All-frans-Retinal

Rhodopsin is a seven ot-helix trans-membrane protein and visual pigment of the vertebrate rod photoreceptor cells that mediate dim light vision. In this photoreceptor, retinal is the chromophore bound by opsin protein, covalently linked to Lys296 by a Schiff base linkage. Kpega et al.64 have studied NMR spectra of Schiff bases being derivatives of all-frans retinal and amino-p-cyclodextrins as a model of rhodopsin, where p-cyclodextrin plays a role of a binding pocket. On the basis of analysis of the chemical shift differences for the model compound in the presence and in the absence of adamantane carboxylate, it has been shown that the derivative of 3-amino-p-cyclodextrin forms dimer in water and retinoid is inserted into p-cyclodextrin cavity [31]. 

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. 

C2g isoprenoids Phytol All frans-retinal Photoautotrophs Proteobacteria... 

A stack of about 1 000 disks in each rod cell contains the lightsensing protein rhodopsin,10 in which the chromophore 11-o i-retinal (from vitamin A) is attached to the protein opsin. When light is absorbed by rhodopsin, a series of rapid transformations releases all-frans-retinal. At this stage, the pigment is bleached (loses all color) and cannot respond to more light until retinal isomerizes back to the 11 -cis form and recombines with the protein. 

Figure 23-46 The photoreaction cycle of bacteriorhodopsin. After Bullough and Henderson.585 The subscript numbers indicate the wavelengths of maximum absorption of each intermediate and the approximate lifetimes are given by the arrows. Resting bacteriorhodopsin as well as intermediates J and O have all-frans retinal but K through N are thought to all be 13-cz s. A proton is transferred from L to aspartate 85 and then to the exterior surface of the membrane. A proton is taken up from the exterior surface via aspartate 96 to form N.

Figure S2.4 shows the structures of 11 -c/.v-retinal and its more stable isomer all-frans-retinal. The reti-nals are related to the alcohol retinol, or vitamin A,. Mammals cannot synthesize these compounds de novo but can form them from dietary carotenoids such as /3-carotene. A deficiency of vitamin A causes night blindness, along with serious deterioration of the eyes and other tissues.

Metarhodopsin II is then recycled back into rhodopsin by a multi-step sequence involving cleavage to all-frans-retinal and cis-trans isomerization back to ll-cts-retinal. 

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. 

In 1958 the American biochemist George Wald and his co-workers discovered that visible light isomerizes 11-cm retinal to ail-trans retinal by breaking a carbon-carbon pi bond. With the pi bond broken, the remaining carbon-carbon sigma bond is free to rotate and does so. Within 200 femtoseconds after it has absorbed a photon, the 11-cm retinal is transformed into all-fran retinal. 

The all-fran retinal does not fit into the 11-cm retinal binding site on opsin therefore, npon isomerization the trans isomer separates from the protein. At this point an electrical impulse is generated and transmitted to the brain. In the absence of light, enzymes mediate the isomerization of all-fran retinal back to 11-cm retinal, and rhodopsin is regenerated by the binding of the cis isomer to opsin, as described above. With the completion of this step, the vision cycle can begin again. 

Other recent examples of CIDNP studies on olefin isomerizations via reverse electron transfer populating the triplet, which all fall into class Ila, concerned all-frans retinal with stilbene as donor or a quinone as an acceptor, and a, P unsaturated ketones with triphenylamine or triphenylphosphine as donors °... 

The detection of light involves the initial conversion of 11-ds-retinal to its all-tmns isomer. This is the only obvious role of light in this process. The high energy of a quantum of visible light promotes the fission of the tt bond between carbons 11 and 12. When the tt bond breaks, free rotation about the a bond in the resulting radical is possible. When the ir bond re-forms after such rotation, all-frans-retinal results. All-irans-retinal is more stable than 11-czs-retinal, which is the reason the isomerization proceeds spontaneously in the direction shown in the following equation. 

The two molecules have different shapes because of their different structures. The 11-ds-retinal has a fairly curved shape, and the parts of the molecule on either side of the cis double bond tend to lie in different planes. Because proteins have complex and specific three-dimensional shapes (tertiary structures), 11-ds-retinal associates with the protein opsin in a particular manner. All-frans-retinal has an elongated shape, and the entire molecule tends to lie in a single plane. This different shape for the molecule, compared with that for the 11-ds isomer, means that all-frans-retinal will have a different association with the protein opsin. 

In fact, all-frans-retinal associates very weakly with opsin because its shape does not fit the protein. Consequently, the next step after the isomerization of retinal is the dissociation of all-frans-retinal from opsin. The opsin protein imdergoes a simultaneous change in conformation as the all-frans-retinal dissodates. 

At some time after the 11-ds-retinal-opsin complex receives a photon, a message is received by the brain. It was originally thought that either the isomerization of 11-czs-retinal to all-frans-retinal or the conformational change of the opsin protein was an event that generated the electrical message sent to the brain. Current research. 

Wald was also able to explain the sequence of events by which the rhodopsin molecules are regenerated. After dissociation of all-frans-retinal from the protein, the following enzyme-mediated changes occur. All-frans-retinal is reduced to the alcohol all-frans-retinol, also called all-frans-Vitamin A. 

By this process, as little light as 10 of the number of photons emitted from a typical flashlight bulb can be detected. The conversion of light into isomerized retinal exhibits an extraordinarily high quantum efficiency. Virtually every quantum of light absorbed by a molecule of rhodopsin causes the isomerization of 11-ds-retinal to all-frans-retinal. 

FIGURE 7-21 The first step in vision occurs when light converts 11-c/s-retinal into its isomer, all-frans-retinal. 

The different dependence on orbital symmetries makes two-photon spectroscopy a useful technique for studying some excited states that are not readily accessible by one-photon excitation. Birge [87] used two-photon excitation to explore the 2 A excited state of retinyl derivatives (Box 4.12) in solution and bound to rhodopsin. Excitation to this state from the ground state is, to a first approximation, forbidden in a one-photon transition but is allowed as a two-photon transition. Comparisons of the one- and two-photon absorption spectra of the unprotonated Schiff base of all-frans-retinal in solution showed that the 2 A state lies below I B. Protonating the Schiff base moves 1 down in energy and inverts the order. In rhodopsin containing a locked 11-cis-retinyl derivative that was unable to undergo photoisomerization, 1 B+ was found to lie below 2 A, in accord with other indications that the Schiff base is protonated [87, 90]. 

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