Retinal isomers

Using single-frequency and noise-modulated resonance and off-resonance proton decoupling, 7] relaxation time measurements, relaxation reagents like Gd (fod)3 and specifically deuterated compounds, all the carbons in retinal isomers, the model compounds a-and /i-ionone, and vitamin A and its isomers [165, 555-557] were assigned. The olefinic ring carbons (C-5 and C-6) could be identified on the assumption that the 13C relaxation times are dominated by intramolecular dipole-dipole interactions with neighboring protons and that the same rotational correlation time characterizes the interactions for both carbons. Consequently the ratio of T/s for C-5 and C-6 can be estimated from eq. (5.1)... 

Studies of retinal isomers and retinal analogs in solution were useful in that they permitted to speculate on the conformation of the opsin-bound 1 l-c/s-retinal, an important problem in the stereochemistry of vision. Although there was a priori no reason to believe that the conformation of 1 l-cis-retinal when bound to opsin will be identical either to its solution conformation or to its conformation in the crystalline... 

Figure 2. Absorption spectra of retinal isomers and rhodopsins. [Retinal spectra (in hexane at room temperature) are reproduced from refs. 52, 168, and 174.] The spectra of rhodopsin and iso-rhodopsin (A > 350 nm) are for digitonin-solubilized preparations in aqueous glycerol mixtures at 4°K (ref. 287), and room temperature (A < 350 nm. ref. 6). (Those of light-adapted (BrML) and dark-adapted (BRjjgg) bacteriorhodopsin, both for aqueous7membrane suspensions at room temperature, are reproduced from refs. 259 and 377.

RetinalS. The structure and photophysics of rhodopsins are intimately related to the spectroscopic properties of their retiny1-polyene chromophore in its protein-free forms, such as the aldehyde (retinal), the alcohol (retinol or vitamin A), and the corresponding Schiff bases. Since most of the available spectroscopic information refers to retinal isomers (48-55), we shall first center the discussion on the aldehyde derivatives. Three bands, a main one (I) around 380 nm and two weaker transitions at 280 nm and 250 nm (II and III), dominate the spectrum of retinals in the visible and near ultraviolet (Fig. 2). Assignments of these transitions are commonly made in terms of the lowest tt, tt excited states of linear polyenes, the spectroscopic theories of which have been extensively discussed in the past decade (56-60). In terms of the idealized C2h point group of, for example, all-trans butadiene, transitions are expected from the Ta ground state to B , A, and A" excited states... 

Two additional noteworthy features of the Bu - Ag transition which have led to important structural information are the 14-nm shift observed for all retinal isomers upon cooling to 77°K (51,53) and the broad, structureless nature of the band. 

Schiff Bases. The suggestion that a protonated Schiff base is the primary form of the retinal-opsin binding in natural pigments has stimulated considerable work on the spectroscopic properties of free Schiff bases of retinal (RSB) in solution, especially in their protonated forms (PRSB). Schiff base formation does not alter substantially the spectrum of retinal isomers, being associated with a 20-nm blue shift in the position of the main band, I. Except for the absence of the low-energy (n, tr ) transition, theory predicts only small changes in the location of all other states (75,121). This 1 s been confirmed by the extensive experimental study of Schaffer et al. (75). It appears from theoretical calculations (121) that Schiff bases lack Ag-lfiJ... 

With the exception of photodissociation to radical ions observed for retinol in polar solvents (144), cis-trans isomerization is the major photochemical transformation undergone by all forms of the free retinyl-polyene chromophore. [Unidentified photochemical damage has been reported to occur with very low quantum yields, e.g., 0.04 in the case of all-trans retinal (177).] We shall subsequently see that critical comparisons between the photochemical behavior of the biopigments and that of the opsin-free chromophore have led to the conclusion that the protein moiety plays a major role in governing the photochemical mechanism in rhodopsin (176). It is, therefore, natural that in parallel to spectroscopic and theoretical investigations, considerable attention has been devoted to the photoisomerization of model compounds, particularly to that of retinal isomers. 

Mechanism (3) is consistent with the observation of two distinct T-T spectra for 11-cis and all-trans retinal (169,175). An additional argument against a "common triplet" or "equilibrated triplets" (171,182) is based on the observation (177,183) that the photosensitized isomerization of different retinal isomers (e.g., all-trans, 11-cis, and 13-cis) leads in each case to a different product distribution (13-cis, trans/9,13 dicis, and... 

As discussed in the preceding sections, there is strong evidence from fluorescence and ISC data showing that the l(n, it ) state is the lowest for all retinal isomers in nonpolar solvents, while a weakly emissive (n, it ) state becomes the lowest in polar solutions. Thus the (n, 7T ) state, rather than a ( IT, tt ) state, may have to be considered in the case of isomerization in nonpolar solvents. Moreover, in methanol the photoisomerization behavior of 11-cis retinal is similar to that of the 9-cis, 13-cis, and all-trans isomers. This is inconsistent with assigning a lowest Bj excited state to the 11-cis isomer which would lead to very different fluorescence and ISC patterns as compared with those of the other cis isomers characterized by a lowest Ag excited state. 

