The ST turn is a structural feature in proteins and polypeptides. Each consists of three amino acid residues (labeled "i", "i" + 1 and "i" + 2) in which residue "i" is a serine (S) or threonine (T) that forms a hydrogen bond from its sidechain oxygen group to the mainchain NH group of residue "i" + 2.
Similar motifs occur with aspartate or asparagine as residue "i", called asx turn. Four types of asx turn and ST turn can be distinguished: types I, I’, II and II’. These categories correspond (via sidechain-mainchain mimicry of residue i) to those of the more abundant 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. 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 ST and asx turns.
Asx and ST turns both occur frequently at the N-termini of α-helices, as part of asx motifs or ST motifs, with the asx, serine or threonine as the N cap residue. They are thus often regarded as helix capping features.
Evidence for a functionally relevant ST turn is provided in the CDR3 region of the T-cell receptor (B chain, V domain) 
A proportion of ST turns are accompanied by a mainchain-mainchain hydrogen bond that qualifies them as ST motifs.
References.
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