Abiotic representative examples

Methylation of both metals and metalloids has been observed for both fungi and bacteria. These metabolites may, however, be toxic to higher biota as a result of their volatility. The Minamata syndrome represents the classic example of the toxicity of forms of methylated Hg to man, even though the formation of Hg(CH3)2 was probably the result of both biotic and abiotic reactions. 

As for waste from the production of chemicals, the array of structures represented by agrochemicals is truly enormous. Only some illustrative examples are provided, and it is important to emphasize that not only the original compound, but also potential metabolites should be considered. The pathways for biodegradation of many of the structures have been presented in Chapter 9 and reference should be made to these for details. There is increased interest in the degradation of agrochemicals after application, and abiotic reactions including photochemical degradation that are important on the soil surface are discussed in Chapter E... 

Metabolites may be produced by biochemical transformation of the substrate rather than by degradation, or may result from partial abiotic reactions. These products may be (a) terminal and persistent or (b) toxic to other components of an ecosystem—including the microorganisms that produce them. Both of these represent important considerations that are illustrated by examples in this book. 

The thioester hypothesis can be summed up as follows the formation of thiols was possible, for example, in volcanic environments (either above ground or submarine). Carboxylic acids and their derivatives were either formed in abiotic syntheses or arrived on Earth from outer space. The carboxylic acids reacted under favourable conditions with thiols (i.e., Fe redox processes due to the sun s influence, at optimal temperatures and pH values) to give energy-rich thioesters, from which polymers were formed these in turn (in part) formed membranes. Some of the thioesters then reacted with inorganic phosphate (Pi) to give diphosphate (PPi). Transphosphorylations led to various phosphate esters. AMP and other nucleoside monophosphates reacted with diphosphate to give the nucleoside triphosphates, and thus the RNA world (de Duve, 1998). In contrast to Gilbert s RNA world, the de Duve model represents an RNA world which was either supported by the thioester world, or even only made possible by it. 

The shining examples of the potential of this approach are the natural enzymes, which represent the most selective reaction systems we presently know of. Thus, it is no surprise that the increasing knowledge about the biocatalysts initiates attempts to adopt enzymic working principles for the introduction of selectivity into abiotic systems. It is admittedly true that we have certainly not a quantitative understanding of biocatalysis and even its molecular basis is still being disputed... 

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