The polypeptide's R groups in alpha helices---------------------and in the β-pleated sheet, hydrogen bonding between atoms on the polypeptide chain's backbone forms the "pleats". The R groups are attached to the carbons and -------------------- the pleat's folds.

Question

BlackTom AI · Accepted Answer

The question is describing how the polypeptide backbone forms hydrogen bonds in alpha helices and beta-pleated sheets, and asks about the position of the R groups (side chains) relative to the structure.
Option 1: 'protude out of the structure; extend above and below' This aligns with standard structural biology: in both alpha helices and beta-pleated sheets, the backbone forms hydrogen bonds that stabilize the folds, and the side chains (R groups) project outward from the backbone, extending above and below the plane of the main chain. The wording conveys that side chains stick out from the structural core rather than being buried in the hydrogen-bond network, which is correct.
Option 2: 'are separated by 4 Aa, and are separated by the same distance in' This option seems to conflate spatial packing with a specific distance between side chains. While there are regularities in helix geometry (e.g., a helical rise per residue and a repeat distance along the axis), stating a fixed separation of 4 Å between all side chains in both structures is misleading and not an accurate general rule. The actual distances are context-dependent and not simply '4 Å' in both motifs.
Option 3: 'protrude out of the pleats; separated by 7A' The phrase 'pleats' refers to the beta-pleated sheet, but side chains do indeed extend from the backbone in beta sheets, not specifically 'out of the pleats' as a separate structural feature. Additionally, the claimed 7 Å separation is not a universal descriptor for side-chain spacing in beta sheets, making this option scientifically dubious and overly specific without justification.
Option 4: 'extend above and below, protrude out of the structure' This option closely mirrors Option 1 but swaps the order of clauses. While the content is similar, the phrasing is a bit awkward. The substantive idea—that R groups extend above and below the backbone plane in these structures—is correct, but the redundancy and inverted phrasing make it less precise than Option 1.
In summary, the core concept to evaluate is where the R groups sit relative to the backbone: they extend outward from the main-chain structure rather than being tucked into the hydrogen-bonded core. Options 2 and 3 introduce specific distances or phrases that misrepresent or overstate the geometry, and Option 4, while not entirely wrong in meaning, is less clearly phrased than Option 1.

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The polypeptide's R groups in alpha helices---------------------and in the β-pleated sheet, hydrogen bonding between atoms on the polypeptide chain's backbone forms the "pleats". The R groups are attached to the carbons and -------------------- the pleat's folds.

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