Assertion (A): All naturally occurring $\alpha$-amino acids except glycine are optically active. Reason (R): Most naturally occurring amino acids have L-configuration. (i) Both A and R are true and R is the correct explanation of A. (ii) Both A and R are true but R is not the correct explanation of A. (iii) A is true but R is false. (iv) Both A and R are false.

> [!TLDR] > The key idea is that optical activity in $\alpha$-amino acids arises from a chiral carbon centre. Glycine lacks this chiral centre, so it is optically inactive, making Assertion (A) true. The Reason (R) is also true — most natural amino acids have the L-configuration — but this fact does not explain why glycine is the only exception. Hence, both statements are true, but R is not the correct explanation of A. 1. **Understanding optical activity and chirality** A molecule is optically active if it rotates plane-polarised light. This happens when the molecule is **chiral** — it has a non-superimposable mirror image. In organic chemistry, the most common source of chirality is a carbon atom bonded to four different groups (a chiral centre). For $\alpha$-amino acids, the general structure is $H_2N-CHR-COOH$, where $R$ is a side chain. The central $\alpha$-carbon is bonded to an amino group ($-NH_2$), a carboxyl group ($-COOH$), a hydrogen atom ($-H$), and the side chain $R$. If $R \neq H$, all four groups are different, so the $\alpha$-carbon is chiral — the molecule exists as two non-superimposable mirror images (enantiomers), and each is optically active. If $R = H$, the molecule is glycine: $H_2N-CH_2-COOH$. Here, the $\alpha$-carbon has two hydrogen atoms (the side chain is just H), so it is bonded to only three *different* groups. This makes glycine **achiral** — it has no mirror image that is different from itself — and therefore optically inactive. 2. **Why Assertion (A) is true** All naturally occurring $\alpha$-amino acids except glycine have $R \neq H$, so they possess a chiral $\alpha$-carbon and are optically active. Glycine, with $R = H$, is the sole exception. Thus, Assertion (A) is correct. 3. **Why Reason (R) is true** It is a well-established fact in biochemistry that almost all naturally occurring amino acids (in proteins) have the **L-configuration** at the $\alpha$-carbon. This refers to the absolute stereo

Assertion (A): All naturally occurring α-amino acids except glycine are optically active. Reason (R): Most nat

NCERT Exemplar · Q68

\alpha

(i) Both A and R are true and R is the correct explanation of A.

(ii) Both A and R are true but R is not the correct explanation of A.

(iii) A is true but R is false.

(iv) Both A and R are false.

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Concept understanding — Amino Acid Classification

Amino Acid Classification: From Intuition to Precision

Imagine you're building with LEGO blocks. You have many different pieces — some are long, some are short, some have bumps on the side, some are flat. But all of them have one thing in common: they all click onto the same base plate. That's exactly what amino acids are like. They are the building blocks of proteins, and every single one of them shares a common "base plate" — a core structure — but differs in a side chain that gives each its unique personality.

The Common Core: What Every Amino Acid Shares

Every amino acid has a central carbon atom (called the $\alpha$ -carbon) bonded to four groups:

An amino group ( $-NH_2$ )
A carboxyl group ( $-COOH$ )
A hydrogen atom ( $-H$ )
A variable side chain (called the R group)

The R group is what makes each of the 20 standard amino acids different. It's like the unique shape and colour of each LEGO piece. The classification of amino acids is really just a way of grouping them based on what their R groups are like.

The Big Picture: Why Classify?

You classify things to understand their behaviour. In a crowded room, you might group people by height, or by what they're wearing. Similarly, amino acids are classified to predict how they will behave in water, how they interact with each other, and what role they play in a protein's structure. The most fundamental classification is based on polarity — essentially, how the R group interacts with water.

The Five Major Classes (with Intuition)

1. Nonpolar (Hydrophobic) Amino Acids

Intuition: These R groups are like oil. They hate water. They prefer to hide inside a protein, away from the watery environment of the cell.

What they look like: Their R groups are made mostly of carbon and hydrogen — no charged or polar groups. They are "greasy."

Examples: Glycine (the smallest, just a hydrogen), Alanine, Valine, Leucine, Isoleucine, Methionine, Proline (has a ring that connects back to the amino group), Phenylalanine, Tryptophan.

