Editing Miller–Urey experiment (section)

==Chemistry of experiment==
In 1957 Miller published research describing the chemical processes occurring inside his experiment.<ref name="Miller-1957">{{Cite journal |last=Miller |first=Stanley L. |date=1957-01-01 |title=The mechanism of synthesis of amino acids by electric discharges |url=https://dx.doi.org/10.1016/0006-3002%2857%2990366-9 |journal=Biochimica et Biophysica Acta |volume=23 |issue=3 |pages=480–489 |doi=10.1016/0006-3002(57)90366-9 |pmid=13426157 |issn=0006-3002|url-access=subscription }}</ref> Hydrogen cyanide (HCN) and [[aldehyde]]s (e.g., formaldehyde) were demonstrated to form as intermediates early on in the experiment due to the electric discharge.<ref name="Miller-1957" /> This agrees with current understanding of [[atmospheric chemistry]], as HCN can generally be produced from reactive [[Radical (chemistry)|radical species]] in the atmosphere that arise when CH<sub>4</sub> and nitrogen break apart under [[Ultraviolet light|ultraviolet (UV) light]].<ref name="Sullivan-2007">{{Cite book |last1=Sullivan |first1=Woodruff Turner |title=Planets and life: the emerging science of astrobiology |last2=Baross |first2=John A. |date=2007 |publisher=Cambridge University Press |isbn=978-0-521-82421-7 |location=Cambridge}}</ref> Similarly, aldehydes can be generated in the atmosphere from radicals resulting from CH<sub>4</sub> and H<sub>2</sub>O decomposition and other intermediates like [[methanol]].<ref name="Ferris-1975">{{Cite journal |last1=Ferris |first1=J. P. |last2=Chen |first2=C. T. |date=1975 |title=Chemical evolution. XXVI. Photochemistry of methane, nitrogen, and water mixtures as a model for the atmosphere of the primitive earth |url=https://pubs.acs.org/doi/abs/10.1021/ja00844a007 |journal=Journal of the American Chemical Society |language=en |volume=97 |issue=11 |pages=2962–2967 |doi=10.1021/ja00844a007 |pmid=1133344 |bibcode=1975JAChS..97.2962F |issn=0002-7863|url-access=subscription }}</ref> Several energy sources in planetary atmospheres can induce these dissociation reactions and subsequent hydrogen cyanide or aldehyde formation, including lightning,<ref>{{Cite journal |last1=Rimmer |first1=P. B. |last2=Helling |first2=Ch |date=2016-05-23 |title=A Chemical Kinetics Network for Lightning and Life in Planetary Atmospheres |journal=The Astrophysical Journal Supplement Series |volume=224 |issue=1 |pages=9 |doi=10.3847/0067-0049/224/1/9 |arxiv=1510.07052 |bibcode=2016ApJS..224....9R |issn=1538-4365 |doi-access=free }}</ref> ultraviolet light,<ref name="Sullivan-2007" /> and [[Cosmic ray|galactic cosmic rays]].<ref>{{Cite journal |last=Huntress |first=W. T. |date=1976 |title=The chemistry of planetary atmospheres |url=https://pubs.acs.org/doi/abs/10.1021/ed053p204 |journal=Journal of Chemical Education |language=en |volume=53 |issue=4 |pages=204 |doi=10.1021/ed053p204 |bibcode=1976JChEd..53..204H |issn=0021-9584|url-access=subscription }}</ref>

For example, here is a set [[Photochemistry|photochemical]] reactions of species in the Miller-Urey atmosphere that can result in formaldehyde:<ref name="Ferris-1975" />

: H<sub>2</sub>O + ''[[Photon energy|hv]]'' → H + OH<ref>{{Cite journal |last1=Getoff |first1=N. |last2=Schenck |first2=G. O. |date=1968 |title=PRIMARY PRODUCTS OF LIQUID WATER PHOTOLYSIS AT 1236, 1470 AND 1849 Å |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1751-1097.1968.tb05859.x |journal=Photochemistry and Photobiology |language=en |volume=8 |issue=3 |pages=167–178 |doi=10.1111/j.1751-1097.1968.tb05859.x |s2cid=97474816 |issn=0031-8655|url-access=subscription }}</ref>
: CH<sub>4</sub> + OH → CH<sub>3</sub> + HOH<ref>{{Cite journal |last=Wilson |first=Wm. E. |date=1972-04-01 |title=A Critical Review of the Gas-Phase Reaction Kinetics of the Hydroxyl Radical |url=https://doi.org/10.1063/1.3253102 |journal=Journal of Physical and Chemical Reference Data |volume=1 |issue=2 |pages=535–573 |doi=10.1063/1.3253102 |bibcode=1972JPCRD...1..535W |issn=0047-2689|url-access=subscription }}</ref>
: CH<sub>3</sub> + OH → CH<sub>3</sub>OH<ref>{{Cite journal |last1=Greenberg |first1=Raymond I. |last2=Heicklen |first2=Julian |date=1972 |title=The reaction of O( 1 D ) with CH 4 |url=https://onlinelibrary.wiley.com/doi/10.1002/kin.550040406 |journal=International Journal of Chemical Kinetics |language=en |volume=4 |issue=4 |pages=417–432 |doi=10.1002/kin.550040406 |issn=0538-8066|url-access=subscription }}</ref>
: CH<sub>3</sub>OH + ''hv'' → CH<sub>2</sub>O (formaldehyde) + H<sub>2</sub><ref>{{Cite journal |last1=Hagege |first1=Janine |last2=Leach |first2=Sydney |last3=Vermeil |first3=Catherine |date=1965 |title=Photochimie du méthanol en phase vapeur A 1 236 et A 1 849 Å |url=https://jcp.edpsciences.org/articles/jcp/abs/1965/01/jcp196562p736/jcp196562p736.html |journal=Journal de Chimie Physique |language=fr |volume=62 |pages=736–746 |doi=10.1051/jcp/1965620736 |bibcode=1965JCP....62..736H |issn=0021-7689|url-access=subscription }}</ref>

