Retinaldehyde absorption spectrum

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. 

The absorption spectrum of retinaldehyde (2) in the visible and near-ultraviolet region shows three bands a main one at about 370 nm and two weaker transitions at 280 nm and 250 nm. It has been concluded from a number of theoretical calculations that the main 370-nm band in all geometric isomers of retinaldehyde (2) is attributable to the allowed transition in the lowest state (Wiesenfeld and Abrahmson, 1968 Langlet et aL, 1969a Christensen and Kohler, 1973 Schulten et a/., 1976). 

When the mixture of these geometric isomers of (2) was chromatographed twice, two fractions were obtained whose compositions were determined on the basis of their H-NMR spectra. The first fraction contained (7Z, 13Z)-reti-naldehyde (353) and (7Z, 9Z, 13Z)-retinaldehyde (354), while the second fraction comprised (all- )-retinaldehyde (2), (7Z)-retinaldehyde (355), and (7Z, 9Z)-retinaldehyde (356). The 7-cis aldehydes (353), (354), (355), and (356) are remarkably stable. An absorption spectrum of an ethanolic solution of these aldehydes, kept under nitrogen at about 0 C, showed no change over a period of 6 months. Of course, such 7Z isomers are extremely sensitive to light. 

In the pigment epithelium of the retina, all-/ra j-retinol is isomerized to 11-cm-retinol and oxidized to 1 l-at-retinaldehyde. This reacts with a lysine residue in opsin, forming the holoprotein rhodopsin. Opsins are cell type specific they shift the absorption of 11-a r-retinaldehyde from the ultraviolet (UV) into what we call, in consequence, the visible range — either a relatively broad spectrum of sensitivity for vision in dim light (in the rods) or more defined spectral peaks for differentiation of colours in bright light (in the cones). 

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