Note

Proline is unique — its R group forms a ring that includes the amino nitrogen, making it rigid and often causing "kinks" in protein chains.

2. Polar, Uncharged Amino Acids

Intuition: These R groups are like sugar. They dissolve in water but carry no net electric charge. They are "friendly" with water but don't have a full positive or negative charge.

What they look like: Their R groups contain oxygen, nitrogen, or sulfur atoms that can form hydrogen bonds with water.

Examples: Serine, Threonine, Cysteine (has a sulfur atom that can form disulfide bonds), Asparagine, Glutamine.

Watch out

Cysteine is often grouped here, but its sulfur atom can form a special covalent bond (disulfide bridge) with another cysteine. This is a strong, permanent link — not a weak interaction like hydrogen bonds.

3. Positively Charged (Basic) Amino Acids

Intuition: These R groups carry a positive charge at physiological pH (around 7.4). They are like magnets with a "+" sign — they attract negatively charged things.

What they look like: Their R groups contain an extra amino group ( $-NH_2$ ) that picks up a proton ( $H^+$ ) to become $-NH_3^+$ .

Examples: Lysine, Arginine, Histidine.

Important

Histidine is special — its charge changes near physiological pH. This makes it a common player in enzyme active sites where it can act as a proton donor or acceptor.

4. Negatively Charged (Acidic) Amino Acids

Intuition: These R groups carry a negative charge at physiological pH. They are like magnets with a "−" sign — they attract positively charged things.

What they look like: Their R groups contain an extra carboxyl group ( $-COOH$ ) that loses a proton to become $-COO^-$ .

Examples: Aspartic acid, Glutamic acid.

5. Aromatic Amino Acids

Intuition: These have a ring structure (a benzene ring) in their R group. They absorb ultraviolet light — a property used to measure protein concentration.

What they look like: They contain a planar, ring-shaped structure.

Examples: Phenylalanine, Tyrosine, Tryptophan.

Tip

Tyrosine and Tryptophan absorb UV light at 280 nm. This is how scientists measure protein concentration in a lab — a quick and dirty method.

--- …

Why this formula?

Amino Acid Classification: Why the Groupings Make Sense

Amino acids are the building blocks of proteins, and their classification isn't arbitrary — it's based on the chemical properties of their side chains (R-groups). Understanding why these groups behave as they do is key to mastering biochemistry for exams.

The Core Idea: The R-Group Dictates Everything

Every amino acid has a common backbone:

Amino group ( $\text{NH}_2$ )
Carboxyl group ( $\text{COOH}$ )
Hydrogen atom ( $\text{H}$ )
Variable side chain ( $\text{R}$ )

The R-group determines:

Polarity
Charge at physiological pH (~7.4)
Hydrogen bonding ability
Hydrophobicity/hydrophilicity

The Five Major Classes (and Why They Exist)

1. Nonpolar, Aliphatic (Hydrophobic) R-Groups

Examples: Glycine, Alanine, Valine, Leucine, Isoleucine, Methionine, Proline

Why they're nonpolar:

The R-group consists of only carbon and hydrogen (hydrocarbon chains).
Carbon and hydrogen have similar electronegativity ( $\chi_C \approx 2.55$ , $\chi_H \approx 2.20$ ), so no significant dipole forms.
Water is polar; "like dissolves like" — these side chains avoid water and cluster in protein interiors.

Key exam point: Methionine has a sulfur atom, but the $\text{CH}_3\text{SCH}_2\text{CH}_2-$ group is still nonpolar because the C–S bond is nearly nonpolar.

2. Aromatic R-Groups

Examples: Phenylalanine, Tyrosine, Tryptophan

Why they're special:

Contain benzene rings (conjugated $\pi$ systems).
The delocalized electrons make them planar and rigid.
Phenylalanine is purely hydrophobic (no polar groups on ring).
Tyrosine has an –OH group → can form hydrogen bonds (partially polar).
Tryptophan has an indole ring with a nitrogen — can donate H-bonds.

Why they absorb UV light: The conjugated $\pi$ system has a small HOMO-LUMO gap, absorbing at ~280 nm — used to measure protein concentration.

