JEE Chemistry Biomolecules MCQs Set 30

Answer: (b) (ii) and (iii)

Explanation:
1. Glycine has no chiral centre and threonine and isoleucine have two, so \( (i) \) is false and (ii) is true.
2. Protein amino acids are all L-configured, so (iii) is true.
3. Proline is a secondary amine (imino acid), so (iv) is false.

Teacher's Note:
1. Glycine is achiral.
2. Proline's nitrogen is part of a ring and has only one \( H \).

Answer: (b) \( \ce{H3N+-CH2-COOH} \)

Explanation:
1. The positively charged ammonium group is strongly electron-withdrawing \( (-I) \).
2. It stabilises the carboxylate ion formed on loss of \( H+ \), so \( \ce{H3N+CH2COOH} \) (pKa about 2.3) is more acidic than ethanoic acid (4.76) or the neutral amino acid.

Teacher's Note:
1. Cationic form is the most acidic species in the amino acid system.
2. \( -I \) effect increases acidity.

Answer: (a) alpha-COO- and NH3+ with the side-chain COOH intact

Explanation:
1. The alpha-COOH is more acidic than the side-chain COOH because of the neighbouring NH3+ group (\( -I \) effect).
2. So the alpha-COOH ionises first: alpha-COO-, NH3+ and side-chain COOH remain.
3. This gives the zwitterion.

Teacher's Note:
1. The carboxyl group closest to the ammonium ion is the most acidic.
2. pKa values: alpha-COOH 2.0, side chain 3.9.

Answer: (c) The basic functional group in amino acids is the carboxylate \( -COO^- \)

Explanation:
1. Amino acids contain both acidic (\( \ce{NH3+} \)/COOH) and basic (NH2) groups, so they are ampholytes.
2. They are classed as neutral, acidic or basic depending on side chains.
3. The characteristic basic group is the amino group, not carboxylate, so \( (c) \) is false.

Teacher's Note:
1. Ampholyte = can act as acid and base.
2. Classification: acidic (extra COOH), basic (extra NH2), neutral.

Answer: (c) alanine does not migrate, while arginine migrates to cathode

Explanation:
1. At pH equal to pI alanine exists as a neutral zwitterion and does not migrate.
2. Arginine's pI (10.2) is above 6, so at pH 6 it is in a more acidic medium than its pI; it takes up \( H+ \) and carries a net positive charge.
3. It therefore migrates towards the cathode.

Teacher's Note:
1. pH < pI: net positive charge (to cathode). pH > pI: net negative (to anode).
2. At pH = pI the net charge is zero.

Answer: (a) formaldehyde

Explanation:
1. Strecker synthesis: aldehyde + \( \ce{NH3} \) + HCN gives an alpha-aminonitrile, which is hydrolysed to the amino acid.
2. The aldehyde carbon becomes the alpha-carbon bearing side chain \( R \).
3. Glycine has \( R = H \), so the aldehyde is HCHO (formaldehyde).

Teacher's Note:
1. Aldehyde R-CHO gives amino acid R-CH(NH2)COOH.
2. Acetaldehyde would give alanine.

Answer: (c) \( \ce{CH3COCOOH} \)

Explanation:
1. This is a reductive amination: a keto acid reacts with ammonia to form an imino acid, which is reduced.
2. Pyruvic acid \( \ce{CH3COCOOH} \) gives alanine.

Teacher's Note:
1. Reductive amination of alpha-keto acids gives amino acids.
2. The carbonyl carbon becomes the alpha-carbon.

Answer: (c) \( \ce{64 x 10^3} \)

Explanation:
1. Mass of 4 Fe atoms = \( 4 x 56 = 224 \) g/mol.
2. This is 0.35% of the molar mass: \( M = 224 x 100 / 0.35 \).
3. \( M = 64000 = \ce{64 x 10^3} \) g/mol.

Teacher's Note:
1. Mass % = (mass of component / total) \( x 100 \).
2. Haemoglobin has 4 haem groups, each with one Fe.

Answer: (d) both amino groups protonated and the carboxyl group as -COO- (net charge +1)

Explanation:
1. Lysine has pKa values about 2.2 (COOH), 9.0 (alpha-NH3+) and 10.5 (epsilon-NH3+); pI is about 9.7.
2. At pH 9, which is below pI, the species with net charge \( +1 \) predominates: -COO- with both amino groups as \( \ce{NH3+} \).
3. So \( (d) \).

