A beta turn occurs where the polypeptide chain makes a sharp reversal by 180 degrees within 4 residues. A turn is defined by 4 consecutive residues (i to i+3) if the C-alpha(i) to C-alpha(i+3) distance is less than 7 angstroms, and if the central two residues are not helical. There is often a hydrogen bond between the CO(i) and NH(i+3). There are several different types of beta turns (see references below).