3. Polar, Uncharged R-Groups

Examples: Serine, Threonine, Cysteine, Asparagine, Glutamine

Why they're polar but uncharged:

Contain electronegative atoms (O, N, S) that create partial charges.
Serine/Threonine: –OH group can donate and accept hydrogen bonds.
Cysteine: –SH group can form disulfide bonds ( $\text{–S–S–}$ ) — critical for protein structure.
Asparagine/Glutamine: –CONH $_2$ group has both carbonyl oxygen (H-bond acceptor) and amide hydrogen (H-bond donor).

Why they're uncharged at pH 7: The –OH, –SH, and –CONH $_2$ groups do not ionize significantly at physiological pH.

4. Positively Charged (Basic) R-Groups

Examples: Lysine, Arginine, Histidine

Why they're basic (proton acceptors):

Lysine: Has an $\epsilon$ -amino group ( $\text{–CH}_2\text{CH}_2\text{CH}_2\text{CH}_2\text{NH}_2$ ). The lone pair on nitrogen accepts a proton: $\text{–NH}_2 + \text{H}^+ \rightleftharpoons \text{–NH}_3^+$ pKa ≈ 10.5 → protonated at pH 7.
Arginine: Guanidino group ( $\text{–NH–C(NH}_2\text{)=NH}$ ) is resonance-stabilized when protonated — extremely basic (pKa ≈ 12.5).
Histidine: Imidazole ring has pKa ≈ 6.0 — unique because it can be neutral or positively charged near physiological pH, making it a key catalytic residue in enzymes.

5. Negatively Charged (Acidic) R-Groups

Examples: Aspartic Acid, Glutamic Acid

Why they're acidic (proton donors):

Contain a second carboxyl group ( $\text{–COOH}$ ).
At pH 7, the carboxyl group loses its proton: $\text{–COOH} \rightleftharpoons \text{–COO}^- + \text{H}^+$ …

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The key idea is that optical activity in $\alpha$ -amino acids arises from a chiral carbon centre. Glycine lacks this chiral centre, so it is optically inactive, making Assertion (A) true. The Reason (R) is also true — most natural amino acids have the L-configuration — but this fact does not explain why glycine is the only exception. Hence, both statements are true, but R is not the correct explanation of A.

Understanding optical activity and chirality

A molecule is optically active if it rotates plane-polarised light. This happens when the molecule is chiral — it has a non-superimposable mirror image. In organic chemistry, the most common source of chirality is a carbon atom bonded to four different groups (a chiral centre).

For $\alpha$ -amino acids, the general structure is $H_2N-CHR-COOH$ , where $R$ is a side chain. The central $\alpha$ -carbon is bonded to an amino group ( $-NH_2$ ), a carboxyl group ( $-COOH$ ), a hydrogen atom ( $-H$ ), and the side chain $R$ .

If $R \neq H$ , all four groups are different, so the $\alpha$ -carbon is chiral — the molecule exists as two non-superimposable mirror images (enantiomers), and each is optically active.

If $R = H$ , the molecule is glycine: $H_2N-CH_2-COOH$ . Here, the $\alpha$ -carbon has two hydrogen atoms (the side chain is just H), so it is bonded to only three different groups. This makes glycine achiral — it has no mirror image that is different from itself — and therefore optically inactive.
Why Assertion (A) is true

All naturally occurring $\alpha$ -amino acids except glycine have $R \neq H$ , so they possess a chiral $\alpha$ -carbon and are optically active. Glycine, with $R = H$ , is the sole exception. Thus, Assertion (A) is correct.
Why Reason (R) is true

It is a well-established fact in biochemistry that almost all naturally occurring amino acids (in proteins) have the L-configuration at the $\alpha$ -carbon. This refers to the absolute stereochemistry (based on the Fischer projection, with the amino group on the left). So Reason (R) is also true.
Does (R) correctly explain (A)? …

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Common Mistakes on This Question (Optical Activity of Amino Acids)

Mistake 1: Confusing Optical Activity with L/D Configuration

The error: Students think "L-configuration" automatically means "optically active."

Why it's wrong: Optical activity depends on the presence of a chiral carbon (four different groups attached), not on the L/D label. L-configuration is a naming convention based on glyceraldehyde, not a guarantee of chirality.

