Oxygen paradox

Darley-Usmar, V., O Leary, V. and Stone, D. (1989). The glutathione status of perfused rat hearts subjected to hypoxia and teoxygenation the oxygen paradox. (Corrected and republished article originally printed in Free Rad. Res. Commun. 5, 283-289. 

CC Winterbourn. In The Oxygen Paradox KJA Davies, F Ursini (eds), Cleup University Press, 1995, pp 23-32. 

In a Lewis-acid catalysed Diels-Alder reaction, the first step is coordination of the catalyst to a Lewis-basic site of the reactant. In a typical catalysed Diels-Alder reaction, the carbonyl oxygen of the dienophile coordinates to the Lewis acid. The most common solvents for these processes are inert apolar liquids such as dichloromethane or benzene. Protic solvents, and water in particular, are avoided because of their strong interactions wifti the catalyst and the reacting system. Interestingly, for other catalysed reactions such as hydroformylations the same solvents do not give problems. This paradox is a result of the difference in hardness of the reactants and the catalyst involved... 

Oxygen is the most abundant element on the earth s surface it occurs both as the free element and combined in innumerable compounds, and comprises 23% of the atmosphere by weight, 46% of the lithosphere and more than 85% of the hydrosphere ( 85.8% of the oceans and 88.81% of pure water). It is also, perhaps paradoxically, by far the most abundant element on the surface of the moon where, on average, 3 out of every 5 atoms are oxygen (44.6% by weight). 

It is intriguing that Mo is an important element in biology despite its scarcity at the Earth s surface—indeed, no element of similar scarcity is so biochemically ubiquitous. The reason for this paradoxical combination is unknown. Possible explanations include the unique chemical character of this element, evolutionary adaptation to the abrmdance of Mo in oxygenated oceans relative to other transition metals, a legacy of prebiotic chemical evolution in Mo-rich environments (e.g., in association with sulfide minerals), or some combination of these factors. Regardless, this paradox has inspired creative hypotheses about the importance of Mo in evolution (Crick and Orgel 1973 Anbar and Knoll 2002). 

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