Thermodynamics of unfolding Protein G: ΔH° (unfolding) = +210.6 kJ/mol. What does the sign and magnitude imply about enthalpic favorability?
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AUnfolding is favored enthalpically
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BFolding is favored enthalpically (unfolding is endothermic)
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CThe entropy is positive at all temperatures
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DThe entropy is negative at all temperatures
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EΔH° says nothing about enthalpy
Answer
Correct Answer: Folding is favored enthalpically (unfolding is endothermic)
Explanation
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:
1) Reaction considered: Folded → Unfolded, ΔH° = +210.6 kJ/mol.2) Positive ΔH° implies energy absorbed to break stabilizing interactions.3) Therefore, enthalpy favors the folded state; unfolding is not enthalpically favored.4) Overall stability depends on ΔG° = ΔH° − T * ΔS°.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)