Question 1

Which of the following statements correctly describes the structure of an α-amino acid at neutral pH?

Accepted Answer

The amino group is protonated and the carboxylic acid group is deprotonated, resulting in a zwitterion.

Question 2

Which amino acid does NOT have a chiral α-carbon?

Accepted Answer

Glycine

Question 3

L-alanine and D-alanine are examples of:

Accepted Answer

Enantiomers

Question 4

Which amino acids are primarily responsible for the UV absorbance of proteins at 280 nm?

Accepted Answer

Tyrosine and tryptophan

Question 5

What is the typical $pK_a$ range for the α-amino group in amino acids?

Accepted Answer

$8.0-9.0$

Question 6

During the titration of histidine, what does the green dot on the titration curve represent?

Accepted Answer

The point where the net charge on the molecule is zero (isoelectric point, $pI$)

Question 7

Which posttranslational modification is important for stabilizing collagen structure?

Accepted Answer

4-Hydroxyproline

Question 8

The formation of a peptide bond between two amino acids is:

Accepted Answer

Thermodynamically unfavorable and requires coupling to ATP hydrolysis.

Question 9

Why is the peptide bond planar and stable?

Accepted Answer

Due to delocalization of electrons between the carbonyl oxygen and amide nitrogen.

Question 10

Which condition is required for non-enzymatic peptide bond cleavage in the laboratory?

Accepted Answer

Boiling in 6N HCl for 12–24 hours.

Question 11

Trypsin is a protease that cleaves peptide bonds after which amino acid residues?

Accepted Answer

Lysine and arginine

Question 12

A peptide chain consisting of 10 amino acid residues is classified as:

Accepted Answer

An oligopeptide

Question 13

Which modification can block the N-terminus of a peptide?

Accepted Answer

N-formyl group

Question 14

As the pH of a solution increases, what happens to the overall charge of a peptide?

Accepted Answer

It becomes more negative.

Question 15

Which of the following amino acids is classified as a nonpolar aromatic amino acid?

Accepted Answer

Phenylalanine

Question 16

Which group has a typical $pK_a$ range of $3.5-4.0$ in amino acids?

Accepted Answer

α-Carboxyl group

Question 17

Which posttranslational modification is involved in gene expression or suppression?

Accepted Answer

N-acetyllysine

Question 18

A protein sample is quantified by measuring its absorbance at 280 nm. Which molecule would interfere with this measurement if present in high concentration?

Accepted Answer

Nucleic acids

Question 19

Which of the following statements about polyampholytes is correct?

Accepted Answer

Their overall charge depends on the pH of the solution.

Question 20

A peptide contains the following sequence: Glu-Ala-Lys-Gly. At pH 7, which residue is most likely to carry a positive charge?

Accepted Answer

Lysine (Lys)

Amino Acids and Peptides: Structure, Properties, and Bonding

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