​The pKa of an amino acid side chain was measured in aqueous solution. Which one of the following is the correct arrangement of the amino acids in the decreasing order of their side chain pKa?​

Correct option is A

The correct answer is (a).

The pKa values of the amino acids are ordered as follows:The pKa values of the side chains of the amino acids in the question are approximately as follows:

• Serine: ~13 (hydroxyl group, very weakly acidic/basic)
• Lysine: ~10.5 (basic, amino group)
• Histidine: ~6.0 (basic, imidazole group)
• Aspartate: ~3.9 (acidic, carboxyl group)

Incorrect statement-

Option (b) Lysine > Histidine > Aspartate > Serine: Incorrect because Serine has a much higher pKa than Aspartate.

Option (c) Aspartate > Histidine > Lysine > Serine: Incorrect because Aspartate has the lowest pKa, not the highest.

Option (d) Serine > Histidine > Lysine > Aspartate: Incorrect because Lysine has a higher pKa than Histidine.