How to avoid: Remember — glycine has two hydrogen atoms on the $\alpha$ -carbon, so it has no chiral centre. All other $\alpha$ -amino acids have four different groups ( $\text{NH}_2$ , $\text{COOH}$ , $\text{H}$ , and a variable R group), making them chiral and optically active.

Mistake 2: Thinking "All L-amino acids are optically active"

The error: Assuming every amino acid with L-configuration rotates plane-polarised light.

Why it's wrong: L-configuration is a relative stereochemical descriptor. Optical activity is an absolute physical property. While most L-amino acids are indeed optically active, the reason is chirality, not the L-label.

How to avoid: Separate the two ideas:

Chirality → optical activity (caused by asymmetric carbon)
L/D → a naming system (based on Fischer projection of glyceraldehyde)

Mistake 3: Misreading the Assertion-Reason Link

The error: Choosing option (A) — "Both true, R is correct explanation of A."

Why it's wrong: The Assertion says "all except glycine are optically active" (true). The Reason says "most have L-configuration" (also true). But L-configuration does not explain optical activity — chirality does. The Reason is a separate fact, not a causal explanation.

How to avoid: Ask yourself: Does the Reason directly cause the Assertion? Here, L-configuration is a consequence of stereochemistry, not the cause of optical rotation.

Mistake 4: Forgetting Glycine is the Exception

The error: Claiming "all $\alpha$ -amino acids are optically active" or "none are." …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

KCET 2026Set D31 markMCQ
$\alpha$
›Reveal solutionSolution

Checking each statement against real amino acid/peptide chemistry shows the peptide-bond count in the tripeptide is what is miscounted.

Step 1 — Statements (A) and (D)

Methionine is indeed one of the essential amino acids that must come from the diet (true), and the zwitterion of valine, like other amino acids, carries both a protonated $-NH_3^+$ and a deprotonated $-COO^-$ group and can react with both acids and bases — i.e. it is amphoteric (true).

Step 2 — Statement (B)

Glycine, $H_2N-CH_2-COOH$ , has two identical hydrogen atoms on its α-carbon, so that carbon is not a stereocentre — glycine is achiral and shows no enantiomerism (true).

Step 3 — Statement (C) …
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KCET 2025Set D-41 markMCQ

\alpha

›Reveal solution

Test each option slot-by-slot against the required sequence (α-amino acid → hormone → vitamin → carbohydrate) and reject the moment one member is misclassified; only option (D) survives all four checks.

Step 1 — Fix the four categories

The stem demands, in this order:

an α-amino acid — a carboxylic acid with an $\ce{-NH2}$ on the α-carbon (the one adjacent to $\ce{-COOH}$ ); the building block of proteins.
a hormone — a chemical messenger secreted by an endocrine gland, carried in blood to a target organ.
a vitamin — an organic micronutrient the body cannot synthesise in adequate amounts and must obtain from the diet.
a carbohydrate — a polyhydroxy aldehyde/ketone (a sugar), general formula ≈ $\ce{C_x(H2O)_y}$ .

Step 2 — Test option (A): Thiamine, Thyroxine, Vitamin A, Glucose

Slot	Given	Actually is	✓/✗
α-amino acid	Thiamine	Vitamin B₁ — a vitamin, not an amino acid	✗
hormone	Thyroxine	hormone (thyroid) ✓	✓
vitamin	Vitamin A	vitamin ✓	✓
carbohydrate	Glucose	carbohydrate ✓	✓

Fails at slot 1 — thiamine is vitamin B₁. Rejected.

Step 3 — Test option (B): Glutamine, Insulin, Aspartic acid, Fructose

Slot	Given	Actually is	✓/✗
α-amino acid	Glutamine	α-amino acid ✓	✓
hormone	Insulin	hormone ✓	✓
vitamin	Aspartic acid	an α-amino acid — not a vitamin	✗
carbohydrate	Fructose	carbohydrate ✓	✓

Fails at slot 3. Rejected.

Step 4 — Test option (C): Arginine, Testosterone, Glutamic acid, Fructose

Slot	Given	Actually is	✓/✗
α-amino acid	Arginine	α-amino acid ✓	✓
hormone	Testosterone	hormone (androgen) ✓	✓
vitamin	Glutamic acid	an α-amino acid — not a vitamin	✗
carbohydrate	Fructose	carbohydrate ✓	✓

Fails at slot 3 — the same trap as (B). Three of four are right, which makes this the most tempting distractor, but glutamic acid is emphatically an amino acid. Rejected.

