A portion of a polypeptide coils into an alpha helix. Which interaction most directly holds this secondary structure in place?

Core Concept

Secondary structure is local folding of the backbone itself. In an alpha helix each carbonyl oxygen of one peptide bond accepts a hydrogen bond from the amide hydrogen of another peptide bond a few residues along the chain, and this repeating network of backbone-to-backbone hydrogen bonds twists the polymer into a coil. Peptide bonds define the primary sequence, while disulfide bridges and salt bridges involve side chains and therefore belong to tertiary or quaternary structure.