Four types of Asx turn can be distinguished:^[8] types I, I’, II and II’. These categories correspond to those of the better-known hydrogen-bonded beta turns, which have four residues and a hydrogen bond between the CO of residue i and the NH of residue i+3. Asx turns and beta turns have structurally similar hydrogen-bonded loops and exhibit sidechain-mainchain mimicry in the sense that the sidechain of residue i of the Asx turn mimics the mainchain of residue i of the beta turn. Regarding their occurrence in proteins, they differ in that type I is the commonest of the four beta turns while type II’ is the commonest of the Asx turns.

Asx and ST turns both occur frequently at the N-termini of α-helices.^{[9][10][11][12]} as part of Asx motifs or ST motifs such that the Asx, serine or threonine is the N cap residue. They are thus often regarded as helix capping features.

Similar motifs occur with serine or threonine as residue i, which are called ST turns.^[13] In spite of serine and threonine having one less sidechain atom, such that the sidechain-mainchain mimicry of β turns is imperfect, these features occur in proteins as the four types in numbers approaching those of Asx turns. They also exhibit a tendency to substitute each other over evolutionary time.^[14]

A proportion of Asx turns are accompanied by a mainchain–mainchain hydrogen bond that qualifies them as Asx motifs.

• Motivated Proteins
• PDBeMotif
• [1]^[1]
• [2].^[2]