Peptides linear conformations, extended backbones

When molecules are composed of several peptide units, they may adopt the extended conformation, in which the entire backbone is nearly planar and each peptide unit is in the trans form. More often, however, peptides, whether they are linear or cyclic, are folded. The folding is accomplished by rotations about the N—C bonds and the C —C bonds. An international commission has established a convention for the nomenclature (lUPAC-IUB Commission on Biochemical Nomenclature, 1970) for torsional angles (Fig. 1). According to this convention, the torsional angles f, and a>i equal 180° for a fully extended chain. For a planar trans peptide unit, a>i = 180°, whereas... 

A review of the known structures of linear peptides in the crystalline state indicates that their conformations are quite unpredictable. Di- and tripeptides are probably greatly affected by packing forces and by the large number of hydrogen bonds usually present between different peptide molecules and to the cocrystallized solvent, usually water. Their backbones are never fully extended but take on a variety of twists and bends. The longer peptides usually contain intramolecular hydrogen bonds, and their conformations appear to be much less influenced by packing forces. The proline residue is often associated with a jS bend however, it is also often associated with a cis conformation for the amide bond. 

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