Amino acids behave like salts rather than simple amines or carboxylic acids. Explain.

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Q. Amino acids behave like salts rather than simple amines or carboxylic acids. Explain.

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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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Amino acids exist as zwitterions (dipolar ions) in the solid state and in neutral solution because the acidic carboxyl group donates a proton to the basic amino group. This internal salt formation explains why they have high melting points, are soluble in water but not in nonpolar solvents, and behave differently from simple amines or carboxylic acids.

Why This Happens — The Concept

A simple amine ( $\ce{R-NH2}$ ) is a base — it accepts protons. A simple carboxylic acid ( $\ce{R-COOH}$ ) is an acid — it donates protons. An amino acid contains both groups in the same molecule. So what happens when you put an acid and a base together? They react.

In an amino acid, the carboxyl group ( $\ce{-COOH}$ ) is acidic enough to transfer its proton to the amino group ( $\ce{-NH2}$ ) on the same molecule. This intramolecular acid-base reaction produces a zwitterion — a molecule with both a positive and a negative charge, but overall neutral.

\ce{H2N-CHR-COOH <=> [^+H3N-CHR-COO^-]}

The equilibrium lies heavily to the right in the solid state and in neutral aqueous solution. So an amino acid is not a neutral molecule with separate amine and acid groups — it is a salt-like dipolar ion.

Step-by-Step Reasoning

1. The structure of an amino acid forces an internal acid-base reaction

Every standard $\alpha$ -amino acid has the general formula $\ce{H2N-CHR-COOH}$ . The amino group is a base ( $\mathrm{p}K_b \approx 3\text{–}4$ for the conjugate acid), and the carboxyl group is an acid ( $\mathrm{p}K_a \approx 2\text{–}3$ ). Because they are on the same molecule, the carboxyl proton can transfer directly to the amino nitrogen.

This is not a hypothetical — it is experimentally confirmed. X-ray crystallography shows that in the solid state, the $\ce{C-O}$ bond lengths in the carboxylate group are equal (both about 1.26 Å), which is characteristic of a carboxylate ion $\ce{-COO^-}$ , not a carboxylic acid $\ce{-COOH}$ (where $\ce{C=O}$ is ~1.20 Å and $\ce{C-OH}$ is ~1.31 Å).

2. This explains the physical properties that seem contradictory

If amino acids were simple amines or carboxylic acids, they would have low melting points (like acetic acid, m.p. 17 °C, or ethylamine, m.p. –81 °C). Instead, glycine melts at 233 °C with decomposition. That is salt-like behaviour — compare sodium chloride (m.p. 801 °C).

Property	Simple amine	Simple carboxylic acid	Amino acid
Melting point	Low	Low	High (decomposes)
Solubility in water	High	High	High
Solubility in organic solvents	High	High	Very low
Electrical conductivity (solid)	None	None	None (ions fixed in lattice)
Electrical conductivity (aqueous)	Weak base	Weak acid	Depends on pH

The high melting point comes from the strong electrostatic attraction between the positive and negative charges in the crystal lattice — just like an ionic salt.

3. The zwitterion explains the amphoteric behaviour

Because the amino acid is already a zwitterion, it can act as both an acid and a base:

In acidic solution (low pH): The carboxylate group accepts a proton, giving $\ce{^+H3N-CHR-COOH}$ — a fully protonated cation.
In basic solution (high pH): The ammonium group donates a proton, giving $\ce{H2N-CHR-COO^-}$ — a fully deprotonated anion.

Watch out

A common mistake is to think that the amino group is "free" to act as a base in neutral solution. It is not — it is already protonated. The zwitterion is the dominant form at physiological pH (~7.4), so the "amino" group is actually $\ce{-NH3^+}$ .

4. The isoelectric point (pI) confirms the zwitterion model …

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Here is a breakdown of the common mistakes students make when explaining why amino acids behave like salts, along with how to avoid each.

The Core Concept: Zwitterionic Nature

First, the "why": In neutral solution (and in the solid state), the amino group ( $-NH_2$ ) is basic enough to grab a proton ( $H^+$ ) from the nearby carboxylic acid group ( $-COOH$ ). This creates an internal salt called a zwitterion (dipolar ion).

Structure: $^{+}H_3N-CHR-COO^{-}$
Result: The molecule has both a positive and a negative charge, making it behave like a salt (high melting point, soluble in water, insoluble in non-polar solvents), not like a simple amine (which is a base) or a simple carboxylic acid (which is an acid).

