Protein folding and hydrogen bonds: During the unfolding of an α-helix, breaking each intramolecular hydrogen bond requires about 2 kJ/mol. What does this indicate about the relative strength of hydrogen bonds in proteins compared with hydrogen bonding in bulk water?

Introduction / Context:
Hydrogen bonding stabilizes secondary structure (α-helices and β-sheets) in proteins. However, in aqueous solution a hydrogen bond broken inside a protein can often be replaced by new hydrogen bonds to water. The small energetic cost reported for bond breakage during helix unfolding helps interpret the balance between intraprotein and water-mediated hydrogen bonding.

Given Data / Assumptions:

• Energetic cost to break each helix hydrogen bond is ~2 kJ/mol.
• Process occurs in water where donor/acceptor groups can hydrogen-bond to solvent.
• We compare relative strengths qualitatively (protein interior vs bulk water).

Concept / Approach:
In thermodynamics of folding, intramolecular hydrogen bonds compete with hydrogen bonds to water. If breaking a helix hydrogen bond required a large energy, that would imply markedly stronger bonds in proteins. A modest ~2 kJ/mol cost indicates that when one H-bond is disrupted, new H-bonds with water almost compensate, making the net cost small. Thus, protein H-bonds are not drastically stronger; if anything, they are slightly weaker or only marginally favorable relative to solvent H-bonding.

Step-by-Step Solution:
Identify the reported bond breakage energy: ~2 kJ/mol (small).Infer that water quickly re-forms H-bonds with exposed donors/acceptors.Conclude that intraprotein H-bonds are slightly less favorable than water H-bonds.

Verification / Alternative check:
Experimental unfolding energetics and molecular dynamics show that hydrogen bonds contribute modestly to stability in water; hydrophobic packing and side-chain interactions often dominate.

Why Other Options Are Wrong:

• Much stronger / slightly stronger: Would imply a large energetic penalty, not observed.
• Not reformed with water: In water, exposed groups readily form new hydrogen bonds.
• Identical in all environments: Local environment modulates H-bond strength.

Common Pitfalls:
Assuming that any hydrogen bond automatically confers large stability; context (solvent competition) matters.

Final Answer:
Slightly weaker in proteins than in water.