Introduction

Proteins are complex biomolecules whose three-dimensional structure is essential for their biological function. The native folded conformation of globular proteins is stabilized by various non-covalent interactions, including hydrogen bonds, ionic interactions, van der Waals forces, and hydrophobic interactions. When treated with organic solvents such as ethanol or acetone, proteins often lose their native conformation—a process known as denaturation.

This article explores which molecular interactions are primarily disrupted by organic solvents during protein denaturation, focusing on the dominant role of hydrophobic interactions.

What Happens During Protein Denaturation by Organic Solvents?

Denaturation refers to the unfolding or disruption of the native structure of proteins without breaking the primary peptide bonds. Organic solvents induce denaturation by altering the protein’s environment, affecting the delicate balance of forces that maintain its folded state.

Main Interactions Stabilizing Globular Proteins

• Hydrophobic interactions: Nonpolar amino acid side chains cluster inside the protein core to avoid contact with water, driving folding and stability.

• Hydrogen bonds: Stabilize secondary structures (α-helices, β-sheets) and contribute to tertiary structure.

• Ionic interactions (salt bridges): Electrostatic attractions between charged side chains.

• van der Waals forces: Weak attractions between closely packed atoms.

How Organic Solvents Affect These Interactions

• Organic solvents are less polar than water and disrupt the hydrophobic effect by reducing the energetic penalty for exposing nonpolar groups to the solvent.

• This weakening of hydrophobic interactions causes the protein core to become destabilized, leading to unfolding.

• Organic solvents do not typically break covalent bonds or directly disrupt ionic or hydrogen bonds initially but alter the protein’s solvation shell and environment.

• The loss of hydrophobic core packing is the key step in solvent-induced denaturation.

Evidence from Studies

• Denaturation by organic solvents like ethanol or acetone is believed to interfere mainly with the mutual attraction of nonpolar groups within the protein.

• Experimental and computational studies show that hydrophobic interactions contribute approximately 60% to protein stability, making them the most sensitive to solvent changes.

• Disruption of hydrophobic interactions leads to exposure of nonpolar residues, loss of compact structure, and unfolding.

Why Other Interactions Are Less Affected

• Hydrogen bonds can be disrupted by denaturants like urea or guanidinium chloride but are less directly affected by organic solvents initially.

• Ionic interactions require changes in pH or ionic strength to be disrupted significantly.

• Covalent bonds (like disulfide bridges) remain intact unless chemically reduced.

Summary Table

Practical Implications

• Understanding that hydrophobic interactions are the main target of organic solvent denaturation helps in protein purification and formulation.

• It guides the design of solvents and additives to stabilize proteins in pharmaceutical and industrial applications.

• It explains why some proteins refold upon removal of solvents, as hydrophobic interactions can re-establish.

Conclusion

When globular proteins are treated with organic solvents, the hydrophobic interactions that stabilize their core structure are primarily disrupted. This leads to unfolding and loss of native conformation, while hydrogen bonds, ionic interactions, and covalent bonds are less directly affected. Recognizing the central role of hydrophobic interactions in solvent-induced denaturation is crucial for understanding protein stability and designing strategies to preserve protein function.

Keywords

protein denaturation, organic solvents, hydrophobic interactions, globular proteins, protein unfolding, hydrogen bonds, ionic interactions, covalent bonds, protein stability, ethanol denaturation, acetone denaturation

Correct answer:
(4) Hydrophobic interactions