Editing Retinal (section)

{{short description|Vitamin A aldehyde, a polyene chromophore}}
{{Distinguish|Retinol}}
{{About|the molecule|the anatomical feature|Retina}}
{{Chembox
| Watchedfields = changed
| verifiedrevid = 444085949
| Name = All-trans-retinal
| ImageFile = All-trans-Retinal.svg
| ImageSize = 250
| ImageAlt = Skeletal formula of retinal
| ImageFile1 = Retinal 3D ball.png
| ImageSize1 = 260
| ImageAlt1 = Ball-and-stick model of the retinal molecule
| IUPACName = Retinal
| SystematicName = (2''E'',4''E'',6''E'',8''E'')-3,7-Dimethyl-9-(2,6,6-trimethylcyclohex-1-en-1-yl)nona-2,4,6,8-tetraenal
| OtherNames = {{Bulleted list|Retinene|Retinaldehyde|Vitamin A aldehyde|RAL}}
|Section1={{Chembox Identifiers
| CASNo_Ref = {{cascite|correct|CAS}}
| CASNo = 116-31-4
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = RR725D715M
|  PubChem = 638015
|  ChemSpiderID = 553582
|  ChEBI = 17898
|  StdInChI = 1S/C20H28O/c1-16(8-6-9-17(2)13-15-21)11-12-19-18(3)10-7-14-20(19,4)5/h6,8-9,11-13,15H,7,10,14H2,1-5H3/b9-6+,12-11+,16-8+,17-13+
|  StdInChIKey = NCYCYZXNIZJOKI-OVSJKPMPSA-N
| SMILES = CC1=C(C(CCC1)(C)C)/C=C/C(=C/C=C/C(=C/C=O)/C)/C}}
|Section2={{Chembox Properties
| C=20 | H=28 | O=1
| Appearance = Orange crystals from [[petroleum ether]]<ref name="Merck">''Merck Index'', 13th Edition, '''8249'''</ref>
| Density =
| MeltingPtC = 61 to 64
| MeltingPt_ref = <ref name="Merck"/>
| BoilingPt = 
| Solubility = Nearly insoluble
| SolubleOther = Soluble
| Solvent = fat}}
|Section7={{Chembox Hazards
| MainHazards = 
| FlashPt = 
| AutoignitionPt = }}
|Section8={{Chembox Related
| OtherFunction = 
| OtherFunction_label = 
| OtherCompounds = [[retinol]]; [[retinoic acid]]; [[beta-carotene]]; [[dehydroretinal]]; 3-hydroxyretinal; 4-hydroxyretinal}}
}}

'''Retinal''' (also known as '''retinaldehyde''') is a [[polyene]] [[chromophore]]. Retinal, bound to proteins called [[opsin]]s, is the chemical basis of [[visual phototransduction]], the light-detection stage of [[visual perception]] (vision).

Some microorganisms use retinal to convert light into metabolic energy. One study suggests that approximately three billion years ago, most living organisms on Earth used retinal, rather than [[chlorophyll]], to convert sunlight into energy. Because retinal absorbs mostly green light and transmits purple light, this gave rise to the [[Purple Earth hypothesis]].<ref>{{Cite journal |last1=DasSarma |first1=Shiladitya |last2=Schwieterman |first2=Edward W. |date=2018 |title=Early evolution of purple retinal pigments on Earth and implications for exoplanet biosignatures |journal=International Journal of Astrobiology |language=en |publication-date=2018-10-11 |volume=20 |issue=3 |pages=241–250 |doi=10.1017/S1473550418000423 |s2cid=119341330 |issn=1473-5504|doi-access=free |arxiv=1810.05150 }}</ref>

Retinal itself is considered to be a form of [[vitamin A]] when eaten by an animal. There are many forms of vitamin A, all of which are converted to retinal, which cannot be made without them. The number of different molecules that can be converted to retinal varies from species to species. Retinal was originally called '''[[retinene]]''',<ref name=Wald1934>{{cite journal |last1=Wald |first1=George |title=Carotenoids and the Vitamin A Cycle in Vision |journal=Nature |date=14 July 1934 |volume=134 |issue=3376 |pages=65 |doi=10.1038/134065a0 |bibcode=1934Natur.134...65W |s2cid=4022911|doi-access=free }}</ref> and was renamed<ref name=Wald1968>{{cite journal |last1=Wald |first1=G. |title=Molecular basis of visual excitation |journal=Science |date=11 October 1968 |volume=162 |issue=3850 |pages=230–9 |pmid=4877437 |doi=10.1126/science.162.3850.230 |bibcode=1968Sci...162..230W}}</ref> after it was discovered to be '''vitamin A [[aldehyde]]'''.<ref name=Morton1944>{{cite journal |last1=MORTON |first1=R. A. |last2=GOODWIN |first2=T. W. |title=Preparation of Retinene in Vitro |journal=Nature |date=1 April 1944 |volume=153 |issue=3883 |pages=405–406 |doi=10.1038/153405a0 |bibcode=1944Natur.153..405M |s2cid=4111460}}</ref><ref name=Ball1946>{{cite journal |last1=Ball |first1=S. |last2=Goodwin |first2=T. W. |last3=Morton |first3=R. A. |title=Retinene1-vitamin A aldehyde. |journal=The Biochemical Journal |date=1946 |volume=40 |issue=5–6 |pages=lix |pmid=20341217}}</ref>

[[Vertebrate]] animals ingest retinal directly from meat, or they produce retinal from [[carotenoid]]s – either from [[alpha-Carotene|α-carotene]] or [[β-Carotene|β-carotene]] – both of which are [[carotene]]s. They also produce it from [[beta-Cryptoxanthin|β-cryptoxanthin]], a type of [[xanthophyll]]. These carotenoids must be obtained from plants or other [[photosynthetic]] organisms. No other carotenoids can be converted by animals to retinal. Some carnivores cannot convert any carotenoids at all. The other main forms of vitamin A – [[retinol]] and a partially active form, [[retinoic acid]] – may both be produced from retinal.

[[Invertebrate]]s such as [[insect]]s and [[squid]] use hydroxylated forms of retinal in their visual systems, which derive from conversion from other [[xanthophylls]].