Miller-Urey

Table 8.4 Typical yield of a Urey-Miller experiment (Miller (2000)) ...

Amino acid formation in the Urey-Miller experiment and almost certainly in the prebiotic environment is via the Stecker synthesis shown in Figure 8.3. This reaction mechanism shows that the amino acids were not formed in the discharge itself but by reactions in the condensed water reservoir. Both HCN and HCO are formed from the bond-breaking reactions of N2 and H2O in a plasma, which then react with NH3 in solution. The C=0 group in formaldehyde or other aldehydes is replaced by to form NH and this undergoes a reaction with HCN to form the cyano amino compound that hydrates to the acid. The Strecker synthesis does not provide stereo-control over the carbon centre and must result in racemic mixtures of amino acids. There is no room for homochirality in this pathway. 

Changing the initial conditions in the Urey-Miller experiment to favour a primordial atmosphere with CO2 as the primary carbon source does not produce such a rich mixture of prebiotic molecules and there is a significantly lower yield of amino acids an argument against an endogenous source of organic molecules. 

Endogenous organic synthesis Urey-Miller experiments as a source of prebiotic molecules via the Strecker synthesis for amino acids, HCN polymerisation for purines and pyrimidines and the formose reaction for sugars... 

Endogeneous organics Organic molecules made on the surface of a planet as part of a prebiotic chemistry, perhaps generated by Urey-Miller syntheses. 

Other pathways are in fact possible, and under this heading the Urey-Miller-type (UMT) synthesis is frequently mentioned. In the UMT synthesis, the initial molecules are highly reduced (CH4, H20, NH3). In the case of hydrocarbons, the reaction is given by... 

At the end of this subsection we are forced to conclude that the origin of the extractable organic matter in chondrites remains fundamentally unknown, though there are at least two plausible scenarios a Fischer-Tropseh-type synthesis or a Urey-Miller-type synthesis. 

Both pathways probably involved quite different conditions, the main difference being the absence of liquid water in the interstellar medium. Nevertheless the basic building blocks and chemical reactions should have been roughly similar, thus leading to important connections between these two routes. While a wide variety of amino acids are prebiotically relevant (as attested by either Urey-Miller experiments or meteorite analysis), we shall focus in this section on a-amino acids (as the most relevant to biochemistry) and closely related compounds. 

Figure 2.1. The Urey-Miller Experiment. An electric discharge (simulating lightning) passed through an atmosphere of CH4, NH3, H2O, and H2 leads to the generation of key organic compounds such as amino acids.

Figure 2.2. Products of Prebiotic Synthesis. Amino acids produced in the Urey-Miller experiment.

Figure 2 The Urey-Miller Experiment. The glass vessel to the lower left of the picture is used for the water reservoir and mimics oceans on the early Earth. An electrical discharge of a gas mixture mimicking the composition of the early Earth s atmosphere takes place in the large bulb at the top right. Molecules formed in the discharge may be condensed and are collected at the bottom ofthe system.

Explain the Urey-Miller experiment, and diagram the apparatus. Describe the major products produced by this experiment. 

RNA bases are built from amino acids. Thus amino acids (which are produced in the Urey Miller experiment) are older than RNA building blocks (which are not produced in this experiment). Is it reasonable that the only use to which amino acids were put was synthesis of RNA building blocks ... 

Summarizing there are basically three different theories about where life has evolved on our planet the primordial soup theory (Urey-Miller experiment), life has originated in the atmosphere (Sagan) and black smokers (hydrothermal vents). The primordial soup theory seems to be no longer acceptable without strong mod-... 

The construction of the basic building blocks for life (monomers) is easy to explain (extraterrestrial origin, Urey-Miller experiment, black smokers...). However, it is much more difficult to explain the formation of polymers out of monomers. According to Darwin s Soup theory, mixtures of monomers should produce polymers. For the process of polymer production water plays also a negative role in an aequous environment, hydrolysis transforms the polymers into their constituent monomers. By hydrolysis the water molecule H2O is split into H+ and OH . 

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