Thermodynamics of unfolding Protein G: ΔH° (unfolding) = +210.6 kJ/mol. What does the sign and magnitude imply about enthalpic favorability?

Introduction:
Protein stability reflects a balance of enthalpy (ΔH) and entropy (ΔS). The enthalpy change for unfolding indicates how favorable intramolecular interactions are in the folded state compared to the unfolded state.

Given Data / Assumptions:

• Unfolding reaction: Folded → Unfolded with ΔH° = +210.6 kJ/mol.
• Sign convention: positive ΔH° means the reaction absorbs heat.

Concept / Approach:

If unfolding is endothermic (positive ΔH°), then favorable enthalpic interactions (H-bonds, van der Waals contacts, salt bridges) are being broken. Hence, folding is enthalpically preferred; unfolding requires heat input.

Step-by-Step Solution:

Verification / Alternative check:

Calorimetry typically shows endothermic unfolding transitions for many proteins, consistent with enthalpically stabilized folded states.

Why Other Options Are Wrong:

“Unfolding enthalpically favored” contradicts positive ΔH°. Statements about entropy cannot be concluded solely from ΔH°. ΔH° certainly conveys enthalpic information.

Common Pitfalls:

Equating enthalpy with spontaneity; forgetting the role of temperature and ΔS° (hydrophobic effect often gives positive ΔS° for unfolding of hydrophobic surfaces into solvent).

Final Answer:

Folding is favored enthalpically (unfolding is endothermic)