βturniil

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### Definition

A motif of four consecutive residues that may contain one H-bond, which, if present, is between the main-chain CO of the first residue and the main-chain NH of the fourth. It is characterized by the dihedral angles of the second and third residues, which are the basis for sub-categorization:

Sub-categories
```Type I
residue(i+1): -140° < φ < -20°   -90° < ψ < 40°
residue(i+2): -140° < φ < -20°   -90° < ψ < 40°
Type I'
Left-handed form of Type I
Type II
residue(i+1): -140° < φ < -20°    80° < ψ < 180°
residue(i+2):   20° < φ < 140°   -40° < ψ < 90°
Type II'
Left-handed form of Type II
```