Beta turn

β turns (also β-bends, tight turns, reverse turns, Venkatachalam turns) are the most common form of turns—a type of non-regular secondary structure in proteins that cause a change in direction of the polypeptide chain. They are very common motifs in proteins and polypeptides.^{[1][2][3][4][5][6][7][8][9]} Each consists of four amino acid residues (labelled i, i+1, i+2 and i+3). They can be defined in two ways:

1. By the possession of an intra-main-chain hydrogen bond between the CO of residue i and the NH of residue i+3;
2. By having a distance of less than 7Å between the Cα atoms of residues i and i+3.

The hydrogen bond criterion is the one most appropriate for everyday use, partly because it gives rise to four distinct categories; the distance criterion gives rise to the same four categories but yields additional turn types.