Beta-Turns (02JACS1203) are sites where the polypeptide chain reverses its overall direction, allowing proteins to adopt globular shape. A beta-turn loop comprises four amino acid residues (numbered as i, i+1, i+2, and i+3), and the different types of beta-turn are distinguished by the backbone torsion angles of the central i+1 and i+2 residues. Several beta-turn types are feasible:

Type I. Having two preferred conformations (fi=-60 and psi=-30) and (fi=-90 and psi=0)

Type II. Having two preferred conformations (fi=-60 and psi=120) and (fi=80 and psi=0)

Type I' and I''.

Characteristic NMR restraints are:

Strong d_NN(i+2,i+3) (NOE (NH(i+2) and NH(i+3)) in both type-I and type-II.

Type II are easily identified because of the absence of other d_NN NOEs and the presence of a strong d_aN(i+2,i+3) interaction.

Type I shows weak d_aN(i+2,i+3) ans there is usually a d_NN(i+1,i+2) NOE.

Type I is similar to alpha-helix except that a d_aN(i+1,i+3) is present and there is absence of a repetitive pattern of d_NN interactions.

NH proton of i+1 and i+2 residues are exposed to the solvent, while the NH(i+3) is usually hydrogen bonded with the CO(i) in both type-I and type-II conformations. Thus, NH(i+3) usually has decreased H/D exchange rate and its chemical shift has a low temperature coefficient.

Identification of beta-turns at the ends of beta-sheet strands is straightforward because they represent a clear interruption of the regular pattern of NOEs characteristic of beta-sheets.