Common Mistake #1: Forgetting the "Internal" Part

The Mistake: Students say "Amino acids react with an acid and a base to form a salt." This is wrong because it implies the salt is formed by an external reaction (e.g., with HCl or NaOH).

Why it's wrong: The salt is formed within the same molecule by an intramolecular acid-base reaction. No external acid or base is needed for the zwitterion to exist in neutral conditions.

How to Avoid:

Use the word "internal": Always say "internal salt" or "zwitterion."
Draw the mechanism: Show the proton transfer from $-COOH$ to $-NH_2$ on the same molecule.
Key phrase: "The amino group acts as a base and accepts a proton from its own carboxylic acid group."

Common Mistake #2: Confusing the pH Condition

The Mistake: Students claim amino acids exist as zwitterions "always" or "in acidic solution."

Why it's wrong:

In acidic solution (low pH): The $-COO^{-}$ group gets protonated. The form is $^{+}H_3N-CHR-COOH$ (a cation).
In basic solution (high pH): The $^{+}H_3N-$ group loses a proton. The form is $H_2N-CHR-COO^{-}$ (an anion).
The zwitterion exists only at the isoelectric point (pI) — the pH where the molecule has no net charge.

How to Avoid:

Memorize the pH zones:
- Low pH: Cation ( $^{+}H_3N-COOH$ )
- Neutral/pI: Zwitterion ( $^{+}H_3N-COO^{-}$ )
- High pH: Anion ( $H_2N-COO^{-}$ )
Use a diagram: Draw the three forms side-by-side with arrows showing proton gain/loss as pH changes.

Common Mistake #3: Ignoring the Physical Properties

The Mistake: Students explain the "salt-like" behavior only in terms of chemical reactions (e.g., "it can react with acids and bases") and forget the physical evidence.

Why it's wrong: The question asks why they behave like salts. The key evidence is:

High melting point: Zwitterions have strong electrostatic attractions (like ionic compounds), unlike simple amines or carboxylic acids which have weaker hydrogen bonds.
Solubility: They are soluble in water (polar) but insoluble in organic solvents (non-polar), exactly like inorganic salts (e.g., NaCl).

How to Avoid:

List the physical properties explicitly:
- High melting point (often >200°C)
- Water soluble, organic solvent insoluble
- Crystalline solid at room temperature
Compare directly: "A simple amine like methylamine is a gas at room temperature; a simple acid like acetic acid is a liquid. Glycine is a crystalline solid — that's salt-like behavior." …

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

Showing the 12 most recent of 26 on this concept.

AP EAPCET 2026Set eng-2026-05-12-FN1 markMCQ
$Q. Match the following List - I (Name of the amino acid) | List - II (Description) A. Tyrosine | I. Basic amino acid B. Lysine | II. Optically inactive amino acid C. Glycine | III. Sulphur containing amino acid D. Cysteine | IV. Aromatic amino acid The correct answer is (A) A - II, B - I, C - IV, D - III (B) A - IV, B - III, C - II, D - I (C) A - IV, B - I, C - II, D - III (D) A - III, B - IV, C - I, D - II$
›Reveal solutionSolution
Tyrosine → aromatic, Lysine → basic, Glycine → optically inactive, Cysteine → sulphur-containing, giving option (C).

Concept and Intuition

Amino acids differ only in their side chains, which set their properties: an aromatic ring (tyrosine), an extra basic amino group (lysine), no chiral centre when both alpha substituents are H (glycine), and a thiol group (cysteine).

Step-by-Step Solution

Tyrosine has a p-hydroxyphenyl side chain ⇒ aromatic amino acid ⇒ IV.

Lysine has a second $-NH_2$ on its side chain ⇒ basic amino acid ⇒ I.

Glycine's alpha carbon bears two H atoms, so it is not chiral ⇒ optically inactive ⇒ II.

Cysteine has a $-CH_2-SH$ side chain ⇒ sulphur-containing ⇒ III. …
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AP EAPCET 2026Set eng-2026-05-12-AN1 markMCQ
$Q. Amino acids containing hetero aromatic ring are I. Pro II. His III. Tyr IV. Trp The correct answer is (A) I, III only (B) II, IV only (C) I, II only (D) III, IV only$
›Reveal solutionSolution
Only His (imidazole) and Trp (indole) carry an aromatic ring containing a heteroatom; Pro's ring is saturated and Tyr's ring is aromatic but carbocyclic (no heteroatom in the ring).

