Synthesis of Conformationally Restricted Peptides

Peptides are very flexible molecules, in contrast to proteins, which are stabilized by disulfide bridges and salt bridges in their tertiary structure. Peptides can adopt several conformations, at least in aqueous solutions. Some limitations, however, are imposed caused by then primary sequence. Two torsion angles characterize the free rotation of each peptide unit: the rotation about the Cα-CO-bond is called ψ-angle, the rotation about the Cα-NH-bond the φ-angle (Fig. 1 [1 ]). The torsion angle of the peptide bond, CO-NH, is about 180� and almost fixed owing to its double-bond character. Only certain combinations of ψ- and φ-angles are possible because of the steric hindering of carbonyl oxygen, amide hydrogen, and side-chain atoms of the amino acids.