Retinal
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| 11- cis -retinal (above), all- trans -retinal (below) | ||||||||||
| General | ||||||||||
| Surname | Retinal | |||||||||
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| Molecular formula | C 20 H 28 O | |||||||||
| Brief description |
orange-red crystals |
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| Molar mass | 284.44 g mol −1 | |||||||||
| Physical state |
firmly |
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| As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions . | ||||||||||
Retinal is one of the carotenoids and, together with other related compounds, represents vitamin A. It is the aldehyde of retinol and is therefore also called vitamin A aldehyde .
Biological function
Retinal is the chromophore of numerous light-sensitive proteins, the opsins . It is covalently bound as an imine to a lysine residue and then forms rhodopsin . In bacterial and animal rhodopsins, this binding site is on the seventh transmembrane helix.
In the rods of the retina , retinal is bound in rhodopsin or visual purple and is in the 11- cis retinal configuration . Light absorbed by the rods leads to a stretching of the retinal molecule (11- cis -retinal becomes all- trans -retinal), which triggers a signal-amplifying signal cascade , which via hyperpolarization of the receptor cell, if there is sufficient light, stimulates the optic nerve.
A slight hypovitaminosis manifests itself in reduced night vision. Greater deficiency leads to faster eye fatigue, night blindness and cornification of the visual cells of the eye.
Visual process
Two isomeric forms of the retinal are important in the visual process: the 11- cis -retinal and the all- trans -retinal. 11- cis -retinal is the light-sensitive component of rhodopsin - the molecule , which in the rods of the eye (see retina ) the key for the neutral color See photosensitive chromophore is. By absorbing light, the 11- cis retinal is converted into the all- trans configuration, in which it separates from the protein component of rhodopsin, scotopsin.
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| 11- cis -retinal takes up a light quantum and rearranges to the all- trans -isomer. |
This triggers a signal transduction cascade , through which the light signal is converted into an electrochemical signal, the receptor potential of the rod cell, by way of the uptake by the retinal and the decay of the rhodopsin. The resulting all- trans -retinal, on the other hand, is then, provided no further light is incident, by an enzyme , the so-called. Retinal isomerase , converted back into the cis form, in which it can combine with the remaining protein component to form a new, “receptive” rhodopsin molecule.
The importance of the retinal to eyesight is also the reason why it is popularly said that carrots are good for the eyes. The β-carotene in many yellow and reddish colored vegetables is required for the formation of retinal. all- trans -Retinal is the aldehyde of vitamin A 1 ( retinol ) and is therefore closely related to it.
Other isomeric forms of retinal are known, but have no significant significance either in nature or in technology.
Individual evidence
- ↑ a b c Entry on retinal. In: Römpp Online . Georg Thieme Verlag, accessed on December 22, 2014.
- ↑ a b data sheet all trans-Retinal from Sigma-Aldrich , accessed on April 22, 2011 ( PDF ).
- ↑ OP Ernst, DT Lodowski, M. Elstner, P. Hegemann, LS Brown, H. Kandori: Microbial and animal rhodopsin: structures, functions, and molecular mechanisms. In: Chemical Reviews . Volume 114, number 1, January 2014, pp. 126-163, doi : 10.1021 / cr4003769 , PMID 24364740 , PMC 3979449 (free full text) (review).
- ^ A b Siegfried Hauptmann: Organic chemistry. 2nd Edition. German publishing house for basic industry, Leipzig 1988, ISBN 3-342-00280-8 , pp. 774–775.