When free in solution (i.e., not bound to opsin), both all-trans and 11-cis reti

Question

When free in solution (i.e., not bound to opsin), both all-trans and 11-cis retinal have absorption maxima of about 370-380 nm. How many electrons can be counted in the conjugated pi-system? Using lambda = 370 nm and the ID particle in a box model, estimate the effective chain length of the retinal chromophore. For color vision, the absorption maximum of 11-cis retinal is about 560 nm when incorporated by a Schiff-base linkage into the red photoreceptor protein. What would the effective chain length be in this ease? Comparing your answers in (b) and (c). evaluate the plausibility that the length of the conjugated chain could be the cause of these differences. When bound to opsin, the 11-cis retinal Schiff-base linkage has an absorption of about 500 nm, while the opsin-bound all-trans retinal absorbs at about 540 nm. Assuming the length of the box cannot be invoked to explain these absorbance's, how else might we modify the particle in a box model to explain these differences considering that the retinal chromophore is now located in a specialized pocket in the interior of the protein?

Explanation / Answer

a) Double bonds which can be delocalised are considered to constitute conjugated pi system. One double bond gives two pi electrons. Retinal have 5 pi bonds= 10 pi electrons

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