title: Empirical rules predicting conformation of cyclic tetrapeptides from primary structure.
abstract: A useful set of empirical rules is put forward to predict the conformations of cyclic tetrapeptides and cyclic tetradepsipeptides on the basis of primary structure, briefly presented as follows: A conformation allowing an intramolecular hydrogen bond (IMHB) of gamma-turn is preferred, and an ester bond always adopts a trans form. On a right-handed peptide ring, the carbonyl group acylating a D residue is oriented to the upper side of the main ring. The carbonyl group acylating a D proline or an N-methyl-D-amino acid residue is oriented to the lower side of the ring, forming a cis bond. The LDDL configurational sequence adopts a cis-trans-cis-trans backbone with Ci symmetry. A glycine residue behaves as a D residue in an L-peptide. Conformations of cyclotetrapeptides containing two glycine residues at diametric positions or containing an N-methyl-dehydroamino acid residue are predicted by use of appendices of rule 5. Almost all conformations of cyclic tetrapeptides are predicted by these rules. Energetical rationalization of the rules and prediction of possible new conformations are described. Conformations of cyclo(-L-Pro-L-Leu-D-Tyr(Me)-L-Ile-)(1) and cyclo (-L-Pro-D-Leu-D-Tyr(Me)-L-Ile)(2) are compared. Results of n.m.r. experiments showed that compound 1 adopts a unique cis-trans-trans-trans backbone with a gamma-turn IMHB, and 2 has a cis-trans-cis-trans backbone with Ci symmetry. These observations confirmed the rules described above. Peptides 1 and 2 are the first diastereomeric peptides with trans (LD) and cis (DD) secondary amide bonds.
mesh_terms: D009682:Magnetic Resonance Spectroscopy; D009842:Oligopeptides; D010456:Peptides, Cyclic; D011487:Protein Conformation; D013329:Structure-Activity Relationship
chemical_list: D009842:Oligopeptides; D010456:Peptides, Cyclic