25. At what pH does poly-Glu in an aqueous solution form α-helical structure?
(A) 3
(B) 7
(C) 9
(D) 12
At What pH Does Poly-Glutamate (Poly-Glu) Form an α-Helical Structure?
Correct Answer
(A) pH 3
Introduction
Proteins acquire their biological functions only after folding into specific three-dimensional structures. One of the most common secondary structural elements found in proteins is the α-helix, a right-handed helical arrangement stabilized by hydrogen bonds between the peptide backbone. Although the peptide backbone forms these hydrogen bonds, the stability of an α-helix is strongly influenced by the chemical properties of the amino acid side chains. Factors such as pH, ionic interactions, electrostatic repulsion, and solvent conditions determine whether a polypeptide remains helical or adopts an unfolded conformation.
Polyglutamate (poly-Glu) is a synthetic polypeptide composed entirely of glutamic acid residues. Because every amino acid in the chain possesses the same side chain, poly-Glu has been widely used to study the relationship between amino acid charge and protein secondary structure. One of the classic observations in protein chemistry is that poly-Glu forms an α-helix under acidic conditions but loses its helical structure at neutral and alkaline pH.
Understanding the Concept Behind the Question
Glutamic acid contains a side-chain carboxyl group (-COOH) with a pKa of approximately 4.1. The ionization state of this group depends on the pH of the surrounding solution.
At low pH, the side-chain carboxyl groups remain protonated (-COOH) and therefore carry no negative charge. Since neighboring glutamate residues are electrically neutral, there is very little electrostatic repulsion between adjacent side chains. Under these conditions, the peptide backbone can fold comfortably into a stable α-helical structure.
At neutral or alkaline pH, however, the side-chain carboxyl groups lose protons and become negatively charged (-COO⁻). Because every glutamate residue now carries a negative charge, strong electrostatic repulsion develops between neighboring side chains. This repulsion destabilizes the α-helix and favors an extended or random coil conformation.
Among the given options, pH 3 is the only strongly acidic condition where most glutamate side chains remain protonated.
Therefore, poly-Glu forms an α-helical structure at pH 3.
Why Option (A) Is Correct
pH 3
At pH 3, the solution is more acidic than the pKa of glutamic acid. Consequently, the side-chain carboxyl groups remain predominantly in their protonated (-COOH) form.
Because these side chains are electrically neutral, neighboring glutamate residues do not repel one another. The peptide backbone is therefore free to establish the characteristic hydrogen-bonding pattern required for α-helix formation.
This acidic environment stabilizes the helical conformation, making pH 3 the correct answer.
Therefore, Option (A) is correct.
Why Option (B) Is Incorrect
pH 7
At physiological pH, glutamic acid side chains exist almost entirely as negatively charged carboxylate ions (-COO⁻).
Since every residue in poly-Glu carries the same negative charge, intense electrostatic repulsion develops along the length of the polypeptide chain. This repulsion disrupts the regular hydrogen-bonding pattern required for α-helix formation and causes the molecule to adopt a more extended conformation.
Therefore, poly-Glu does not form a stable α-helix at pH 7.
Hence, Option (B) is incorrect.
Why Option (C) Is Incorrect
pH 9
At pH 9, the environment is strongly alkaline, and all glutamate side chains remain fully deprotonated (-COO⁻).
The negative charges repel one another even more strongly than at neutral pH, making α-helix formation highly unfavorable. Instead of folding into a compact helix, the polypeptide chain remains unfolded because electrostatic repulsion exceeds the stabilizing effect of backbone hydrogen bonds.
Therefore, Option (C) is incorrect.
Why Option (D) Is Incorrect
pH 12
At pH 12, the solution is extremely alkaline. Every glutamate side chain is completely ionized and negatively charged.
The resulting electrostatic repulsion between adjacent residues is maximal, producing one of the least favorable conditions for α-helical structure. Under these conditions, poly-Glu exists predominantly in an extended random-coil conformation rather than as an α-helix.
Therefore, Option (D) is incorrect.
Why Does pH Affect Protein Secondary Structure?
