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The α-helix structure is formed when the chain of α-amino acids coils as a right-handed screw (called α-helix) because of the formation of hydrogen bonds between amide groups of the same peptide chain.

1. Hydrogen bonding between different units is responsible for holding helix in a position.
2. NH group in one unit is linked to carbonyl oxygen of the third unit by hydrogen bonding.
3. Haemoglobin acts as an oxygen carrier in the blood since it reacts with oxygen to form unstable oxyhaemoglobin which easily breaks to give back haemoglobin and oxygen.
4. All proteins are not found in L-form but they may be present in form of D or L.

Correct answer: NH group in one unit is linked to carbonyl oxygen of the third unit by hydrogen bonding.

Solution

The α-helix structure is stabilized by intramolecular hydrogen bonding between the NH group of one amino acid and the carbonyl oxygen of the amino acid located three residues earlier in the chain.

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