Q3. Glycine is amphoteric. What does its zwitter ion become in strongly acidic solution, and in strongly basic solution?
In acid the –COO− group takes up a proton, giving H3N+–CH2–COOH. In base the –N+H3 group loses one, giving H2N–CH2–COO− (reasoned from NCERT page 292: the zwitter ion reacts with both acids and bases).
Q4. Classify lysine, aspartic acid and serine as acidic, basic or neutral amino acids.
Lysine is basic: its side chain, H2N–(CH2)4–, adds a second amino group. Aspartic acid is acidic: HOOC–CH2– adds a second carboxyl group. Serine is neutral: HO–CH2– adds neither (side chains from NCERT Table 10.2; rule from NCERT page 291).
Q5. How many water molecules are lost when six amino acids join into a hexapeptide? What is the molar mass of the hexapeptide of glycine, (Gly)6? (Glycine = 75 g mol−1.)
Six units in a chain are joined by five peptide bonds, and each bond releases one water, so 5 water molecules are lost. M = 6 × 75 − 5 × 18 = 450 − 90 = 360 g mol−1 (reasoned from NCERT page 292).
Q6. How do the hydrogen bonds of an α-helix differ from those of a β-pleated sheet?
In the α-helix the –NH of each residue is hydrogen-bonded to a >C=O on the next turn of the same chain. In the β-pleated sheet, stretched chains lie side by side and are held by intermolecular hydrogen bonds between the chains (NCERT page 293).