light energy and photolysis of water to the creation of chemical energy during photosynthesis.^[1]

Oxygen-evolving complex

  Water oxidation is catalyzed by a manganese-containing enzyme complex associated with thylakoid membranes known as the oxygen evolving complex (OEC) or water-splitting complex. Manganese is an important cofactor, and calcium and chloride are also required for the reaction to occur.^[1]

X-ray crystallography studies have recently provided detailed models of the structure of the oxygen-evolving complex and its manganese cluster.^[3] Based on structural and spectroscopic experiments, oxygen evolution involves a core three-plus-one cluster of three manganese ions and one calcium ion, with one additional manganese, which are oxidized via intermediate states called S-states. The O-O bond of molecular oxygen is formed between manganese-ligated oxygen atoms at the most oxidized, or S4, state.^[4]

Evolution of oxygen evolution

Oxygen production during photosynthesis evolved on earth around 2.7 to 3.5 billion years ago.^[5] Oxygen was not only a waste product of this reaction, but was also toxic to many metabolic processes such as nitrogen fixation. Consequently, it was released into the atmosphere as a means of detoxification. This contributed to the conversion of earth's atmosphere from anaerobic to its current aerobic composition, triggering the Oxygen Catastrophe and the evolution of aerobic metabolism utilizing the oxygen that was released by photosynthetic organisms as part of the oxygen cycle.

History of discovery

It wasn't until the end of the 18th century that Joseph Priestley discovered by accident the ability of plants to "restore" air that had been "injured" by the burning of a candle. He followed up on the experiment by showing that air "restored" by vegetation was "not at all inconvenient to a mouse." He was later awarded a medal for his discoveries that: "...no vegetable grows in vain... but cleanses and purifies our atmosphere." Priestley's experiments were followed up by Jan Ingenhousz, a Dutch physician, who showed that "restoration" of air only worked in the presence of light and green plant parts.^[1]

Ingenhousz suggested in 1796 that CO₂ (carbon dioxide) is split during photosynthesis to release oxygen, while the carbon combined with water to form carbohydrates. While this hypothesis was attractive and reasonable and thus widely accepted for a long time, it was later proven incorrect. Graduate student C.B. Van Niel at Stanford University found that purple sulfur bacteria reduce carbon to carbohydrates, but accumulate sulfur instead of releasing oxygen. He boldly proposed that in analogy to the sulfur bacteria forming elemental sulfur from H₂S (hydrogen sulfide), plants would form oxygen from H₂O (water). In 1937, this hypothesis was corroborated by the discovery that plants are capable of producing oxygen in the absence of CO₂. This discovery was made by Robin Hill, and subsequently the light-driven release of oxygen in the absence of CO₂ was called the Hill reaction. Our current knowledge of the mechanism of oxygen evolution during photosynthesis was further established in experiments tracing isotopes of oxygen from water to oxygen gas.^[1]

Technological oxygen evolution

  Oxygen evolution occurs as a byproduct of hydrogen production via electrolysis of water. While oxygen production is not the main focus of industrial applications of water electrolysis, it becomes essential for life support systems in situations that require the generation of oxygen for air revitalization. Human exploration of regions that lack breathable oxygen, such as the deep sea or outer space, requires means of reliably generating oxygen apart from earth's atmosphere. Submarines and spacecraft utilize either an electrolytic mechanism (water or solid oxide electrolysis) or chemical oxygen generators as part of their life support equipment.

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