# Miller-Urey experiment

Schematic experimental setup of the Miller-Urey experiment

The Miller-Urey experiment (also Urey-Miller experiment or Miller experiment ) serves to confirm the hypothesis that under the conditions of a postulated primordial atmosphere, the formation of organic molecules ( chemical evolution ) as they occur in living beings today is possible.

In 1953, Stanley Miller and Harold Clayton Urey simulated a hypothetical early earth atmosphere in the laboratory of the University of Chicago . He described the experiment in his publication: Production of amino acids under the possible conditions of a simple earth .

In the Miller-Urey experiment, simple chemical substances from a hypothetical early earth atmosphere -  water (H 2 O), methane (CH 4 ), ammonia (NH 3 ), hydrogen (H 2 ) and carbon monoxide (CO) - are mixed and this mixture is set electrical discharges, which are supposed to simulate the energy supply from thunderstorm lightning. As in the primordial atmosphere, there must be no free oxygen in the apparatus. In the experiment, organic molecules are created after a certain time. The analysis of the resulting mixture of molecules was carried out by means of chromatography .

## Results

With an initial quantity of 59,000 micromoles of CH 4 :

product formula Yield
(amount of substance in μmol)
C atoms Amount of substance
of carbon atoms in μmol
Formic acid H-COOH 2330 1 2330
Glycine * H 2 N-CH 2 -COOH 630 2 1260
Glycolic acid HO-CH 2 -COOH 560 2 1120
Alanine * H 3 C-CH (NH 2 ) -COOH 340 3 1020
Lactic acid H 3 C-CH (OH) -COOH 310 3 930
β-alanine H 2 N-CH 2 -CH 2 -COOH 150 3 450
acetic acid H 3 C-COOH 150 2 300
Propionic acid H 3 C-CH 2 -COOH 130 3 390
Iminodiacetic acid HOOC-CH 2 -NH-CH 2 -COOH 55 4th 220
Sarcosine H 3 C-NH-CH 2 -COOH 50 3 150
α-amino- n -butyric acid H 3 C-CH 2 -CH (NH 2 ) -COOH 50 4th 200
α-hydroxy- n -butyric acid H 3 C-CH 2 -CH (OH) -COOH 50 4th 200
Succinic acid HOOC-CH 2 -CH 2 -COOH 40 4th 160
urea H 2 N-CO-NH 2 20th 1 20th
N -methyl urea H 2 N-CO-NH-CH 3 15th 2 30th
3-azaadipic acid HOOC-CH 2 -NH-CH 2 -CH 2 -COOH 15th 5 75
N -methylalanine H 3 C-CH (NH-CH 3 ) -COOH 10 4th 40
Glutamic acid * HOOC-CH 2 -CH 2 -CH (NH 2 ) -COOH 6th 5 30th
Aspartic acid * HOOC-CH 2 -CH (NH 2 ) -COOH 4th 4th 16
α-aminoisobutyric acid H 3 C-C (CH 3 ) (NH 2 ) -COOH 1 4th 4th

Total:
4916   8945

In total, 18% of the methane molecules are converted into biomolecules, the rest is turned into a tar-like mass.

Originally carried out in 1953, this experiment has since produced comparable results in many variants. It is seen as evidence that the early Earth's atmosphere contained organic molecules in non-negligible concentrations.

Investigations carried out in 2008 on the original vessels used by Miller led to the identification of eight additional, mostly hydroxylated amino acids that had been overlooked by the analytical methods of the 1950s. However, the experiment cannot make any statements about how these molecules would have combined to form large structures.

## Modifications of the test conditions

• As a carbon source: carbon monoxide (CO) or carbon dioxide
• As a nitrogen source: molecular nitrogen N 2
• As an energy source: UV light and fire as a heat source

## Which the Miller experiment alone does not explain

• The amino acids produced as 1: 1 racemate , in the organisms but are mostly only the L -amino acids to find. The problem can be solved by using minerals as catalysts , which Miller did not use.
• In addition to some amino acids, compounds are also formed that are not found in organisms living today, for example the two amino acids β-alanine and sarcosine isomeric to alanine (see table). The absence of these compounds in today's organisms could possibly be explained by selection in the evolution of the metabolic pathways, whereby all variants except the amino acids used by organisms today have been eliminated.

## Reaction pathways in the Miller experiment

Initially, aldehydes (R – CHO) and hydrocyanic acid ( hydrogen cyanide HCN) are the first intermediate products from the starting materials .

In a subsequent multi-stage reaction, the aldehydes react with ammonia as a catalyst to form amino acids:

Sum equation:
${\ displaystyle \ mathrm {R {-} CHO + HCN + H_ {2} O \ longrightarrow H_ {2} N {-} CHR {-} COOH}}$
Aldehyde, hydrogen cyanide and water react to form the amino acid.

The amino acid glycine is created from the aldehyde methanal (HCHO ), and alanine is created from ethanal (CH 3 -CHO).

Sum equation:
${\ displaystyle \ mathrm {R {-} CHO + HCN + 2 \ H_ {2} O \ longrightarrow HO {-} CHR {-} COOH + NH_ {3}}}$
Aldehyde, hydrogen cyanide and water react to form α-hydroxy acids.

Glycolic acid (α-hydroxyethanoic acid) is formed from methanal, lactic acid (α-hydroxypropanoic acid) from ethanal and α-hydroxybutyric acid from propanal (CH 3 -CH 2 -CHO).

## Criticism of the requirements

The results of the experiment are reproducible. However, there are serious doubts as to whether the early Earth assumptions are realistic . The German chemist Günter Wächtershäuser makes this clear: “The theory of the prebiotic primordial soup faces devastating criticism because it is illogical, incompatible with thermodynamics , chemically and geochemically implausible, not in accordance with biology and biochemistry and experimentally refuted. "