Th e β sheet is formed through interac- tions between noncontiguous amino acids in the polypeptide chain, in contrast to the α helix, which is formed from one contiguous segment of the chain. β strands are usually from fi ve to 10 residues long and are in an almost fully extended con- formation. To satisfy their hydrogen-bonding capacity, they form sheets in which β strands are aligned adjacent to each other (Figure 1.37 and Figure 1.38) such that hydrogen bonds can form between C=O groups and NH groups across the strands. Th e β sheets that are formed from several such β strands are “pleated”, with C α atoms alternately a little above and below the plane of the β sheet. Th e sidechains follow this pattern, pointing alternately above and below the β sheet.