CD Spectra. In spite of their being twisted about the C -Cj single bond (262,263), retinal isomers have no optical activity in solution, presumably because of the presence of comparable amounts of both enantiomers of each of the twisted forms. However, both rhodopsin and bacteriorhodopsin are optically active in their visible and uv transitions. The origin of the optical activity in rhodopsin (264-276) is still subject to speculation. 

Recently Inoue and coworkers also reported ab initio study of shieldings for linear jr-conjugated systems. A photoreceptive protein such as rhodopsin (Rh) or bacte-riorhodopsin (bR) possesses a retinal isomer bound to a lysine residue via the protonated Schiff base linkage. Rh exists in the rod cell of the retina of vertebrate and possesses 11 -c/s-rcli iial (Figure 2), which is isomerized into the al l-Zraw.v form by the absorption of photons, finally leading to signal transduction. 

Figure 3.9 shows the effects of double deuteration of the C7=C8 or C11=C12 double bond and that of the C14—C15 single bond on the triplet-state CTI starting from the set of four ds isomers. The results can be summarized as follows (1) The 7,8-deuteration (7,8-D2) reduces the quantum yield of isomerization from the 7-cis to the all-trans isomer that includes rotation around the particular double bond to which deuterium substitution was made, and also, the quantum yield of isomerization from the 9-cis to the all-trans isomer around the neighboring double bond on the right-hand side of the retinal molecule (see Scheme 3.1). (2) The 11,12-deu-teration (11,12-D2) reduces the quantum yields of isomerization from the 7-cis, 9-cis, and 11-cis isomers to the all-trans isomer that include rotation around the particular cis-double bond to which deuterium substitution was made, and also, that around the neighboring double bonds on the left-hand side of the molecule. (3) The 14,15-deuteration (14,15-D2) slightly reduces the quantum yield of isomerization from the 11-cis isomer. (4) Practically no deuteration effects on the quantum yields of isomerization are seen at all starting from the 13-cis isomer [13]. Table 3.1 lists the quantum yields of isomerization per triplet species generated for the undeuterated and variously deuterated retinal isomers.

Hie photophysics and photochemistry of several retinal isomers have been studied with respect to the potential energy surfaces of isomerization by transient spectroscopy as well as product analyses [127-129]. 

Review the material on the chemistry of vision and, with respect to the isomers of retinal, discuss the changes in structure that occur as the nerve impulses (that result in vision) are produced. Provide complete structural formulas of the retinal isomers that you discuss. 

The conversion of a cis isomer to the trms isomer (or vice versa) is a process called cis-trans isomerization. Such conversions are crucial in the vision process. The molecules in the retina that respond to light are rhodopsin, which has two components called 11-cw-retinal and opsin. Retinal is the light-sensitive component, and opsin is a protein molecule. Upon receiving a photon in the visible region, 11-cw-retinal isomer-izes to all-trans retinal by breaking a carbon-carbon tt bond. With the TT bond broken, the atoms connected by the carbon-... 

In three separation schemes, retinyl, retinal, and retinol palmitate isomos were analyzed [665]. A silica colunm was used for all separations. Seven retinal isomers (7-, 9-, 11-, n-cis, 7,9-, ll,13-di-c/s, and all-rnms) were eluted and baseline resolved in 18 min with a 97/3 -heptane/methyl /-butyl ether (A = 371nm) mobile phase. The seven retinol isomers (9-, 11-, 13-cis, 7,9-, 7,13-, 9,13-, 11,13-di-cis, 9,11,13-tri-cij, and all-/ru s) were incompletely resolved and eluted in 22 min using a 94/6 n-heptane/methyl /-butyl ether (2 = 325 nm) mobile phase. Finally, retinyl isomers (9-, 11-, 13-c , 7,9-9,13-di-c/y) were incompletely resolved in 5 min using a 99/1 n-heptane/methyl /-butyl ether (A = 325 nm) mobile phase. For this study 0.1 ng injections were made. 

An enzyme (isomerase) converts the trans isomer back to the cis-11-retinal isomer and the rhodopsin re-forms. If there is a deficiency of rhodopsin in the rods of the retina, night blindness may occur. One common cause is a lack of vitamin A in the diet. In our diet, we obtain vitamin A from plant pigments containing /3-carotene, which is found in foods such as carrots, squash, and spinach. In the small intestine, the /3-carotene is converted to vitamin A, which can he converted to cis-ll-retinal or stored in the hver for future use. Without a sufficient quantity of retinal, not enough rhodopsin is produced to enable us to see adequately in dim light. 

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