2.5.3.2. The Pleated Sheet of β-Peptides

Pleated sheets of α- and β-peptides differ in the following features: (a) the amide planes in an extended conformation are separated by one and two tetrahedral carbons, respectively; (b) thus, the amide planes are arranged in a zig-zag and in a parallel-displaced fashion; (iii) while the C=O and N–H bonds are pointing up and down in the α-peptidic sheet, they are unidirectional i.e. all C=O up, all N–H down, in the β-peptide; (iv) the side chains of an antiparallel and the parallel pleated sheet of an α-peptide pointing perpendicular, above and below, the average plane of the sheet, while the sheet of a β-peptide built from homochiral (all-β2 or all-β3) components is obscured on only one face by the side chains; (v) the α-peptides containing the twenty proteinogenic amino acids may show a certain tendency for forming either a helix or a β-sheet, the chain of a β-peptide constructed from 2,3-disubstituted β-amino acids can be prevented from forming a 31-helix and forced to adopt an extended conformation.

Figure 2.32: Parallel (a) and antiparallel (b) sheets in short β-peptides.

2.5.3.3. The Turn of β-Peptides
The 4 → 1 hydrogen-bonded two residue structure in αα segments is the extremely well studied and is known as β-turn. Polypeptide chain folding about the two backbone residues (i + 1, i + 2) generate β-turn characterized by backbone conformational angles (Φ, ψ). The type I/III turn is characterized by Φ_(i+1) = −30°, ψ_(i+1) = −60°, Φ_(i+2) = −60°/−90°, and ψ_(i+2) = −30°/0°, while the type II turn is characterized by (Φ_(i+1) = −60°, ψ_(i+1) = 120°, Φ_(i+2) = 80°, and ψ_(i+2) = 0°). Turns with different ring size may arise because of the hydrogen bonding in two residue with normal (4 → 1) and reversed (1 → 2) directionalities.

Figure 2.33: Various types of turns in short β-peptides.