# Beta turns

A beta turn is defined for 4 consecutive residues (denoted by i, i+1, i+2 and i+3) if the distance between the Calpha atom of residue i and the Calpha atom of residue i+3 is less than 7Å and if the central two residues are not helical (either using the Kabsch and Sander criteria or using author defined criteria), Lewis et al. (1973).

The turns are assigned to one of 9 classes on the basis of the phi, psi angles of residues i+1 and i+2. The ideal angles for each of the turn types are as follows:

```
Type        Phi(i+1)     Psi(i+1)       Phi(i+2)        Psi(i+2)
I            -60           -30            -90                0
II           -60           120             80                0
VIII         -60           -30           -120              120
I'            60            30             90                0
II'           60          -120            -80                0
VIa1         -60           120            -90                0 cis-proline(i+2)
VIa2        -120           120            -60                0 cis-proline(i+2)
VIb         -135           135            -75              160 cis-proline(i+2)
IV          turns excluded from all the above categories
```

With the exception of the type VI turns these angles were originally defined by Venkatachalam (1968). The angles for the type VI turns were originally defined by Richardson (1981). We have used the nomenclature VIa1 and VIa2 to distinguish between two subclasses of type VIa turns with the phi, psi angles of residue i+1 in the beta and polyproline region of the Ramachandran plot Hutchinson & Thornton (1994).

The phi and psi angles are allowed to vary by +/- 30 degrees from these ideal values with the added flexibility of one angle being allowed to deviate by as much as 40 degrees. Types VIa1, VIa2 and VIb turns are subject to the additional condition that residue i must be a cis-proline. Turns which do not fit any of the above criteria are classified as type IV.

Click here for details of residue preferences at each position in the various turn types.

## Plots

The plots for each turn show a Ramachandran plot with residues i+1 (brown circle) and 1+2 (green square) plotted on it . Below is a schematic plot of the turn with the 4 residues and the Calpha(i)-Calpha(i+3) distance marked. A red arrow, if present, indicates that residue i donates a hydrogen bond to residue i+3. The residue numbers and turn type are indicated above the Ramachandran plot.

A PostScript or PDF version of the plots (showing 16 turns per page) can be generated by clicking on the appropriate icon below the Table.

## Table

The following data about each beta turn are shown: The residue numbers of residues i and i+3 in the turn, the one-letter amino acid code of residues i, i+1, i+2 and i+3 in the turn, and the turn type. For each of the central two residues (i+1 and i+2) phi, psi and chi1 are recorded. The final colums show the distance between the Calpha atoms of residues i and i+3 and whether or not a hydrogen bond exists between these two residues.

References

 Hutchinson EG and Thornton JM (1994). A revised set of potentials for beta turn formation in proteins. Protein Science, 3, 2207-2216. Kabsch W and Sander C (1983). Dictionary of protein secondary structure: pattern recognition of hydrogen-bonded and geometrical features. Biopolymers, 22, 2577-2637. Lewis PN, Momany FA and Scheraga HA (1973). Chain reversals in proteins. Biochem. Biophys. Acta, 303, 211-229. Richardson JS (1981) The anatomy and taxonomy of protein structure. Adv. Protein Chem. 34, 167-339. Venkatachalam CM (1968). Stereochemical criteria for polypeptides and proteins. V. Conformation of a system of three linked peptide units. Biopolymers, 6, 1425-1436.

Gail Hutchinson 22nd May, 1996
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