Short answer: A beta turn (β-turn) is a four-residue segment, usually labelled i to i+3, in which the polypeptide chain folds back on itself so that the α-carbons of the first and fourth residues sit less than about 7 Å apart while the two middle residues are not part of a helix [1][2]. Many β-turns are also closed by a backbone hydrogen bond between the C=O of residue i and the N–H of residue i+3, the feature Venkatachalam used when he first described them in 1968 [1][3]. Turns are classified by the backbone φ and ψ angles of the two central residues: the classical scheme names Types I, I′, II, II′, VIa1, VIa2, VIb, VIII and a catch-all Type IV, and a 2019 high-resolution survey proposed 18 types with a simpler two-letter naming system [1].
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Where beta turns sit among peptide secondary structures
Folded chains are usually described in terms of repetitive elements — the alpha helix and the strands of a beta sheet — joined by irregular loops. In de Brevern's summary, α-helices account for about a third of residues, β-strands about a fifth, and the "coil" state for almost half; the β-turn is described as the third most important secondary structure after helices and β-strands [2].
Turns are grouped by how many residues it takes to bring the chain back on itself, with the Cα of residue i and residue i+n closer than 7 Å (some authors use 7.5 Å) [2]:
- γ-turn: three residues (n = 3).
- β-turn: four residues (n = 4).
- α-turn: five residues (n = 5).
- π-turn: six residues (n = 6).
How a beta turn is defined
Two criteria have been used, and they are not identical [1][2][3]:
- Hydrogen-bond criterion. Venkatachalam's 1968 analysis identified a backbone hydrogen bond between the carbonyl oxygen of the first residue and the amide hydrogen of the fourth [1][2]. Marcelino and Gierasch describe the β-turn as the turn type in which this hydrogen bond links residues i and i+3 [3].
- Distance criterion. In 1973 Lewis and co-workers observed that many chain segments that still look like turns lack that 1→4 hydrogen bond. They redefined a β-turn as four residues with a Cα1–Cα4 distance under 7 Å and central residues 2 and 3 not in a helix [1]. Wilmot and Thornton used the same "less than 7 Å" standard when they extracted turns from 59 protein structures [5].
The 7 Å cutoff is a convention, not a law. Shapovalov et al. found that their clustering with the 7 Å cutoff located all significant β-turn types in their data, but several types (AB2, dD, dN and pG) have significant density above 7 Å, and they suggest that running their assignment tool with cutoffs up to 8.5 Å may be useful in some circumstances [1].
The classical turn types (I, II, their mirror images, VI, VIII and IV)
β-turns are classified by the φ and ψ dihedral angles of the two central residues, i+1 and i+2 — the same backbone angles plotted on a Ramachandran map (see our peptide backbone guide) [2]. In the classical assignment, three of those four angles may deviate by ±30° from the canonical values and the fourth by ±45° [2].
The scheme grew in stages [1][2]:
- 1968 — Venkatachalam: Types I, II and III and their mirror images I′, II′ and III′. He predicted the mirror-image forms would be disfavoured by steric clashes [1].
- 1973 — Lewis et al.: with the distance definition, ten types, including V, VI (a cis-proline turn), VII and a miscellaneous Type IV for turns that fit no definition [1].
- Richardson et al.: reduced the list to I, I′, II, II′, VIa, VIb and Type IV, splitting the cis-proline turns into VIa and VIb and merging Type III into Type I because the two occupy similar Ramachandran regions [1].
- 1988 — Wilmot and Thornton: added Type VIII, in which the central residues adopt αR and β conformations respectively [1][5].
Type IV is not a small leftover bin. The classical types describe only about two-thirds of the β-turns observed in proteins, and de Brevern reports that miscellaneous Type IV turns account for a third of β-turn residues [1][2]. de Brevern's clustering of Type IV defined four new groups, IV1–IV4, which together cover half of the Type IV turns [2].
A newer nomenclature: 18 turn types from ultra-high-resolution structures
In 2019 Shapovalov, Vucetic and Dunbrack clustered 13,030 β-turns from 1,074 protein structures solved at 1.2 Å resolution or better [1]. They defined 18 turn types, 11 of them new (five of those being sub-types of classical types), and proposed naming each type with two letters taken from the Ramachandran-map regions of residues 2 and 3 [1]. Upper-case letters denote the left (negative φ) side of the map and lower-case letters the right (positive φ) side [1].
Selected clusters, with their share of the 13,030 turns, classical equivalents and modal (φ, ψ) values for residues 2 and 3 [1]:
- AD (classical Type I): 49.22% of the turns; residue 2 about (−62°, −23°), residue 3 about (−96°, −2°).
- Pd (classical Type II): 11.94%; residue 2 about (−55°, 133°), residue 3 about (91°, −6°).
- Pa (new, previously grouped with Type II): 6.16%.
- ad (classical Type I′): 5.74%; residue 2 about (47°, 45°), residue 3 about (83°, 1°).
- AB2 (classical Type VIII): 4.63%.
- pD (classical Type II′): 3.09%; residue 2 about (57°, −130°), residue 3 about (−95°, 11°).
Which residues favour turns — and why glycine and proline keep appearing
Turn propensity is positional: each of the four slots favours different residues, and the preferences differ by turn type.
