A polypeptide chain can turn corners in a number of ways, to go from one segment or helix to the next. One kind of compact turn is called a turn (Figure 6.18). There are several varieties of turn, each able to accomplish a complete reversal of the polypeptide chain direction in only four residues; the carbonyl of residue i hydrogen-bonds to the amide hydrogen of residue i + 3.

In the even tighter turn, bonding is to residue i + 2 (Figure 6.19). Proline often plays a role in turns, as in Figure 6.19, and also as a breaker of helices, because this residue cannot be accommodated in the helix. Bends and turns most often occur at the surface of proteins.

-Helix

-Sheets