Millers Experiment

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... 

Figure 3.3 The famous Stanley Miller experiment a strongly reducing atmosphere consisting of the four gases, and electric discharges as energy source.

Bada, J. L. and Lazcano, A. (2003). Prebiotic soup - revisiting the Miller experiment. Science, 300, 745-6. 

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. 

Lazcano A, Bada JL. The 1953 Stalney L. Miller experiment fifty years of prebiotic organic chemistry. Origins Life Evol Biosphere 2003 33 235-42. 

J. H. Miller, Experiments in Molecular Genetics. Cold Spring Harbor Laboratory, Cold Spring Harbor, New York, 1972. 

This requirement is fulfilled for electric discharges in a reduced atmosphere containing methane, ammonia, and water, as in the original Miller experiment. It has also been observed for atmospheres based on N2 and CO or CO2 on the condition that H2 or methane is also present in snfflcient amonnts (19). A neutral atmosphere (based on N2, CO2, and water) wonld produce much lower yields of organics (by several orders of magnitude). In the absence of other species to be oxidized, the rednction of CO2 reqnires the concomitant thermodynamically nnfavorable conversion of water into O2 (as in photosynthesis). However, even if the atmosphere was nentral when life arose, as nsnaUy believed, the Earth was not nniform with respect to redox state simply becanse the rednced state of the mantle and the high volcanic activity favored the occnrrence of locally rednced environments (for instance, in hydrothermal vents in the oceans). Then, a preservation of the hydrogen content of the early atmosphere or the diversity of environments on the early Earth is likely to have made amino acid formation possible, at least at specific places. 

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.

Lazcano, A. Bada, J. L. The 1953 Stanley L. Miller Experiment Fifty Years of Prebiotic Organic Chemistry. Origins of Life Evol. Biosphere 2003, 33,235-242. 

It has been found that dihydropyrimidines are converted to the corresponding pyrimidines by irradiation with ultraviolet light in the presence of water vapor. The reaction is catalyzed by clay minerals, particularly montmorillo-nite, and an evaporating pond environment has been suggested as a reasonable prebiotic locale for such a reaction (Chittenden and Schwartz, 1976). If acetate salts are added to the system, thymine is formed in addition to uracil, presumably by addition of a methyl radical (Schwartz and Chittenden, 1977). An alternative route to thymine involves the addition of formaldehyde to uracil and subsequent reduction by formic acid (Choug-huley et al., 1977). (Formic and acetic acids are major products of the Miller experiment and are probably also abundantly formed from CH4—N2 mixtures.)... 

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 ... 

Crash-Landing of the Murchison Meteorite The Miller Experiment Revisited... 

During the whole chemical evolution, water is the most important substance. As a reactant in the Miller experiment and the only solvent in Level 2, water directly participated in the early prebiotic chemical evolution. In addition, as a selector from Level 3 to Level 4, most of the macromolecules in Level 3 were screened out by the water environment. Water is the primary enviroimient of terrestrial life. 

As noted in the A Word about... Methane, Marsh Gas, and Millers Experiment on page 60, methane can be formed in muddy sediments because of the reducing environment (i.e., lack of oxygen). Write a balanced reaction for the conversion of glucose (C5H12O5, Sec. 2.9) with molecular hydrogen (H2) to form methane and water. 

Since ET takes place before the Marcus inverted region is reached, it is unlikely that the latter will be observed. The reaction rate remains at its maximum. The greatest chance to find the rednction of rate typical for the inverted region is to use intramolecular ET reactions. Alternatively, it is also possible to use solids or frozen solvents (J. R. Miller experiment). When such experiments were carried out, the inverted region appears. 

Albert Eschenmoser I think these aspects should not be taught at this moment simply because they are too uncertain. One should teach important fundamental facts and central ideas supported by experiments. What you say is often done, by the way. Students hear a lot about Miller experiments and such things. I think this is dangerous. Earlier, Duilio referred to probably the most beautiful case of constitutional self-assembly in which the adenine nucleus is formed. This is science which I believe fulfills the criteria of the type of phenomena which can be taught... 

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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