Concept and Intuition

"Heteroaromatic" needs both conditions at once: the ring must be aromatic (planar, cyclic, conjugated, obeying Hückel's rule) AND contain at least one heteroatom (N, O, S, …) in the ring itself.

Step-by-Step Solution

Proline (Pro): its side chain forms a pyrrolidine ring fused to the α-amino group — this ring is fully saturated, not aromatic. Excluded.

Histidine (His): side chain has an imidazole ring — a 5-membered aromatic ring with two nitrogens in the ring. Aromatic + heteroatom = heteroaromatic. Included.

Tyrosine (Tyr): side chain is a phenol ring (benzene ring with an -OH) — aromatic, but the ring itself is all carbon; the heteroatom (O) is a substituent, not a ring member. Not heteroaromatic. Excluded. …
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AP EAPCET 2026Set eng-2026-05-14-FN1 markMCQ
$H_2N-CH(R)-COOH$
›Reveal solutionSolution
Matching each amino acid to its side chain (R group) gives A-III, B-I, C-II, D-IV.

Concept and Intuition

Every standard amino acid has the general structure $H_2N-CH(R)-COOH$ , differing only in the R group. Recognizing these R groups is a matter of recalling each amino acid's characteristic side chain.

Step-by-Step Solution

Alanine: simplest amino acid after glycine, R = $-CH_3$ → matches III.

Leucine: branched aliphatic side chain, R = $-CH_2-CH(CH_3)_2$ (isobutyl) → matches I.

Serine: hydroxyl-bearing side chain, R = $-CH_2-OH$ → matches II.

Cysteine: thiol-bearing side chain, R = $-CH_2-SH$ → matches IV. …
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AP EAPCET 2026Set eng-2026-05-18-FN1 markMCQ
$-NH_2$
›Reveal solutionSolution
This tests recall of amino acid side-chain functional groups: serine carries a hydroxyl ( $-OH$ ) side group and cysteine carries a thiol ( $-SH$ ) side group, so X, Y = $-OH$ , $-SH$ .

Concept and Intuition

All standard $\alpha$ -amino acids share the $-NH_2$ and $-COOH$ groups on the $\alpha$ -carbon; what distinguishes each amino acid is its side chain (R group), which often carries an additional functional group responsible for special chemistry (H-bonding, disulfide bond formation, etc.).

Step-by-Step Solution

Serine's structure is $HOCH_2-CH(NH_2)-COOH$ : besides $-NH_2$ and $-COOH$ , its side chain is $-CH_2OH$ , i.e. it carries an extra $-OH$ (hydroxyl) group. So X = $-OH$ .

Cysteine's structure is $HSCH_2-CH(NH_2)-COOH$ : its side chain is $-CH_2SH$ , i.e. it carries an extra $-SH$ (thiol) group. So Y = $-SH$ .

Matching X, Y = $-OH$ , $-SH$ against the options identifies (B).

Common Mistakes …
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AP EAPCET 2026Set ap-2026-05-19-FN1 markMCQ
$\underline{X}$
›Reveal solutionSolution
Match each amino acid's side-chain functional group to identify the –OH-bearing set (Ser, Tyr) and the –CONH2-bearing set (Gln, Asn).

Concept and Intuition

Amino acid side chains carry many different functional groups that determine their chemical behaviour. Hydroxyl (–OH) groups appear in Serine (aliphatic –CH2OH) and Tyrosine (aromatic phenolic –OH); amide (–CONH2) groups appear in Glutamine and Asparagine, the amide derivatives of glutamic acid and aspartic acid respectively.

Step-by-Step Solution

List side chains: Lys – aminobutyl (–(CH2)4NH2, basic); Gln – –CH2CH2CONH2; Ser – –CH2OH; Cys – –CH2SH; Tyr – phenol ring with –OH; Asn – –CH2CONH2.

Amino acids with –OH: Ser (III) and Tyr (V) → this is set X. …
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AP EAPCET 2025Set eng-2025-05-24-FN1 markMCQ
$Q. The list given below contains essential amino acids that are basic (X) and also non essential amino acids that are neutral (Y). X and Y, respectively are$
a) Lysine b) Alanine c) Serine d) Arginine e) Tyrosine (A) X = b, c, e; Y = a, d (B) X = a, d; Y = b, c, e (C) X = a, c; Y = b, d, e (D) X = a, b, c; Y = d, e
›Reveal solutionSolution
This tests classification of amino acids by both essentiality and acid-base character; X (essential + basic) = lysine, arginine; Y (non-essential + neutral) = alanine, serine, tyrosine.

