Question

question
how is it possible for the α - helix to have amino acids with bulky side chains while the β - pleated sheets cannot?
select the correct answer below:
the r groups in the α - helix stabilize the structure, while they interfere with the hydrogen bonds in a β - pleated sheet.
the r groups are contained inside the α - helix structure, while they interfere with the hydrogen bonds in a β - pleated sheet.
the r groups lie outside the α - helix structure, while they interfere with the hydrogen bonds in a β - pleated sheet.
the α helix structure allows any size r group to help stabilize the form, while only small r groups can fit and form the bonds necessary to stabilize a β - pleated sheet.

Explanation:

In an α - helix, R groups (side chains) are oriented outward. This orientation means that bulky R groups do not disrupt the internal hydrogen - bonding pattern that stabilizes the α - helix. In a β - pleated sheet, hydrogen bonds form between adjacent strands. Bulky R groups on adjacent strands would sterically interfere with the formation of these hydrogen bonds. Option A is incorrect because R groups in the α - helix do not stabilize the structure through their presence (the hydrogen bonds do), and they interfere with β - sheet hydrogen bonds due to steric reasons, not just some undefined "interference". Option B is wrong as R groups are not inside the α - helix. Option D is incorrect because the α - helix does not use R groups to stabilize its form (hydrogen bonds do), and the issue with β - sheets is steric hindrance (not that only small R groups "fit" in some bond - forming sense).

Answer:

C. The R groups lie outside the α - helix structure, while they interfere with the hydrogen bonds in a β - pleated sheet.