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In the alpha helix, a hydrogen bond forms between the carbonyl oxygen of one amino acid and an amino acid that lies:
AOne position further along the chain
BTwo positions further along the chain
CThree positions further along the chain
DFour positions further along the chain
Answer & Solution
Correct answer: D. Four positions further along the chain
1. Secondary structure comes from hydrogen bonding along the polypeptide backbone.
2. In the alpha helix these bonds run between the oxygen atom in a carbonyl group and a second amino acid.
3. That partner sits four amino acids farther along the chain.
4. This fixed spacing is what makes the turn of the helix regular.
5. A spacing of one, two or three would not close the helical turn.
_Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 3 'Biological Macromolecules', sections 3.1-3.5_
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