Q.10 The reaction of 11-cis-retinal with the lysine residue of a specific protein forms the light-
sensitive pigment in the cells of retina. The light-sensitive pigment is an
(A) amide. (B) acid.
(C) anhydride. (D) imine.
The light‑sensitive pigment formed by reaction of 11‑cis‑retinal with a lysine residue of opsin is an imine (a protonated Schiff‑base linkage between retinal and the lysine ε‑amino group).
Correct option: imine (Schiff base)
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11‑cis‑retinal is an aldehyde; its terminal –CHO group reacts with the ε‑NH₂ of a lysine side chain in opsin.
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This condensation between an aldehyde and a primary amine eliminates water and forms a C=N bond, called a Schiff base, which is a type of imine; in rhodopsin this exists as a protonated Schiff base (–NH⁺=CH–).
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This retinal–lysine imine is the chromophore of rhodopsin and is responsible for light sensitivity in rod cells.
So, the correct choice is (D) imine.
Why other options are wrong
Option (A) amide
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Amides form when a carboxylic acid (or its derivative) reacts with an amine to give a C=O–NH– linkage.
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11‑cis‑retinal contributes an aldehyde group, not a carboxylic acid or acyl derivative, and the product with lysine has a C=N (imine), not a C=O–NH– (amide), bond.
Option (B) acid
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11‑cis‑retinal is the aldehyde of vitamin A, not a free carboxylic acid.
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On binding opsin, no new carboxylic acid group is generated; instead, an imine (Schiff base) is formed, so the light‑sensitive pigment cannot be classified as an acid.
Option (C) anhydride
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Anhydrides are derived from two carboxylic acids (RCO–O–COR′); they are not formed by reaction of an aldehyde with an amine.
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The retinal–lysine linkage in rhodopsin contains no acid anhydride (RCO–O–COR′) function, only the C=N of the Schiff base.
Detailed, exam‑oriented explanation
Structure of 11‑cis‑retinal and opsin
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11‑cis‑retinal is the 11‑cis isomer of retinaldehyde, the chromophoric aldehyde derived from vitamin A; it contains a polyene chain ending in a terminal –CHO group.
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Opsin is a G‑protein–coupled receptor in rod cells; one lysine residue (Lys296 in bovine rhodopsin) provides an ε‑amino group that reacts covalently with 11‑cis‑retinal.
Mechanism: formation of the imine (Schiff base)
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The nucleophilic ε‑NH₂ of lysine attacks the electrophilic carbonyl carbon of the retinal aldehyde; after proton transfers and loss of water, a C=N double bond forms, giving a retinylidene Schiff base.
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In the dark state, this imine is protonated (protonated Schiff base, –NH⁺=CH–), stabilized by a counter‑ion (Glu113) and a hydrogen‑bond network in the retinal binding pocket.
Role in visual transduction
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The 11‑cis‑retinal–lysine imine keeps rhodopsin in an inactive conformation and serves as the light‑absorbing chromophore.
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Absorption of a photon causes isomerization of 11‑cis‑retinal to all‑trans‑retinal within the imine linkage, triggering conformational changes that activate transducin and initiate the visual signal cascade.
SEO‑aware introduction (example paragraph)
The reaction of 11‑cis‑retinal with lysine forming imine in rhodopsin is a classic concept in biochemistry and an important topic for competitive exams in life sciences. This covalent bond between the aldehyde group of 11‑cis‑retinal and the ε‑amino group of a lysine residue in opsin produces a protonated Schiff‑base imine that acts as the light‑sensitive chromophore of rod photoreceptors. Understanding why this linkage is an imine rather than an amide, acid, or anhydride helps clarify organic reaction mechanisms and the molecular basis of vision.
