Beta-Turn (β-Turn): Definition

Detailed Explanation

A beta-turn (also called a reverse turn or β-bend) is a structural motif where the peptide backbone reverses direction over four consecutive residues. The turn is stabilized by a hydrogen bond between the C=O of the first residue and the N–H of the fourth residue (i→i+3). Turns are essential for creating compact, globular peptide and protein structures.

There are several types classified by their backbone dihedral angles. Type I and Type II turns are the most common. Glycine and proline are frequently found in turns — glycine because its small size allows unusual backbone angles, and proline because its rigid cyclic structure naturally introduces chain direction changes. The sequence Asn-Gly is a classic turn-promoting motif.

In peptide drug design, turns are attractive structural features. Many biologically active peptides present their key residues on a turn structure. Cyclic peptides often incorporate turn-inducing elements (D-amino acids, proline) to pre-organize the molecule into the bioactive conformation, improving both target affinity and metabolic stability.

Key Facts