Question 1

Which of the following statements best describes the structure of an α-amino acid at physiological pH (pH 7)?

Accepted Answer

The amino group is protonated ($$\mathrm{NH_3$^+}) $$and the carboxyl group is deprotonated ($$\mathrm{COO^-}$$), forming a zwitterion.

Question 2

Which property of the α-carbon in amino acids is responsible for the existence of L- and D-isomers?

Accepted Answer

It is a chiral center, bonded to four different groups.

Question 3

Given the following structure, assign the configuration:

$$\mathrm{COO^-} ($$top), $$\mathrm{CH_2OH} ($$bottom), $$\mathrm{NH_3$^+} ($$left), H (right).

Accepted Answer

L-Serine

Question 4

Which of the following best explains why proteins are almost exclusively composed of L-amino acids?

Accepted Answer

The enzymes responsible for protein synthesis are stereospecific for L-amino acids.

Question 5

Which of the following equations correctly represents the ionization of the carboxyl group of an amino acid in water?

Accepted Answer

$$\mathrm{R-COOH + H_2O \rightarrow R-COO^- + H_3O^+}$$

Question 6

How many stereoisomers are possible for an amino acid with two chiral centers, according to the Van’t Hoff rule?

Accepted Answer

4

Question 7

Which of the following best describes the role of proteins in living systems?

Accepted Answer

They have diverse roles including catalysis, structure, transport, signaling, and defense.

Question 8

Which of the following interactions is NOT a non-covalent interaction involving the carboxyl group of an amino acid?

Accepted Answer

Peptide bond formation

Question 9

A patient’s blood pH drops to 6.8. What is the predominant form of an amino acid’s carboxyl group at this pH? (Assume $$pK_a of $$carboxyl group is 2.2)

Accepted Answer

Mostly deprotonated ($$\mathrm{COO^-})$$

Question 10

Which of the following statements about zwitterions is correct?

Accepted Answer

A zwitterion has both a positive and a negative charge but is overall neutral.

Question 11

Which of the following best explains why glycine is not optically active?

Accepted Answer

Its side chain is a hydrogen atom, making the α-carbon achiral.

Question 12

Which of the following is a correct statement about the peptide bond?

Accepted Answer

It is a covalent bond formed between the α-carboxyl group of one amino acid and the α-amino group of another.

Question 13

Which of the following best describes the difference between configuration and conformation in amino acids?

Accepted Answer

Configuration refers to the fixed spatial arrangement of atoms that can only be changed by breaking covalent bonds, while conformation refers to changes by rotation around single bonds.

Question 14

Which of the following amino acids is achiral?

Accepted Answer

Glycine

Question 15

A mutation causes a protein to incorporate D-amino acids instead of L-amino acids. Which of the following is a likely consequence?

Accepted Answer

The protein will likely lose its biological activity due to altered structure.

Question 16

Which of the following best describes the relationship between the structure and function of proteins?

Accepted Answer

Protein function is determined by the three-dimensional structure, which is dictated by the amino acid sequence.

Question 17

Which of the following is true regarding the optical activity of amino acids?

Accepted Answer

All amino acids except glycine are optically active.

Question 18

Which of the following best explains why amino acids can act as buffers?

Accepted Answer

They have both acidic and basic groups that can donate or accept protons.

Question 19

Which of the following is a correct statement about enantiomers of amino acids?

Accepted Answer

They have identical physical properties except for the direction in which they rotate plane-polarized light.

Question 20

Which of the following best describes the Fischer projection of an L-amino acid?

Accepted Answer

The amino group is on the left when the carboxyl group is at the top.

Amino Acids: Structure, Stereochemistry, and Properties

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