## **Core Concept**
The α-helix is a type of secondary structure in proteins characterized by a spiral conformation. This structure is stabilized by hydrogen bonds between the carbonyl oxygen of one amino acid and the amide hydrogen of another, four residues away. The α-helix has a specific geometry that determines how many amino acids are present in a single turn.

## **Why the Correct Answer is Right**
In a single turn of an α-helix, there are approximately 3.6 amino acids. This number arises from the helical rise per residue (approximately 1.5 Å) and the total rise per turn (approximately 5.4 Å), which results in 3.6 residues per turn. This specific number is crucial for understanding the structure and stability of α-helices in proteins.

## **Why Each Wrong Option is Incorrect**
- **Option A:** 2 amino acids per turn would imply a much tighter helix than what is observed in proteins, which does not align with the known structural parameters of α-helices.
- **Option B:** 4 amino acids per turn might seem plausible but does not accurately reflect the calculated 3.6 residues per turn, which is a well-established value in structural biology.
- **Option D:** 5 amino acids per turn would suggest a looser structure than the actual α-helix, which again does not match the structural data.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the α-helix structure is critical in understanding protein function and stability. The 3.6 amino acids per turn in an α-helix is a fundamental aspect of protein secondary structure, and alterations in this structure can significantly affect protein function and stability.

## **Correct Answer:** . 3.6