The secondary structure of proteins depends not only on hydrogen bonding but also on interactions among amino acid side chains. Changes in pH alter the ionization state of acidic and basic amino acids, modifying electrostatic attractions and repulsions within the polypeptide.
When like charges accumulate on neighboring residues, repulsive forces destabilize compact structures such as α-helices. Conversely, neutralization of these charges reduces repulsion and allows hydrogen bonding to dominate, promoting stable secondary structures.
Poly-Glu is an ideal example because every residue possesses the same ionizable side chain, making the effect of pH particularly dramatic.
Biological Importance of α-Helices
The α-helix is one of the most abundant secondary structures found in proteins. It contributes to the stability and function of enzymes, membrane proteins, transport proteins, receptors, and structural proteins. Many transmembrane proteins contain long α-helical segments that span lipid bilayers, while fibrous proteins such as α-keratin derive much of their strength from extensive α-helical organization.
The ability of environmental conditions such as pH to alter α-helical stability is also important in protein folding, denaturation, and disease processes. Changes in intracellular pH can influence enzyme activity and protein conformation, highlighting the biological significance of this phenomenon.
Relationship Between pH and Poly-Glu Structure
| pH | Side Chain Charge | α-Helix Formation |
|---|---|---|
| 3 | Mostly protonated (-COOH) | Stable |
| 7 | Negatively charged (-COO⁻) | Unstable |
| 9 | Fully deprotonated (-COO⁻) | Unstable |
| 12 | Completely deprotonated (-COO⁻) | Highly unstable |
Common Mistakes in Competitive Examinations
Many students assume that proteins are most stable at physiological pH and therefore choose pH 7. While this may be true for many natural proteins, poly-Glu behaves differently because every residue is glutamic acid. At neutral pH, all side chains become negatively charged, creating strong electrostatic repulsion that destabilizes the α-helix.
Another common mistake is memorizing the answer without understanding the role of pKa. Remember that when the pH is below the pKa, acidic side chains remain protonated and electrically neutral, favoring α-helix formation.
High-Yield Points
- Poly-Glu consists entirely of glutamic acid residues.
- Side-chain pKa of glutamic acid ≈ 4.1.
- Below pKa → COOH (neutral).
- Above pKa → COO⁻ (negative).
- Electrostatic repulsion destabilizes α-helices.
- Poly-Glu forms a stable α-helix at acidic pH (≈3).
Frequently Asked Questions
Why does poly-Glu form an α-helix only at acidic pH?
At acidic pH, glutamic acid side chains remain protonated and electrically neutral. Without electrostatic repulsion, backbone hydrogen bonds stabilize the α-helical structure.
Why is the α-helix unstable at neutral pH?
At neutral pH, glutamate side chains become negatively charged. Repulsion between adjacent negative charges destabilizes the helix and favors an unfolded conformation.
Does pH affect all proteins in the same way?
No. The effect of pH depends on the amino acid composition of the protein. Poly-Glu is especially sensitive because every residue contains the same ionizable acidic side chain.
Key Takeaways
Polyglutamate is an excellent model for understanding how pH influences protein secondary structure. At acidic pH (around 3), the glutamic acid side chains remain protonated and electrically neutral, allowing backbone hydrogen bonds to stabilize the α-helical conformation. As the pH increases above the pKa of glutamic acid, the side chains become negatively charged, producing strong electrostatic repulsion that disrupts the helix. Thus, acidic conditions promote α-helix formation, whereas neutral and alkaline conditions favor an unfolded structure.
Final Answer
Correct Option: (A) pH 3
Explanation
Polyglutamate (poly-Glu) forms a stable α-helical structure under acidic conditions because the side-chain carboxyl groups of glutamic acid remain protonated (-COOH) and therefore electrically neutral. The absence of negative charges minimizes electrostatic repulsion between adjacent glutamate residues, allowing the peptide backbone to form stable hydrogen bonds characteristic of an α-helix. At neutral or alkaline pH, the side chains become negatively charged (-COO⁻), leading to strong electrostatic repulsion that destabilizes the helix. Therefore, poly-Glu forms an α-helical structure at pH 3, making Option (A) the correct answer.
