Visual pigment chromophores

Weingart, O., Schapiro, I., 8c Buss, V. (2007). Photochemistry of visual pigment chromophore models by ab initio molecular dynamics. The Journal of Physical Chemistry B, 111(14), 3782-3788. 

Matsui S, Seidou M, Uchiyama I, Sekiya N, Hiraki K, Yoshihara K, Kito Y (1988) 4-Hydroxyretinal, a new visual pigment chromophore found in the bioluminescent squid Watasenia scintillans. Biochim Biophy Acta 966 370-374... 

Extraction of visual pigments (chromophore and/or protein), either by means of detergents such as digitonin and Triton [57], or by organic solvents, could be the best method for providing large quantities of photoreceptive pigments in an accessible in vitro form for subsequent detailed biochemical analysis. 

Natural Sensitizing Dyes and Photodynamic Therapy. The chlorophylls are, of course, among the natural sensitizers for photosynthesis. Considerable iaterest exists ia chlorophyll and related pigments as photosensitizers ia biology and medicine (75), isomeric retinal chromophores as visual pigments (76,77), and the use of synthetic photosensitizers ia neurobiology (9), hematology (78), and photodynamic therapy (79). 

Apart from lutein and zeaxanthin, a number of other xanthophylls are present in the retina at low concentrations, including 3 -cpi-lutein, lactucaxanthin, 3 -dehydrolutein, and (3,(3-carotene-3,3 -dione (Khachik et al. 1997a, Bernstein et al. 2001). Interestingly, (3-carotene, the provitamin A precursor of the chromophore of the visual pigments, has only been identified in traces in the retina, if at all. 

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]. 

Photochemical cis-trans isomerization in a conjugated polyene system is thought to be the crucial primary process in vision. The visual pigment (rhodopsin) is derived from 11 -crs-retinal by reaction of the aldehyde group with an amino substituent in a protein (opsin). There is considerable distortion in the geometry of this chromophoric group anyway, because of the spatial requirements of the protein... 

The all-trams forms of the vitamins A predominate but 11-c/s-retinal is the lightabsorbing chromophore of the visual pigments... 

The (4R)-absolute configuration of a new chromophore of native visual pigment (159) (negative Cotton effect at 375 nm, negative Cotton effect at 254 nm) was established by the CD exciton chirality method applied to the 4-(dimethylamino)cinnamate (160). The split negative (381 nm) and positive (338 nm) exciton effects of 160 show a counterclockwise helicity between pentaenal and a-4-(dimethylamino)cinnamate chromophores355. 

Vogt, K. Kirschfeld, K. (1984) Chemical identity of the chromophores of fly visual pigment Naturwissenschaften vol. 71, pp 211-212 (in English)... 

Saari has made the bold statement that Three derivatives of 11-cis-retinaldehyde serve as the chromophores of all known visual pigments. They are complexed with a protein component (an opsin), and the resulting protein-retinoid interactions determine the spectral sensitivity of the visual pigment. 85 No reference is given for this statement nor is any explanation of how these complexes exhibit an absorption spectrum in the visual region. However, except for the substitution of all-trans for 11 -cis in the above quotation and a slight modification to the retinoid involved, this work agrees completely with the statement and provides an explanation for how it is applied. 

Visual pigments display characteristic absorption spectra which result from the very specific interactions between protein and chromophore in the binding site, i.e., the absorbance spectrum of retinal at ca. 380 nm is red-shifted to ca. 500 nm in bovine rhodopsin. However, depending on species, rhodopsins absorb from 440 to ca. 600 nm. Porphyropsins show a similar spread in their absorption maxima, absorb at longer wavelengths than the corresponding rhodopsins, and have lower extinction coefficients (ca. 75%) than rhodopsins (e.g., bovine rhodopsin e, ca. 40500) [17] as shown in Fig. 2. 

Since the CD of oriented molecules are extremely sensitive to the interaction of chromophores, such data could be very useful in the investigation of exciton phenomena. The technique also allows one to determine the degree of order and orientation of chromophores in a macromolecule. Thus, CD studies on oriented visual pigments, and possibly on some selected visual pigment analogs, could be very informative in clarifying the role played by induced optical activity of the chromo-phore and/or exciton coupling. 

The study of visual pigment analogs serves to clarify the nature of chromophore-opsin interactions in visual pigments. Visual pigment analogs are modified retinals bound to opsins, which can be investigated by spectroscopic and biochemical methods. 

Both the 9-cw and 9,13-dim isomers of 30 formed visual pigment analogs [165], with Amax at 460 nm and 455 nm (mixed diastereomers), respectively. Moreover, upon incubation of a ca. 1 1 diastereomeric mixture of 30 (the 9-cis isomers) followed by extraction of the bound chromophore by the CH2C12 procedure and... 

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. 

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