Oxygen may be considered one of the most important element in chemistry. Not counting hydrocarbons, there is a greater diversity of molecules with oxygen than with carbon. Given its role in the molecular world, very little time is spend on the chemistry of oxygen in undergraduate chemistry classes. Why is oxygen so special?
Oxygen reacts with atoms of all elements except the Noble gases to form molecules. One of the most important molecules of course, from a biological sense is water. It :
- provides a perfect solvent for biomolecules
- moderates the earths climate
- is the source of almost all the dioxygen in the air
From a chemical point of view, water is a(n):
- nucleophile and electrophile
- acid and base
- oxidizing agent and reducing agent
- a protic solvent that can form H-bonds
The formation of earth and the development of life:
The gaseous and dusty environment from which earth formed contained metals and water, which as you remember from introductory chemistry, can react to form hydrogen gas. H2 reacts with nonmetals (under various conditions of temperature and pressure) to form H2S, HCl, CH4, and NH3 which contributed to the reducing nature of the early atmosphere. This kept the transition metals in their lowest oxidation states. Many metals, including the coinage metals (Cu, Ag, and Au) and the platinum group (Ru, Rh, Pd, Pt) were stable in elemental form.
Then, around 2.7-2.8 billion years ago, photosynthetic organisms (blue/green algae- also called cyanobacteria) developed which could oxidize water to form dioxygen. Oxygen was generally unavailable for redox chemistry before then as photosynthesis, the process that would evolve to oxidize water to produce dioxygen, was unavailable. Remember that to oxidize water to dioxygen, itself a strong oxidizing agent, requires a stronger oxidizing agent than dioxygen and lots of energy. Fossilized remains of cyanobacteria are found in stromatolites. Using knowledge of how atmospheric oxygen can alter the chemistry of different sulfur isotopes of SO2, it has been shown that O2 did not exist in the atmosphere as a whole above 1 ppm earlier than 2.4 billion years ago, although there might have been isolated pockets with higher concentrations. After that it rose, presumably as a result of cyanobacteria. Before this time, bacteria oxidized a similar molecule, H2S to form elemental sulfur. It could do this through photosynthetic reduction of CO2 by H2S. It is probable that volcanic gases like H2 might have kept oxygen levels from rising between 2.7 billion year ago and 2.4 billion years ago, when its build-up started. Hydrogen in the form of H2 and methane, probably decreased around 2.4 billion years ago as methane with its hydrogen atoms escaped to the upper atmosphere and space. Methane levels would also be decreased by its easy reaction with dioxygen in the presence of UV light to form CO2. This would paradoxically lead to a cooling of the earth and pronounced glaciation as a more potent greenhouse gas, methane, was replaced with a less potent one, carbon dioxide.
Over the next billion years, dioxygen rose to perhaps 0.2 - 2% (compared to the present levels of 20%) Why? Because the early atmosphere was reducing, the added oxygen combined with a large "sink" of reduced metals (like elemental Cu and Fe) or nonmetals (like C and ammonia), preventing a large buildup. Only after these reduced substances were "titrated" did dioxygen build up to present levels. In addition, the oxygen might have increased weathering (by oxidation) of sulfur deposits which can lead to sulfides entering the ocean, where they could precipitate ocean iron ions that are necessary for cyanobacterial chemistry. This would place constraints on cyanobacterial growth until dioxygen levels in the atmosphere increased enough so sulfides were converted to sulfates. This first increase in atmospheric oxygen is often called the Great Oxidation Event as it correlated and presumably caused one of the greatest mass extinctions (of anerobic organisms) of all time.
Around 2.3 billion years ago, as trace dioxygen had accumulated in the atmosphere, redox chemistry changed, although isotope evidence suggest that little dioxgen was found in water. Around 1.8 - 1.5 billion years ago, the earth's atmosphere became somewhat oxygenated, which was also coincident with the development of eukaryotic organisms. Until then, life was restricted to the oceans since there was no ozone to absorb dangerous UV radiation. The buildup of dioxygen in the air must have led to another extinction of anaerobic organisms, since as we shall see, products of oxygen metabolism are very toxic. Some evolved to use dioxygen. Ozone developed, and life could then migrate from the sea to the land. It wasn't until around 600 million years ago that animals arose, however. Was this event associated with the development of a fully oxygenated (20%) atmosphere? Recent evidence, which shows that substantial oxygen wasn't available in the deep sea until about 600 million years, seems to suggest that. Based on analysis of iron compounds in waters in Newfoundland, it appears that oxygen was very low in the sea 580 million years ago, during the Gaskiers glaciation period. Immediately after that it rose to levels consistent with atmospheric dioxygen levels of 15%, levels necessary for large animals. Similar trends in carbon and sulfur isotopes in marine rocks in Oman also suggest large increases in oxygen at the end of the Gaskiers glaciation period. What caused this second great oxygenation event? One possibility is that organic matter was sequestered from reaction with atmospheric dioxygen, as clays bound organic molecules in the ocean and lichens and zooplankton facilitated weather and production of insoluble organic material in the oceans.
Dioxygen is obviously critically important for higher organisms, so an understanding of its chemistry becomes important. This chapter will show that dioxygen is a ground state diradical that has low solubility in aqueous solution, reacts in a kinetically sluggish fashion in oxidation reaction, and forms toxic byproducts as it gets reduced. Life forms hence evolved ways to deal with these problems, including ways to increase its solubility (with dioxygen binding and transport proteins), and enzymes (that could activate it kinetically and also detoxify oxygen by-products). Dioxygen is toxic to many cells. Obligate aerobes die in an oxygen environment as many of their cellular components get oxidized by this excellent oxidizing agent. Several strains of bacteria actually swim away from high levels of dioxygen. A graph showing log of survival vs log pO2 is linear with a negative slope for a variety of organisms, including mice, fish, rats, rabbits, and insects. Pure oxygen can induce chest soreness, coughs, and sore throats in people. Premature infants put in pure dioxygen environments often developed blindness due to retrolental fibroplasia (a build-up of fibrous tissue behind the lens). The trade off for this toxicity is clear. Energy is derived from organic molecule through oxidation. Before dioxygen became available to power aerobic catabolism of reduced molecules like fatty acids and the less reduced sugars, such molecules were only partially oxidized. The glycolytic pathway, found in most organisms, oxidizes glucose (6 Cs) to two molecules of pyruvate (3 Cs). It was only with the availability of dioxygen did pathways evolve (Kreb Cycle, mitochondrial electron transport/oxidative phosphorylation) that allowed pyruvate to be fully oxidized to carbon dioxide, with the release of much more energy.