[[File:Evidence_for_Strecker-type_amino_acid_synthesis_in_the_Miller-Urey_experiment.webp|thumb|392x392px|A) [[Cyanohydrin reaction|Cyanohydrin]] (top) and [[Strecker amino acid synthesis|Strecker]] (bottom) schemes for synthesis of hydroxy acids and amino acids, respectively. B) Concentrations of [[ammonia]], [[aldehyde]]s, [[hydrogen cyanide]], and [[amino acid]]s during a Miller–Urey experiment, reproduced from [[Stanley Miller|Miller]] (1957)<ref name="Miller-1957" /> by Cleaves (2012).<ref>{{Cite journal |last=Cleaves |first=H. James |date=2012 |title=Prebiotic Chemistry: What We Know, What We Don't |journal=Evolution: Education and Outreach |language=en |volume=5 |issue=3 |pages=342–360 |doi=10.1007/s12052-012-0443-9 |issn=1936-6434|doi-access=free }}</ref> The concentrations of aldehydes and hydrogen cyanide during amino acid production in aqueous solution provided strong evidence that Strecker synthesis occurs in Miller-Urey chemical environments. Production of hydroxy acids through the cyanohydrin scheme also likely occurs. From: Cleaves, H.J. [[doi:10.1007/s12052-012-0443-9|Prebiotic Chemistry: What We Know, What We Don't]]. ''Evo Edu Outreach'' 5, 342–360 (2012). Licensed under [[CC-BY 2.0]].]]
A photochemical path to HCN from NH<sub>3</sub> and CH<sub>4</sub> is:<ref>{{Cite journal |last=Hu |first=Renyu |date=2021-11-01 |title=Photochemistry and Spectral Characterization of Temperate and Gas-rich Exoplanets |journal=The Astrophysical Journal |volume=921 |issue=1 |pages=27 |doi=10.3847/1538-4357/ac1789 |arxiv=2108.04419 |bibcode=2021ApJ...921...27H |issn=0004-637X |doi-access=free }}</ref>

: NH<sub>3</sub> + ''hv'' → NH<sub>2</sub> + H
: NH<sub>2</sub> + CH<sub>4</sub> → NH<sub>3</sub> + CH<sub>3</sub>
: NH<sub>2</sub> + CH<sub>3</sub> → CH<sub>5</sub>N
: CH<sub>5</sub>N + ''hv'' → HCN + 2H<sub>2</sub>

Other active intermediate compounds ([[acetylene]], [[cyanoacetylene]], etc.) have been detected in the aqueous solution of Miller–Urey-type experiments,<ref>{{Cite journal |last=Orgel |first=Leslie E. |date=2004 |title=Prebiotic Adenine Revisited: Eutectics and Photochemistry |url=https://doi.org/10.1023/B:ORIG.0000029882.52156.c2 |journal=Origins of Life and Evolution of the Biosphere |language=en |volume=34 |issue=4 |pages=361–369 |bibcode=2004OLEB...34..361O |doi=10.1023/B:ORIG.0000029882.52156.c2 |pmid=15279171 |s2cid=4998122|url-access=subscription }}</ref> but the immediate HCN and aldehyde production, the production of amino acids accompanying the plateau in HCN and aldehyde concentrations, and slowing of amino acid production rate during HCN and aldehyde depletion provided strong evidence that [[Strecker amino acid synthesis]] was occurring in the aqueous solution.<ref name="Miller-1957" />

Strecker synthesis describes the reaction of an aldehyde, ammonia, and HCN to a simple amino acid through an [[aminoacetonitrile]] intermediate:

: CH<sub>2</sub>O + HCN + NH<sub>3</sub> → NH<sub>2</sub>-CH<sub>2</sub>-CN (aminoacetonitrile) + H<sub>2</sub>O
: NH<sub>2</sub>-CH<sub>2</sub>-CN + 2H<sub>2</sub>O → NH<sub>3</sub> + NH<sub>2</sub>-CH<sub>2</sub>-COOH ([[glycine]])

Furthermore, water and formaldehyde can react via [[Formose reaction|Butlerov's reaction]] to produce various sugars like [[ribose]].<ref>{{Cite journal |last=Mense |first=Thorben H. |date=December 2019 |title=A Closer Look at Reactions in the Miller-Urey-Experiment using Coupled Gas Chromatography – Mass Spectrometry |url=https://www2.physik.uni-bielefeld.de/fileadmin/user_upload/theory_e6/Master_Theses/MasterMiller1912.pdf |journal=Bielefeld University}}</ref>

The experiments showed that simple organic compounds, including the building blocks of proteins and other macromolecules, can abiotically be formed from gases with the addition of energy.