- Type I vs Type II (Wilmot and Thornton). Type I turns favoured Asp, Asn, Ser and Cys at i; Asp, Ser, Thr and Pro at i+1; Asp, Ser, Asn and Arg at i+2; and Gly, Trp and Met at i+3. Type II turns preferred Pro at i+1, Gly and Asn at i+2, and Gln and Arg at i+3 [5].
- Proline. Proline suits positions that need φ near −60°, such as position 2 of Types I, II and VIII; it is required for the cis-peptide turn types; and because it prevents the residue before it from adopting an α conformation, it is completely absent from positions 3 and 4 of AD (Type I) turns [1].
- Glycine (and Asn/Asp). Glycine lets residues reach the positive-φ regions of the map, which is why it dominates position 3 of Pd (Type II) turns; Asp and Asn dominate position 2 of ad (Type I′) turns [1].
Wilmot and Thornton noted that because these preferences reflect local interactions, their predictions should apply to peptides as well as proteins [5]. One structural note for peptide chemists: turns are frequently sites of post-translational modification such as phosphorylation and glycosylation in proteins [3].
Beta turns and beta hairpins
A β-turn flanked by two β-strands forms a β-hairpin, the basic building block of a β-sheet [3]. The mirror-image turn types are strongly associated with hairpins: Wilmot and Thornton found 83% of Type I′ and 50% of Type II′ turns in β-hairpins [5], and Hutchinson and Thornton tied sequence preferences in I′ and II′ turns to their involvement in hairpins [4].
Hairpin stability depends on more than the turn. Marcelino and Gierasch list cross-strand aromatic–aromatic, aromatic–polar, hydrogen-bonding, hydrophobic and salt-bridge interactions, together with loop conformational propensity and loop-length entropy terms [3].
The tryptophan zipper ("trpzip") peptides from Cochran, Skelton and Starovasnik illustrate how turn sequence and cross-strand pairs work together [6]:
- trpzip1 (SWTWEGNKWTWK) uses a Type II′ turn sequence (EGNK).
- trpzip2 (SWTWENGKWTWK) uses a Type I′ Asn-Gly turn.
- trpzip3 (SWTWEpNKWTWK) uses a Type II′ turn built on D-proline ("p") followed by Asn — see our D-amino acids guide for the configurational background.
Studying turns in short peptides
Because a β-turn is local, it is well suited to study in small model peptides [3]. Marcelino and Gierasch describe how cyclic peptides, pioneered as turn models by the Blout laboratory, served as constrained models whose crystal and NMR structures confirmed the original β-turn types; their rigidity also made them useful for defining spectroscopic signatures of turns (our cyclic peptides guide covers the broader class) [3]. Later work by Dyson, Wright and Lerner showed that certain sequences gave highly populated turn conformations even in short linear peptides in water [3].
A long-running hypothesis is that turns can nucleate folding: turns with significant stability as isolated segments would actively promote structure formation, while turn sequences that merely permit chain reversal would be passive [3]. Marcelino and Gierasch conclude that intrinsic turn stability, topological context and participation in hydrophobic networks all correlate with how important a turn is in folding [3].
In the laboratory, solution spectroscopy such as circular dichroism reports average conformation rather than assigning each residue; in the trpzip work, for example, well-defined near-UV CD bands of trpzip1 were read as evidence that its indole side chains sit in a defined chiral environment [6].
FAQ
How many residues are in a beta turn?
Four, conventionally numbered i to i+3. A three-residue reversal is a γ-turn and a five-residue one an α-turn [2]. A β-bulge can add a fifth residue to a β-turn [3].
Does every beta turn have a hydrogen bond?
No. The i→i+3 hydrogen bond was Venkatachalam's original criterion, but Lewis et al. found many turn-like segments without it and switched to a Cα1–Cα4 distance under 7 Å [1]. Turn-like segments that lack the hydrogen bond but have φ, ψ angles within 30° of a standard type are called open turns [3].
What is the difference between a Type I and a Type II beta turn?
Mainly the orientation of the peptide bond between the two corner residues, which changes the φ, ψ angles of residues 2 and 3 and the direction their side chains point [1][3]. Type II turns prefer Pro at i+1 and Gly or Asn at i+2 [5].
What do the primes in Type I′ and Type II′ mean?
They mark the mirror-image forms of Types I and II, first described by Venkatachalam [1]. In the 2019 two-letter scheme, Type I is "AD" and Type I′ is "ad" [1].
What is a Type IV beta turn?
The miscellaneous category for turns that do not match any defined type. It accounts for about a third of β-turn residues, which is why researchers have proposed extending the classification [1][2].
How is a beta turn different from a beta hairpin?
A β-turn is the four-residue reversal itself; a β-hairpin is a β-turn flanked by two β-strands that pair with each other [3].
Bottom line
A beta turn is the four-residue unit that lets a peptide chain reverse direction: residues i and i+3 sit within about 7 Å, the middle two are non-helical, and an i→i+3 hydrogen bond is often, but not always, present [1][2][3]. Types are assigned from the φ, ψ angles of the two corner residues; the classical I/I′/II/II′/VI/VIII/IV scheme is still the most widely used, while a 2019 survey of ultra-high-resolution structures proposed 18 types with Ramachandran-based names [1][2]. Glycine, proline, asparagine and aspartic acid recur in turns for conformational reasons, mirror-image I′ and II′ turns are common in β-hairpins, and designed peptides such as the trpzips show how turn choice and cross-strand interactions combine to give a stable fold in water [1][3][5][6].
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