Retinal
1010189
205299373
2008-04-13T09:54:16Z
Wimvandorst
142418
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| ImageFile = Retinal structure.png
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| Formula = C<sub>20</sub>H<sub>28</sub>O
| PubChem = 1070
| SMILES =
| MolarMass= 284.436
| Appearance =
| CASNo = 116-31-4
| Density=
| MeltingPt= 63 °C
| BoilingPt=
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'''Retinal''', technically called '''retinene<sub>1</sub>''' or '''retinaldehyde''', is a light-sensitive [[retinene]] molecule found in the [[photoreceptor cell]]s of the [[retina]]. Retinal is the fundamental [[chromophore]] involved in the transduction of [[light]] into visual signals, i.e. nerve impulses, in the [[visual system]] of the [[central nervous system]].
==Overview==
The molecule that takes part in the initial step in the [[visual cycle|vision process]], [[rhodopsin]], has two components called 11-cis retinal and [[opsin]]. Retinal is a light-sensitive derivative of [[vitamin A]], and opsin is a protein molecule. Rhodopsin is found in the [[rod cell]]s of the eye. 11-cis retinal is a powerful absorber of light because it is a polyene; its 6 alternating single and double bonds make up a long conjugated electron network. When no light is present, the 11-cis retinal molecule is found in a "bent (cis) configuration" (''fig A''), and as such it is attached to the opsin molecule in a stable arrangement:
[[Image:RetinalCisandTrans.png|375px|left|thumb|'''Retinal molecule''' - straightens in response to a [[photon]] γ (light), of the correct wavelength]]
When light strikes the retina, a retinal molecule may absorb a [[photon]], promoting it into an excited electronic state. The nature of the excited state is not well understood, but it is known that within 200 [[femtosecond]]s it returns to the ground electronic state.<ref name='Van Dorst'>{{cite journal | title = Quantumchemische berekeningen aan retinal-modelstoffen | first = W.C.A. | last = van Dorst | coauthors = Buck, H.M., Dormans, G.J.M. | publisher = Eindhoven University of Technology, Faculty of Chemical Technology, Organic Chemistry dept | date = 28 Januari 1987 | language = Dutch | pages = 35 pages}}</ref> One third of these events cause no net change, while the remaining two thirds induce a rotation in the [[pi bond]] found between the eleventh and twelfth carbon atoms. In other words, the 11-cis retinal is transformed into the all-trans retinal (''fig B'') in a straightened configuration.<ref>{{cite book|author=Chang, Raymond |title=Chemistry, 6th Ed.|location=New York | publisher=McGraw Hill|year=1998|id=ISBN 0-07-115221-0}}</ref>
The all-trans retinal configuration, subsequently, does not fit into the binding site of the opsin molecule; as a result, upon [[isomerization]], the trans isomer separates from the protein, which triggers a [[G protein]] signaling pathway' including [[transducin]], that results in the generation of an [[electrical impulse]], which is transmitted through the [[optic nerve]] to the [[brain]] for processing.
It takes a minimum of five photons to trigger a nerve impulse.<ref>{{cite book|author=Feynman, Richard |title=QED - The Strange Theory of Light and Matter|location=Princeton, New Jersey | publisher=Princeton University Press|year=1985|id=ISBN 0-691-02417-0}}</ref> In the absence of light, enzymes mediate the isomerization of all-trans back to the 11-cis configuration, and rhodopsin is regenerated by a new formation of a [[Schiff base]] linkage, which actuates the binding of the cis isomer to opsin. This is the basic mechanism of the vision cycle.
All-trans-retinal is also an essential component of type I, or microbial, opsins such as [[bacteriorhodopsin]], [[channelrhodopsin]], and [[halorhodopsin]]. In these molecules, light causes the all-trans-retinal to become 13-cis retinal,<ref>J Photochem Photobiol B. 2002 Apr;66(3):188-94.</ref> which then cycles back to all-trans-retinal in the dark state.
<center>
{|align="center" class="wikitable"
|<center>[[Image:Cis-retinal-3D-balls.png|300px]]</center>||<center>[[Image:Trans-retinal-3D-sticks.png|300px]]</center>
|-
|<center>11-''cis''-retinal</center>||<center>all-''trans''-retinal</center>
|}
</center>
==History==
This photon induced retinal-bending mechanism was discovered in 1958 by the American biochemist [[George Wald]] and his co-workers. For his work, Wald won a share of the [[1967]] [[Nobel Prize in Physiology or Medicine]] with [[Haldan Keffer Hartline]] and [[Ragnar Granit]].<ref>[http://nobelprize.org/nobel_prizes/medicine/laureates/1967/ 1967 Nobel Prize in Medicine]</ref>
==See also==
*[[Sensory system]]
*[[Visual perception|Vision]]
*[[Visual cycle|The Visual Cycle]]
==References==
{{reflist}}
==External links==
*[http://www.accessexcellence.org/AE/AEC/CC/vision_background.html First Steps of Vision] - National Health Museum
*[http://www.chemistry.wustl.edu/~edudev/LabTutorials/Vision/Vision.html Vision and Light-Induced Molecular Changes]
*[http://palaeo-electronica.org/2000_1/retinal/vision.htm Retinal Anatomy and Visual Capacities]
*[http://www.ch.ic.ac.uk/vchemlib/mim/bristol/retinal/retinal_text.htm Retinal]
{{Carotenoids}}
[[Category:Photoreceptor cells]]
[[Category:Vision]]
[[Category:Signal transduction]]
[[de:Retinal]]
[[pl:Retinal]]
[[zh:视黄醛]]