Turns in Proteins

From Proteopedia

Since turns are classified as a type of secondary structure, they have an ordered, repetitive structure. There are about six different classes of β-turns with all of them containing four residues, but in the different classes the second and third residues have different values for psi and phi. Gly and Pro are commonly found in β-turns because their unique side chains permit a sharp bend in the peptide chain. A hydrogen bond formed between the backbond atoms of residues two and three is the major force maintaining the conformation of the bend in the chain. In one class a Pro in the third position has the cis configuration which maintains the sharp bend without the aid of a hydrogen bond.

. of first turn.