INVOLVEMENT OF SIDE FUNCTIONS IN PEPTIDE STRUCTURES - THE ASX TURN - OCCURRENCE AND CONFORMATIONAL ASPECTS

The Asx turn is a local conformation in peptide chains that is topologically equivalent to the beta-turn. It is characterized by an interaction closing a 10-membered cycle and involving the C-gamma-O group of an Asn or Asp residue and the peptide NH group two residues ahead in the sequence. Its conformational features have been deduced from its statistical occurrence in crystallized proteins, and from theoretical and experimental analysis of model peptides reproducing this local conformation. Four Asx-turn types have been considered and their relative stability is discussed in relation with the sequence and the existence of longer range interactions.