Miller–Urey experiment

The experiment

The Miller–Urey experiment^[1] (or Miller experiment)^[2] was a chemical experiment that simulated the conditions thought at the time to be present on the early Earth, and tested the chemical origin of life under those conditions. The experiment supported Alexander Oparin's and J. B. S. Haldane's hypothesis that putative conditions on the primitive Earth favoured chemical reactions that synthesized more complex organic compounds from simpler inorganic precursors. Considered to be the classic experiment investigating abiogenesis, it was conducted in 1952^[3] by Stanley Miller, with assistance from Harold Urey, at the University of Chicago and later the University of California, San Diego and published the following year.^[4][5][6]
After Miller's death in 2007, scientists examining sealed vials preserved from the original experiments were able to show that there were actually well over 20 different amino acids produced in Miller's original experiments. That is considerably more than what Miller originally reported, and more than the 20 that naturally occur in life.^[7] More-recent evidence suggests that Earth's original atmosphere might have had a different composition from the gas used in the Miller experiment. But prebiotic experiments continue to produce racemic mixtures of simple to complex compounds under varying conditions.^[8]

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Experiment

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Descriptive video of the experiment

The experiment used water (H₂O), methane (CH₄), ammonia (NH₃), and hydrogen (H₂). The chemicals were all sealed inside a sterile 5-liter glass flask connected to a 500 ml flask half-full of liquid water. The liquid water in the smaller flask was heated to induce evaporation, and the water vapour was allowed to enter the larger flask. Continuous electrical sparks were fired between the electrodes to simulate lightning in the water vapour and gaseous mixture, and then the simulated atmosphere was cooled again so that the water condensed and trickled into a U-shaped trap at the bottom of the apparatus.
After a day, the solution collected at the trap had turned pink in colour.^[9] At the end of one week of continuous operation, the boiling flask was removed, and mercuric chloride was added to prevent microbial contamination. The reaction was stopped by adding barium hydroxide and sulfuric acid, and evaporated to remove impurities. Using paper chromatography, Miller identified five amino acids present in the solution: glycine, α-alanine and β-alanine were positively identified, while aspartic acid and α-aminobutyric acid (AABA) were less certain, due to the spots being faint.^[4]
In a 1996 interview, Stanley Miller recollected his lifelong experiments following his original work and stated: "Just turning on the spark in a basic pre-biotic experiment will yield 11 out of 20 amino acids."^[10]
As observed in all subsequent experiments, both left-handed (L) and right-handed (D) optical isomers were created in a racemic mixture. In biological systems, almost all of the compounds are non-racemic, or homochiral.
The original experiment remains today under the care of Miller and Urey's former student Jeffrey Bada, a professor at the UCSD, Scripps Institution of Oceanography.^[11] The apparatus used to conduct the experiment is on display at the Denver Museum of Nature and Science.^[12]

Chemistry of experiment

One-step reactions among the mixture components can produce hydrogen cyanide (HCN), formaldehyde (CH₂O),^[13][14] and other active intermediate compounds (acetylene, cyanoacetylene, etc.):^{[citation needed]}

CO₂ → CO + [O] (atomic oxygen)

CH₄ + 2[O] → CH₂O + H₂O

CO + NH₃ → HCN + H₂O

CH₄ + NH₃ → HCN + 3H₂ (BMA process)

The formaldehyde, ammonia, and HCN then react by Strecker synthesis to form amino acids and other biomolecules:

CH₂O + HCN + NH₃ → NH₂-CH₂-CN + H₂O

NH₂-CH₂-CN + 2H₂O → NH₃ + NH₂-CH₂-COOH (glycine)

Furthermore, water and formaldehyde can react, via Butlerov's reaction to produce various sugars like ribose.
The experiments showed that simple organic compounds of building blocks of proteins and other macromolecules can be formed from gases with the addition of energy.