Retinal

Not to be confused with Retinol.
This article is about the molecule. For the anatomical feature, see Retina.
All-trans-retinal
Skeletal formula of retinal
Ball-and-stick model of the retinal molecule
Names
IUPAC name
Retinal
Systematic IUPAC name
(2E,4E,6E,8E)-3,7-Dimethyl-9-(2,6,6-trimethylcyclohex-1-en-1-yl)nona-2,4,6,8-tetraenal
Other names
  • Retinene
  • Retinaldehyde
  • Vitamin A aldehyde
  • RAL
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.003.760 Edit this at Wikidata
UNII
  • InChI=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+
    Key: NCYCYZXNIZJOKI-OVSJKPMPSA-N
  • CC1=C(C(CCC1)(C)C)/C=C/C(=C/C=C/C(=C/C=O)/C)/C
Properties
C20H28O
Molar mass 284.443 g·mol−1
Appearance Orange crystals from petroleum ether[1]
Melting point 61 to 64 °C (142 to 147 °F; 334 to 337 K)[1]
Nearly insoluble
Solubility in fat Soluble
Related compounds
Related compounds
 retinol; retinoic acid; beta-carotene; dehydroretinal; 3-hydroxyretinal; 4-hydroxyretinal
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)

Retinal (also known as retinaldehyde) is a polyene chromophore. Retinal, bound to proteins called opsins, 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.[2]

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,[3] and was renamed[4] after it was discovered to be vitamin A aldehyde.[5][6]

Vertebrate animals ingest retinal directly from meat, or they produce retinal from carotenoids — either from α-carotene or β-carotene — both of which are carotenes. They also produce it from 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.

Invertebrates such as insects and squid use hydroxylated forms of retinal in their visual systems, which derive from conversion from other xanthophylls.

  1. ^ a b Merck Index, 13th Edition, 8249
  2. ^ DasSarma, Shiladitya; Schwieterman, Edward W. (2018). "Early evolution of purple retinal pigments on Earth and implications for exoplanet biosignatures". International Journal of Astrobiology. 20 (3) (published 2018-10-11): 241–250. arXiv:1810.05150. doi:10.1017/S1473550418000423. ISSN 1473-5504. S2CID 119341330.
  3. ^ Wald, George (14 July 1934). "Carotenoids and the Vitamin A Cycle in Vision". Nature. 134 (3376): 65. Bibcode:1934Natur.134...65W. doi:10.1038/134065a0. S2CID 4022911.
  4. ^ Wald, G (11 October 1968). "Molecular basis of visual excitation". Science. 162 (3850): 230–9. Bibcode:1968Sci...162..230W. doi:10.1126/science.162.3850.230. PMID 4877437.
  5. ^ MORTON, R. A.; GOODWIN, T. W. (1 April 1944). "Preparation of Retinene in Vitro". Nature. 153 (3883): 405–406. Bibcode:1944Natur.153..405M. doi:10.1038/153405a0. S2CID 4111460.
  6. ^ BALL, S; GOODWIN, TW; MORTON, RA (1946). "Retinene1-vitamin A aldehyde". The Biochemical Journal. 40 (5–6): lix. PMID 20341217.