The β-turn is a common secondary structure in biologically active peptides and globular proteins, where it is widely thought to serve as a molecular recognition site for many biological processes. Although the primary β-turn recognition requirements are thought to be straightforward, relating mainly to the relative positions of the peptide sidechains, current classifications of β-turns are complex and are based solely upon the very variable geometry of the peptide backbone. We demonstrate here that β-turns can be described in terms of a single dihedral angle, which we have called β, which provides a complete description of the spatial relationship between the entry and exit peptide bonds as well as the relative orientations of the intervening sidechains for any β-turn. This description should prove particularly useful in the development and application of novel peptide mimetic drugs, compounds for which a classification based on a peptide backbone geometry may be entirely irrelevant.
