four types of asx turn can be distinguished: types i, i’, ii and ii’. these categories correspond to those of the better-known hydrogen-bonded beta turns, which have four residues and a hydrogen bond between the co of residue i and the nh of residue i+3. asx turns and beta turns have structurally similar hydrogen-bonded loops and exhibit sidechain-mainchain mimicry in the sense that the sidechain of residue i of the asx turn mimics the mainchain of residue i of the beta turn. regarding their occurrence in proteins, they differ in that type i is the commonest of the four beta turns while type ii’ is the commonest of the asx turns.