Q.Proteins are found to have two different types of secondary structures viz. $\alpha$-helix and $\beta$-pleated sheet structure. $\alpha$-helix structure of protein is stabilised by:

Concept: Hydrogen Bonding – The $\alpha$-helix is a coiled secondary structure held together primarily by intramolecular hydrogen bonds between the carbonyl oxygen of one amino acid and the amide hydrogen of another, four residues away.

Reasoning:

1. Peptide bonds form the primary backbone but do not stabilize the helical shape itself.
2. van der Waals forces and dipole-dipole interactions contribute weakly but are not the dominant stabilizing force. …

The question asks what stabilises the $\alpha$-helix — a specific, regular coiling pattern in a protein chain. To answer this, you need to understand what holds the helix together once it has formed.

A protein’s primary structure is just a linear chain of amino acids linked by peptide bonds. But the $\alpha$-helix is a secondary structure — a local, repeating shape that the chain folds into. The peptide bonds themselves are already formed; they don’t provide the extra stabilisation for the helix. So option (i) is out.

What about van der Waals forces or dipole-dipole interactions? These exist everywhere in molecules, but they are weak and non-directional. The $\alpha$-helix is a very precise, regular structure — it needs a specific, strong, and directional interaction to hold its shape. That interaction is the hydrogen bond.

Here’s the key insight: In an $\alpha$-helix, the backbone of the protein (not the side chains) forms a spiral. Every amino acid has a carbonyl group ($\ce{C=O}$) and an amide group ($\ce{N-H}$). In the helix, the $\ce{C=O}$ of one residue points directly toward the $\ce{N-H}$ of the residue four positions ahead in the chain. This creates a hydrogen bond between the oxygen (electronegative, partial negative) and the hydrogen (attached to nitrogen, partial positive).

These hydrogen bonds run parallel to the helix axis and are repeated regularly along the entire length of the helix. They are the main force that keeps the coil from unwinding.

Let’s walk through the reasoning step by step:

1. Identify the structure: The $\alpha$-helix is a right-handed coil where each turn contains about 3.6 amino acid residues. The side chains point outward, away from the helix core.

2. What holds the coil together? The backbone atoms are involved in a repeating pattern of hydrogen bonding. Specifically, the $\ce{C=O}$ of residue $n$ forms a hydrogen bond with the $\ce{N-H}$ of residue $n+4$.

3. Why not the other options?

   • Peptide bonds are covalent links between amino acids — they form the primary structure, not the secondary structure’s stabilisation.
   • van der Waals forces are weak and contribute to packing but are not the dominant stabiliser of the helix. …

Concept: Protein Secondary Structure Stabilisation

The $\alpha$-helix is a right-handed coiled conformation of a polypeptide chain. The key stabilising force comes from hydrogen bonding between the backbone amide groups.

Method: Identify the Bonding Pattern in $\alpha$-Helix

Step 1: Recall the $\alpha$-helix geometry

• The polypeptide backbone coils tightly.
• Each turn contains about 3.6 amino acid residues.

Step 2: Locate the stabilising interactions

• The $\ce{C=O}$ group of residue n forms a hydrogen bond with the $\ce{N-H}$ group of residue n+4.
• These bonds run parallel to the helix axis and hold the coil together.

Step 3: Eliminate other options

• (A) Peptide bonds — These are covalent bonds linking amino acids; they form the primary structure, not stabilise the helix. …

Common Mistakes & How to Avoid Them

Mistake 1: Choosing Peptide Bonds (Option A)

Why students pick it:

Students confuse the primary structure (held by peptide bonds) with the secondary structure. Since the question mentions "protein structure," they assume peptide bonds are always the answer.

How to avoid:

Remember the hierarchy of protein structure:

• Primary → peptide bonds (covalent)
• Secondary → hydrogen bonds (between $\ce{-NH}$ and $\ce{-CO}$ groups of the backbone)
• Tertiary → various interactions (ionic, hydrophobic, disulfide, etc.)
• Quaternary → same as tertiary, but between subunits

Key rule: Peptide bonds are already formed before the chain folds into $\alpha$-helix or $\beta$-sheet. They do not stabilise the helix — they are the backbone itself.

Mistake 2: Choosing van der Waals forces (Option B)

Why students pick it:

They think "any weak force stabilises a helix" and van der Waals forces are weak. Also, some recall that van der Waals forces help in tertiary structure packing.

How to avoid:

Ask yourself: What specifically holds the turns of the helix together?

In an $\alpha$-helix, the $\ce{-NH}$ of one amino acid forms a hydrogen bond with the $\ce{-CO}$ of the amino acid four residues earlier. This is a regular, repeating pattern — only hydrogen bonds can provide this directional, repeating stabilisation.

Memory aid: "Helix is held by H-bonds" — both start with H.

Mistake 3: Choosing Dipole-dipole interactions (Option D)

Why students pick it:

The $\ce{-NH}$ and $\ce{-CO}$ groups are polar, so students think dipole-dipole interactions are at work.

How to avoid:

Dipole-dipole interactions are non-specific and weaker. In the $\alpha$-helix, the interaction is a specific type of dipole-dipole interaction — hydrogen bonding. In exam contexts, "hydrogen bonds" is the precise, accepted term. "Dipole-dipole" is too broad and not the standard answer for this question.

Exam tip: When a specific, named interaction exists (like H-bonding), always choose that over a general category.

Mistake 4: Confusing $\alpha$-helix with $\beta$-sheet …