Concept and Intuition

Amino acids are classified along two independent axes: whether the body can synthesise them (non-essential) or must obtain them from diet (essential), and their side-chain acid-base character (acidic, basic, or neutral). A question combining both axes requires checking each amino acid against both criteria simultaneously — an amino acid could be essential yet neutral, or non-essential yet basic, so care is needed not to conflate the two classifications.

Step-by-Step Solution

Lysine (a): essential, and basic (extra amino group in side chain) → fits X.

Alanine (b): non-essential, neutral (simple methyl side chain) → fits Y.

Serine (c): non-essential, neutral (hydroxymethyl side chain) → fits Y.

Arginine (d): essential (or semi-essential, classically grouped as essential in this context), and basic (guanidino side chain) → fits X. …
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AP EAPCET 2025Set eng-2025-05-26-AN1 markMCQ
$Q. Identify the essential amino acids from the following (only = only). A) Leucine B) Tyrosine C) Cysteine D) Histidine (A) A & B only (B) B & C only (C) B & D only (D) A & D only$
›Reveal solutionSolution
Tests recall of essential vs non-essential amino acids; Leucine and Histidine are essential, Tyrosine and Cysteine are not.

Concept and Intuition

"Essential" amino acids are those the human body cannot synthesise in adequate amounts through its own metabolic pathways, so they must be obtained from the diet. "Non-essential" amino acids can be synthesised in the body, often by simple transformation of another amino acid or metabolic intermediate — for instance Tyrosine is made in the body by hydroxylation of Phenylalanine, and Cysteine is made from Methionine (via the transsulfuration pathway) and serine. Leucine, being a branched-chain amino acid, and Histidine both lack the biosynthetic machinery in humans and must be supplied externally, so they are classified essential.

Step-by-Step Solution

List the standard essential amino acids for humans: Valine, Leucine, Isoleucine, Threonine, Methionine, Phenylalanine, Tryptophan, Lysine, Histidine (mnemonic "PVT TIM HALL"). …
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AP EAPCET 2025Set eng-2025-05-26-FN1 markMCQ
$Q. Consider the following. Statement-I : Lysine, arginine are essential and basic amino acids Statement-II : Leucine, phenyl alanine are non essential and neutral amino acids. Correct answer is (A) Both statement-I and statement-II are correct (B) Both statement-I and statement-II are not correct (C) Statement-I is correct, but statement-II is not correct (D) Statement-I is not correct, but statement-II is correct$
›Reveal solutionSolution
This tests classification of amino acids as essential/non-essential and acidic/basic/neutral. The answer is (C): Statement I is correct; Statement II is wrong because leucine and phenylalanine are essential, not non-essential.

Concept and Intuition

Amino acids are classified along two independent axes: (i) essential vs non-essential — whether the human body can synthesize them internally (non-essential) or must obtain them from diet (essential); and (ii) acidic/basic/neutral — based on the side-chain functional groups (extra –COOH makes it acidic, extra –NH $_2$ /basic group makes it basic, otherwise neutral).

Step-by-Step Solution

Statement I: Lysine and arginine both carry an extra basic side-chain amino/guanidino group, making them basic amino acids, and both are classified as essential amino acids (the body cannot synthesize them in sufficient amounts and they must come from the diet). This statement is CORRECT. …
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AP EAPCET 2024Set ap-2024-05-16-AN1 markMCQ
$\mathrm{COOH}$
›Reveal solutionSolution
Tests identifying two amino acids from their Fischer projections and classifying them by

essentiality; I is leucine (essential), II is serine (non-essential).

Concept and Intuition

Essential amino acids cannot be synthesised by the human body in adequate amounts and must come

from the diet; non-essential ones can be synthesised endogenously from other metabolites.

Branched-chain amino acids (leucine, isoleucine, valine) are classic essential amino acids used

directly in muscle metabolism, while simple hydroxyl-bearing amino acids like serine are readily

made in the body (e.g. from the glycolytic intermediate 3-phosphoglycerate) and are non-essential.

Step-by-Step Solution

Structure I: side chain $\mathrm{-CH_2CH(CH_3)_2}$ (isobutyl group) on the standard amino-acid backbone — this identifies it as leucine, a branched-chain essential amino acid.