Teacher's Note:
1. Below the pI the net charge is positive.
2. Basic amino acids have a pI near 10.

Answer: (a) \( \ce{H3N+CH2COO^-} \)

Explanation:
1. Amino acids are dipolar ions in solution: the carboxyl proton transfers to the amino group.
2. So glycine exists as the zwitterion \( \ce{H3N+CH2COO^-} \) near neutral pH.

Teacher's Note:
1. The zwitterion explains high melting point and solubility in water.
2. Cation in strong acid, anion in strong base.

Answer: (b) soluble in water but insoluble in non-polar organic solvents

Explanation:
1. Zwitterionic structure makes amino acids behave like salts.
2. They dissolve in polar water but not in non-polar ether or benzene.

Teacher's Note:
1. High melting points and water solubility come from ionic character.
2. Compare with halo-acids, which are covalent.

Answer: (c) 6

Explanation:
1. A decapeptide has 9 peptide bonds; complete hydrolysis adds 9 water molecules: \( 9 x 18 = 162 \).
2. Total mass of hydrolysed products = \( 796 + 162 = 958 \).
3. Mass of glycine = \( 0.47 x 958 = 450.3 \), and \( 450.3/75 = 6 \) glycine units.

Teacher's Note:
1. Hydrolysis of an n-peptide uses \( (n-1) \) water molecules.
2. Divide glycine mass by its molar mass.

Answer: (b) Ninhydrin

Explanation:
1. Ninhydrin reacts with free alpha-amino groups to give a deep violet-blue compound (Ruhemann's purple).
2. Sanger's reagent (2,4-dinitrofluorobenzene) gives yellow DNP derivatives of the N-terminal amino acid.

Teacher's Note:
1. Ninhydrin is also used to detect amino acids on chromatograms.
2. Proline gives a yellow colour.

Answer: (b) reacts with the amino group, making the carboxyl group titratable with alkali

Explanation:
1. In amino acids the -NH3+ is acidic enough to interfere with titration against NaOH.
2. Formaldehyde reacts with the -NH2 group (forming methylol derivatives), removing the amine's basic character.
3. The carboxyl group can then be titrated sharply with alkali.

Teacher's Note:
1. Sorensen method: blocking of the amino group by HCHO.
2. Van Slyke method uses HONO to measure \( N2 \).

Answer: (b) 6

Explanation:
1. A tripeptide has three positions.
2. Arranging 3 different amino acids in a sequence gives \( 3! = 6 \) different tripeptides.
3. The N-terminal to C-terminal order matters.

Teacher's Note:
1. Peptides are directional: Gly-Ala differs from Ala-Gly.
2. For \( n \) different amino acids, \( n! \) sequences when each is used once.

Answer: (a) 2-amino-3-chloropropanoic acid (3-chloroalanine)

Explanation:
1. The amino acid is serine, \( \ce{HOCH2CH(NH2)COOH} \) with three functional groups.
2. Esterification, then PCl5 replaces the OH by Cl; hydrolysis of the ester gives \( \ce{ClCH2CH(NH2)COOH} \).
3. Reduction of C-Cl to \( C-H \) gives alanine.

Teacher's Note:
1. PCl5 converts alcoholic OH to Cl.
2. Alanine = 2-aminopropanoic acid.

Answer: (c) 3

Explanation:
1. Serine is \( \ce{HO-CH2-CH(NH2)-COOH} \).
2. The functional groups are: hydroxyl (-OH), amino (-NH2) and carboxyl (-COOH).

Teacher's Note:
1. Count different groups, not the number of atoms.
2. Serine is a hydroxy amino acid.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. Amino acids melt with decomposition at high temperature and dissolve in water because of their ionic (dipolar) structure.
2. Halo acids are covalent molecules with weaker intermolecular forces.
3. So the reason correctly explains the assertion.

Teacher's Note:
1. Ionic lattices have high melting points.
2. Zwitterions are salt-like.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. At the pI the net charge is zero.
2. This is because the amino acid exists as a zwitterion with equal positive and negative charges.
3. Zero net charge means no migration, so the reason explains the assertion.

Teacher's Note:
1. pI is the pH at which net charge is zero.
2. Solubility is minimum at pI.