Schiff bases retinal pigment

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]. 

Do we know all of the special chemistry of vitamin A that is involved in its functions Retinal could form Schiff bases with protein groups as it does in the visual pigments. Redox reactions could occur. Conjugative elimination of water from retinol to form anhydroretinol is catalyzed nonenzymatically by HC1. Anhydroretinol occurs in nature and... 

Proton translocation to the Schiff base nitrogen was proposed to occur by concerted double proton transfer (as shown in Fig. 15) leading to a retro retinal structure in the batho intermediate [127, 196], However, this model can be eliminated as it is inconsistent with the formation of batho intermediates from pigment analogs based on 5-desmethylretinal [145] and y-retroretinal [146,147], It also disagrees with the resonance Raman results. 

While 11 -cis-retinal absorbs at 380 nm and a model protonated Schiff base of retinal with n-butylamine in methanol [210,211] absorbs at 440 nm, the absorption maxima of visual pigments based on 11 -cis-retinal span a wide range of values, from 430 to ca. 600 nm. Therefore, the problem consists of determining the nature of the... 

Akhtar et al. [74] proposed that, in rhodopsin, an acceptor group on the protein forms a charge-transfer complex with the unprotonated Schiff base of retinal furthermore, upon ll-cis to trans isomerization, separation of donor and acceptor moieties would occur and the Schiff base linkage would be exposed to hydrolysis. This model can now be discarded as unrealistic the resonance Raman experiments have shown that it is not an unprotonated Schiff base, but a protonated base which is bound to opsin. Further, this and related models were examined by Komatsu and Suzuki [223] using theoretical calculations, who found that charge-transfer type models cannot satisfactorily explain the red shifts seen in visual pigments. 

A series of hydroretinals were synthesized and were bound to bovine opsin to form visual pigment analogs, the hydrorhodopsins [92]. Model protonated Schiff bases were also prepared from each of these retinals, and the magnitudes of the opsin shifts were determined. Table 1 summarizes the data obtained and shows the opsin shift for bovine rhodopsin for comparative purposes. Binding studies were also carried out to ascertain that the hydroretinals occupy the same binding site as 11-cK-retinal in rhodopsin. This is required to show the relevance of the data to the natural system. 

Data is also shown for bovine rhodopsin, 63, for comparison. The hydroretinals 66-68 presumably assume 9-cis or 11 -cis like conformations when bound to opsin. Retinals 67 and 68 form non-bleachable pigments, i.e., no change in their A, occurs upon exposure to room light irradiation by UV light leads to decomposition products instead of separation of the chromophore from opsin. a In MeOH. In case of split chromophores the absorption maxima of the enal moieties are given. b Protonated Schiff base with n-butylamine in MeOH. ... 

Visual excitation (1) in both vertebrates and invertebrates is initiated via light absorption by visual pigments consisting of a chromophore covalently bound to an apo-protein, opsin. Biochemical extraction studies have shown that in all pigments the chromophore is a Schiff base derivative of 11-cis retinal (Fig. 

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... 

Figure 7. Correlation between the ethylenic (C=C) stretching frequency and the main absorption maximum of the retinyl moiety in a variety of pigments and in free retinal Schiff bases in solution. (Data, based on resonance-Raman experiments at room temperature, from refs. 221, 225, 226, and 323.)...

Infrared and Resonance Raman Spectroscopy. Reviewson the uses of resonance Raman spectroscopy in biochemistry and biology include sections on carotenoproteins, visual pigments, and bacteriorhodopsin. The resonance Raman spectrum of the lowest excited triplet state of /3-carotene has been reported.A resonance Raman method has been used for the quantitative analysis of /3-carotene and lutein (20) in tobacco.The mechanism of carotenoid-protein interactions in the carotenoproteins ovoverdin and /3-crustacyanin has been investigated by resonance Raman spectroscopy. " 2 axanthin (24) has been used as a resonance Raman probe of membrane structure. " The resonance Raman spectra have been reported of all-frans-anhydrovitamin A (194), " /3-ionone, retinals, and Schiff bases.The technique has been used extensively to study... 

Retinal Schiff bases and visual pigments have also been studied by i.r. spectroscopy/ ... 

Retinal as Visual Pigment Model Spectroscopy and Physical Chemistry. As in previous years, several theoretical, spectroscopic, and photochemical studies of retinal (136) and related compounds, especially Schiffs bases, have been reported,and in many cases the main aim was to obtain information relevant to the functioning of rhodopsin and related visual pigments. Particularly valuable are surveys of the year s literature on the photochemistry of polyenes, excited states of biomolecules,and recent developments in the molecular biology of vision. 

The aldehyde form, retinal, is an essential component of the visual pigment found in the rods of the eye. A very brief outline of the rho-dopsin cycle is shown in Fig. 8.4. Retinol is transported from the liver to the eye, where it is converted to 11-cis-retinal. In the rod, the aldehyde forms an enamine (Schiffs base) with a lysine on opsin forming rhodopsin (Fig. 8.5). In the presence of light, trans-retinal forms with cleavage of the enamine, sending a nerve impulse to the brain along the optic... 

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