Which statement best describes the hydrogen bonding patterns of alpha-helices and beta-sheets?

Which statement best describes the hydrogen bonding patterns of alpha-helices and beta-sheets?

• A. Alpha-helix has intra-chain hydrogen bonds every fourth residue in a right-handed geometry, while beta-sheets are held together by inter-strand hydrogen bonds and can be parallel or antiparallel.
• B. Alpha-helix forms inter-strand hydrogen bonds between different polypeptide chains; beta-sheets rely on intra-chain bonds within a single strand.
• C. Alpha-helix is stabilized primarily by disulfide linkages; beta-sheets rely on ionic bonds between side chains.
• D. Alpha-helix and beta-sheet use identical hydrogen bonding patterns; the only difference is their overall shape.

Answer: (A)

The main idea is how hydrogen bonds govern the distinct patterns of alpha-helices and beta-sheets. In an alpha-helix, the hydrogen bonds form within the same polypeptide chain: the carbonyl oxygen of residue i accepts a hydrogen from the amide hydrogen of residue i+4. This creates a stable, right-handed helical structure with about 3.6 residues per turn. In contrast, a beta-sheet is built from stretches of backbone from adjacent strands hydrogen-bonding to each other, not within a single strand. These inter-strand hydrogen bonds link neighboring strands, and the strands can run parallel or antiparallel to one another, giving the sheet-like arrangement. So this description matches the statement that alpha-helices are stabilized by intrachain hydrogen bonds between i and i+4 in a right-handed geometry, while beta-sheets are stabilized by inter-strand hydrogen bonds and can be parallel or antiparallel. The other options misstate where the bonds form (inter- vs intra-chain), or emphasize disulfide or ionic interactions, which are not the primary hydrogen-bond patterns defining these secondary structures.

The main idea is how hydrogen bonds govern the distinct patterns of alpha-helices and beta-sheets. In an alpha-helix, the hydrogen bonds form within the same polypeptide chain: the carbonyl oxygen of residue i accepts a hydrogen from the amide hydrogen of residue i+4. This creates a stable, right-handed helical structure with about 3.6 residues per turn. In contrast, a beta-sheet is built from stretches of backbone from adjacent strands hydrogen-bonding to each other, not within a single strand. These inter-strand hydrogen bonds link neighboring strands, and the strands can run parallel or antiparallel to one another, giving the sheet-like arrangement.

So this description matches the statement that alpha-helices are stabilized by intrachain hydrogen bonds between i and i+4 in a right-handed geometry, while beta-sheets are stabilized by inter-strand hydrogen bonds and can be parallel or antiparallel. The other options misstate where the bonds form (inter- vs intra-chain), or emphasize disulfide or ionic interactions, which are not the primary hydrogen-bond patterns defining these secondary structures.