Step 5 — Test option (D): Aspartic acid, Insulin, Ascorbic acid, rhamnose

|---|---|---|---| …

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COMEDK 2025Set 2025-A1 markMCQ
$-\mathrm{SH},-\mathrm{CONH}_2,-\mathrm{NH}_2$
›Reveal solutionSolution
At pH 13.2, all acidic groups are deprotonated and all basic groups are neutral, so the net negative charge equals the number of acidic groups: two –COOH groups give two negative charges, and the –SH group also deprotonates to give a third negative charge, for a total of 3 negative charges.

The key here is understanding ionization states of functional groups at high pH. At pH 13.2, we are far above the pKa of most acidic groups, so they will be fully deprotonated (negatively charged). Basic groups, however, will be deprotonated and neutral. Let’s walk through each group in the pentapeptide.

Identify all ionizable groups

The problem states the pentapeptide has:

one –SH (thiol) group

one –CONH₂ (amide) group

one –NH₂ (amino) group

two –COOH (carboxyl) groups

Note: –CONH₂ is an amide; it is not ionizable under normal conditions (it does not gain or lose protons in aqueous solution). So it contributes zero charge at any pH.

Determine the pKa of each ionizable group (approximate values):

–COOH: pKa ≈ 2–5 (typical for side chains like glutamic/aspartic acid)

–SH: pKa ≈ 8–10 (cysteine side chain)

–NH₂: pKa ≈ 9–11 (lysine side chain or N-terminus)

At pH 13.2, which is > 11, every group with pKa < 13.2 will be in its conjugate base form.

Assign charges at pH 13.2:

–COOH: pKa ~4 → at pH 13.2, fully deprotonated to –COO⁻ → –1 charge each. Two such groups → –2 total.

–SH: pKa ~9 → at pH 13.2, deprotonated to –S⁻ → –1 charge. …
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COMEDK 2025Set 2025-E1 markMCQ
$\alpha-\mathrm{D}-(+)$
›Reveal solutionSolution
The question tests knowledge of biomolecules; only option (B) is correct because leucine and tryptophan are indeed essential amino acids that humans cannot synthesize.

Concept & Intuition

This problem checks your grasp of four distinct biochemistry facts: invert sugar, essential amino acids, milk curdling, and starch structure. The trick is to recall precise definitions—many students confuse “invert sugar” with a single sugar, or think curdling is purely denaturation. Let’s examine each statement systematically.

Option (A): Fructose is an example of Invert sugar

Invert sugar is a mixture of equal parts glucose and fructose, produced by hydrolyzing sucrose. Fructose alone is just a monosaccharide, not a mixture.

Therefore, (A) is false.

Option (B): Leucine and Tryptophan are classified as Essential amino acids

Essential amino acids cannot be synthesized by the human body and must come from diet. Leucine and tryptophan are both on the standard list of nine essential amino acids (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine).

This statement is correct.

Option (C): Curdling of milk is not a denaturation process

Curdling involves the coagulation of casein proteins when milk is acidified (e.g., by lactic acid bacteria or lemon juice). This is a classic example of protein denaturation—the protein’s structure unfolds and aggregates.

So (C) is false. …
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COMEDK 2025Set 2025-M1 markMCQ
$-\mathrm{NH}_2$
›Reveal solutionSolution
Both statements are true and R explains A: a zwitterion forms only when the acid group is a strong enough proton donor and the $-NH_2$ is basic enough to grab that proton — true for glycine and sulphanilic acid, but not for $p$ -aminobenzoic acid whose $-COOH$ is too weak.

Assertion.

