Hydrogen bonding capacity — Which amino acids listed can form hydrogen bonds through their side-chain (R-group) functional groups in aqueous proteins?

Introduction / Context:
Hydrogen bonds are critical for protein folding, enzyme–substrate recognition, and macromolecular assemblies. Side-chain functional groups that can donate or accept hydrogen bonds frequently determine specificity and stability.

Given Data / Assumptions:

• Asparagine (amide), aspartic acid (carboxylate), and glutamine (amide) are considered.
• We evaluate their R-groups for hydrogen bond donor/acceptor roles.
• Physiological pH context applies.

Concept / Approach:
Asparagine and glutamine have carboxamide side chains capable of donating (via –NH) and accepting (via C=O) hydrogen bonds. Aspartic acid, when protonated (–COOH), can donate; when deprotonated (–COO−), it can accept hydrogen bonds through its carboxylate oxygens and engage in electrostatic interactions.

Step-by-Step Solution:

Verification / Alternative check:
Protein crystal structures show these residues frequently located at active sites or solvent-exposed regions making H-bond networks.

Why Other Options Are Wrong:

• Asparagine only or glutamine only: incomplete; ignores aspartate's roles.
• Aspartic acid only: incomplete; amides are strong H-bond participants too.

Common Pitfalls:
Assuming deprotonated carboxylates cannot hydrogen bond; while less effective donors, they are strong acceptors and engage in intricate networks.

Final Answer:
All of these