Structure II: side chain $\mathrm{-CH_2OH}$ — this identifies it as serine, whose hydroxyl …
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AP EAPCET 2024Set ap-2024-05-17-FN1 markMCQ
$\mathrm{H_2N-CH(R)-COOH}$
›Reveal solutionSolution
Only the lysine side chain is correctly given; the other three options each swap in

the side chain of a different, similarly-named amino acid.

Concept and Intuition

Amino acid side chains ("R" groups) must be memorized precisely, and this question

tests classic mix-ups between structurally-related amino acids:

Lysine (Lys): $-(\mathrm{CH_2})_4-\mathrm{NH_2}$ — a 4-carbon chain ending in a primary amine, giving it a basic side chain.

Valine (Val): $-\mathrm{CH(CH_3)_2}$ (isopropyl) — a branched 3-carbon group, NOT the isobutyl group of leucine.

Threonine (Thr): $-\mathrm{CH(OH)CH_3}$ — a secondary alcohol side chain, NOT the simple $-\mathrm{CH_2OH}$ of serine.

Aspartic acid (Asp): $-\mathrm{CH_2COOH}$ — an acidic carboxylic acid side chain, NOT the amide $-\mathrm{CH_2CONH_2}$ of asparagine (its amide derivative).

Step-by-Step Solution

Check (A): Lys — $-(\mathrm{CH_2})_4\mathrm{NH_2}$ . This matches the real lysine side chain. TRUE.

Check (B): Val — $-\mathrm{CH_2CH(CH_3)_2}$ . This is actually leucine's side chain (isobutyl, one extra $\mathrm{CH_2}$ compared to valine's isopropyl). FALSE for Val.

Check (C): Thr — $-\mathrm{CH_2OH}$ . This is actually serine's side chain; threonine has an extra methyl and is $-\mathrm{CH(OH)CH_3}$ . FALSE for Thr. …
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AP EAPCET 2024Set eng-2024-05-18-FN1 markMCQ
$_2$
›Reveal solutionSolution
Matching side chains to amino acid names and recalling which are "essential" (diet-only): the isopropyl side chain identifies Valine, an essential amino acid — option (C).

Concept and Intuition

Essential amino acids are the ones the human body cannot synthesise on its own; they must be obtained from food. The standard list (memorised in NCERT biology/biomolecules chemistry) includes Valine, Leucine, Isoleucine, Phenylalanine, Tryptophan, Threonine, Methionine, Lysine, Histidine. Alanine, Serine, Cysteine, Glycine, etc. are non-essential (the body makes them from other metabolites).

Step-by-Step Solution

Identify each amino acid from its side chain (all four share the same $H_2N-CH(-)-COOH$ backbone):

(A) $-CH_3$ : this is Alanine — non-essential.

(B) $-CH_2OH$ : this is Serine — non-essential.

(C) $-CH(CH_3)_2$ (isopropyl): this is Valine — a branched-chain amino acid, and it is essential.

(D) $-CH_2SH$ : this is Cysteine — non-essential. …
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AP EAPCET 2024Set eng-2024-05-19-AN1 markMCQ
$\mathrm{-NH_2}$
›Reveal solutionSolution
Asparagine uniquely carries a backbone $\mathrm{-NH_2}$ and $\mathrm{-CO_2H}$ plus a side-chain amide $\mathrm{-C(=O)NH_2}$ group.

Concept and Intuition

Every $\alpha$ -amino acid has a backbone $\mathrm{-NH_2}$ and $\mathrm{-CO_2H}$ . The question is really about the side chain: which amino acid's side chain is itself an amide ( $\mathrm{-CONH_2}$ )? That is asparagine, the amide of aspartic acid.

Step-by-Step Solution

Alanine's side chain is just $\mathrm{-CH_3}$ — no extra functional groups.

Arginine's side chain is a guanidino group ( $\mathrm{-NH-C(=NH)-NH_2}$ ), not an amide.

Aspartic acid's side chain is a second $\mathrm{-CO_2H}$ (carboxylic acid), not an amide. …
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Lys	Gln	Ser	Cys	Tyr	Asn
I	II	III	IV	V	VI

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Q.Amino acids behave like salts rather than simple amines or carboxylic acids. Explain.

Why This Happens — The Concept

Step-by-Step Reasoning

1. The structure of an amino acid forces an internal acid-base reaction

2. This explains the physical properties that seem contradictory

3. The zwitterion explains the amphoteric behaviour

4. The isoelectric point (pI) confirms the zwitterion model …

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