$2$ -aminoethanoic acid (glycine) exists as $^+H_3N\text{-}CH_2\text{-}COO^-$ — a zwitterion. ✓

$p$ -aminobenzenesulphonic acid (sulphanilic acid): $-SO_3H$ is a strong acid, so it donates $H^+$ to the ring $-NH_2$ , giving $^+H_3N\text{-}C_6H_4\text{-}SO_3^-$ — a zwitterion. ✓

$p$ -aminobenzoic acid: $-COOH$ on benzene is a relatively weak acid and the aromatic $-NH_2$ is weakly basic, so it exists predominantly as the neutral molecule, not the dipolar ion. ✓

So the Assertion is correct. …
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COMEDK 2022Set 20221 markMCQ
$Q. The essential amino acids are$
(i) Leucine
(ii) Glutamic acid
(iii) Asparagine
(iv) Valine correct option is (A) (i),
(ii) (B) (ii),
(iii) (C) (iii),
(iv) (D) (i), (iv)
›Reveal solutionSolution

So the essential ones are (i) and (iv).

Concept: Essential vs non-essential amino acids. Essential amino acids cannot be synthesised by the human body and must come from the diet. The ten essential ones are: valine, leucine, isoleucine, threonine, methionine, phenylalanine, tryptophan, lysine, histidine, arginine.

Checking the list:

(i) Leucine - ESSENTIAL …
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COMEDK 2021Set 20211 markMCQ
$_2$
›Reveal solutionSolution
Only glutamine carries a polar functional group in its R chain.

Concept: Classification of amino-acid side chains (R groups) as polar / non-polar.

Alanine: R = -CH3 (non-polar)

Valine: R = -CH(CH3)2 (non-polar, branched alkyl)

Glycine: R = -H (non-polar / neutral) …
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COMEDK 2021Set 2021-B1 markMCQ
$Q. Which of the following is NOT correct? (A) α-Keratin is insoluble in water. (B) Isoleucine is an essential amino acid (C) All naturally occurring amino acids except lysine are optically active (D) Keratin and myosin are fibrous proteins..$
›Reveal solutionSolution
Statement (C) is wrong: the achiral (optically inactive) natural amino acid is glycine, not lysine.

Checking each statement:

(A) α-Keratin (a fibrous protein) is insoluble in water — correct.

(B) Isoleucine is an essential amino acid — correct. …
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Q.Assertion (A): All naturally occurring $\alpha$ -amino acids except glycine are optically active.
Reason (R): Most naturally occurring amino acids have L-configuration.

Concept understanding — Amino Acid Classification

Amino Acid Classification: From Intuition to Precision

The Common Core: What Every Amino Acid Shares

The Big Picture: Why Classify?

The Five Major Classes (with Intuition)

1. Nonpolar (Hydrophobic) Amino Acids

2. Polar, Uncharged Amino Acids

3. Positively Charged (Basic) Amino Acids

4. Negatively Charged (Acidic) Amino Acids

5. Aromatic Amino Acids

Why this formula?

Amino Acid Classification: Why the Groupings Make Sense

The Core Idea: The R-Group Dictates Everything

The Five Major Classes (and Why They Exist)

1. Nonpolar, Aliphatic (Hydrophobic) R-Groups

2. Aromatic R-Groups

3. Polar, Uncharged R-Groups

4. Positively Charged (Basic) R-Groups

5. Negatively Charged (Acidic) R-Groups

Method: Assertion–Reason Analysis (Conceptual Verification)

Steps

Common Mistakes on This Question (Optical Activity of Amino Acids)

Mistake 1: Confusing Optical Activity with L/D Configuration

Mistake 2: Thinking "All L-amino acids are optically active"

Mistake 3: Misreading the Assertion-Reason Link

Mistake 4: Forgetting Glycine is the Exception

Step 1 — Statements (A) and (D)

Step 2 — Statement (B)

Step 3 — Statement (C) …

Step 1 — Fix the four categories

Step 2 — Test option (A): Thiamine, Thyroxine, Vitamin A, Glucose

Step 3 — Test option (B): Glutamine, Insulin, Aspartic acid, Fructose

Step 4 — Test option (C): Arginine, Testosterone, Glutamic acid, Fructose

Step 5 — Test option (D): Aspartic acid, Insulin, Ascorbic acid, rhamnose

Q.Assertion (A): All naturally occurring α\alphaα-amino acids except glycine are optically active. Reason (R): Most naturally occurring amino acids have L-configuration.

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Q.Assertion (A): All naturally occurring $\alpha$ -amino acids except glycine are optically active.
Reason (R): Most naturally occurring amino acids have L-configuration.

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