Retinal imine

The eyes of arthropods, mollusks, and vertebrates use the cis-trans isomerization reaction to detect light. When light enters the eye, it is absorbed by an imine of 11-cA-retinal, which isomerizes to the lower energy n -lrans-retinal imine. The isomerization is detected by various enzymes that initiate an electrical impulse that enters the brain via the optic nerve. Meanwhile, the all-frans-retinal is transported to the liver ( ), where the enzyme retinal iso-merase uses acid catalysis and ATP to convert it back to the higher energy 11 -cis form. The 11 -r/.v-retinal is then sent back to the eye, ready to receive the next photon. 

Restricted Hartree-Fock theory (RHF), 23, 234-236 energy, 35 operator, 35 Retinal imine, 270, 272 Rhodopsin, 270, 272... 

In the biochemistry of vision (see Chapter 17), the interaction of rhodopsin, a GCPR, with light results in the photo-induced isomerization of cis retinal imine to trans retinal imine. This causes conformational changes in rhodopsin, which ultimately result in the closing of an ion channel, polarization of the cell membrame, and a nerve impulse that is transmitted to the brain in vision. 

FIGURE 17 11 Imine formation between the aldehyde function of 11 as retinal and an ammo group of a protein (opsin) is involved in the chemistry of vision The numbering scheme in retinal is specifically developed for carotenes and related compounds... 

The values of the 15N CP MAS chemical shift of Lys296 nitrogen bonded to retinal via the —C=N bond ( Schiff base) was equal to 155.4 ppm for rhodopsin and 282.8 ppm for metarhodopsin (relative to 5.6 M aqueous NH4C1).70 The results proved the imine bond polarisation, which facilitates Schiff base hydrolysis. The comparison between chemical shifts for metarhodopsin and model compounds suggested that Schiff base linkage of the all-frans retinal chromophore in Metall is in a polar environment. 

A deficiency of vitamin A leads to vision defects, including a visual impairment at low light levels, termed night blindness. For the processes of vision, retinol needs to be converted first by oxidation into the aldehyde retinal, and then by enzymic isomerization to cw-retinal. c -Retinal is then bound to the protein opsin in the retina via an imine linkage (see Section 7.7.1) to give the red visual pigment rhodopsin. 

The structure of opsin is unknown, but its prosthetic group (1 l-c/.v-retinal) is bonded to it through an imine (Schiff base) function formed between the aldehyde group of the retinal and the side-chain amino function of a lysine unit of opsin ... 

There ensues a series of dark reactions or conformational changes that have the effect of greatly activating the imine linkage of the all-frans-rhodopsin towards hydrolysis. On hydrolysis, all-frawj-retina] is released and is unable to recombine with opsin until it is reconverted to the 11-cis isomer. The trans-to-cis rearrangement is a thermal rather than a photochemical reaction and is catalyzed by the enzyme retinal isomerase. The cycle of reactions is summarized in Figure 28-13. 

One example of this process is found in the chemistry of vision. Ill the rods and cones of the eye, the aldehyde 11 -cis-retinal forms an imine by reaction with an amino group of the protein opsin. Studies have shown that the nitrogen of the imine of the product, called rhodopsin. is protonated. 

When rhodopsin absorbs light in the vision process, the cis double bond between carbons 11 and 12 isomerizes to a trans double bond. This isomerization triggers a nerve impulse telling the brain that light has been absorbed by the eye. The imine of the isomerized product is unstable and is hydrolyzed to opsin and the all-trans form of retinal (also known as vitamin A aldehyde). All-trans retinal is converted back to 11 -c/s-retinal by enzymes so that it can be used again in rhodopsin formation. 

The human eye uses a c/s-alkene, 11-c/s-retinal, to detect light, and a cis-trans isomerism reaction is at the heart of the chemical mechanism by Which we see. The light-sensitive pigment in the cells of the retina is an imine, formed by reaction of ll-ci ... 

The pyrolysis and photolysis of fran5-retinoic acid in aqueous ethanol has been studied/ Mechanisms of thermal equilibration of retinylidene imines (Schiff bases) and their protonated immonium salts have been investigated Electrochemical studies on trans- and cw-retinal have been reported/ ... 

Many imines play vital roles in biological systems. A key molecule in the chemistry of vision is the highly conjugated imine rhodopsin, which is synthesized in the rod cells of the eye from 11-cis-retinal (the molecule introducing Chapter 21) and a 1° amine in the protein opsin. 

Rearrangements.- Examples of, E-photoisomerization in imines have again been reported. The photoisomerizations of methanimine and of fluorinated methanimines have been examined and the study of the photoisomerization of protonated and unprotonated imines of 9-cis-, 11-cis-, 15-cis-, and all trans-retinals has attracted attention in view of the continuing interest in the mechanism of the visual process. Evidence from flash photolysis studies indicates that the photoisomerization of the E-hydrazone (1) to... 

The retina of the eye contains cone cells and rod cells. The cone cells are responsible for color vision and for vision in bright light. The rod cells are responsible for vision in dim light. In rod cells, vitamin A is oxidized to an aldehyde and the trans double bond at C-11 is isomerized to a cis double bond. The mechanism for the enzyme-catalyzed interconversion of cis and trans double bonds is discussed in Section 18.15. The protein opsin uses a lysine side chain (Lys 216) to form an imine with (llZ)-retinal, resulting in a complex known as rhodopsin. When rhodopsin... 

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