Section 22.2.5.3.3. Turns

By far the most common type of turn is the β-turn, a sequence of four residues that brings about a reversal in the polypeptide chain direction. Hydrogen bonding does not seem to be essential for turn formation, but a common feature is a hydrogen bond between the C=O group of residue 1 and the NH group of residue 4, a 3₁₀-type, interaction. Turns are also often associated with characteristic side-chain–main-chain hydrogen-bond configurations (see below). The hydrogen bonds in turns tend to be longer and less linear than those in helices and β-sheets; in particular, the angle at the acceptor oxygen atom C—O···H is around 120° (Baker & Hubbard, 1984).

In addition to β-turns, a small but significant number of γ-turns are found. In these three-residue turns, a hydrogen bond is formed between the C=O of residue 1 and the NH of residue 3, an interaction. Although the approach to the acceptor oxygen atom is highly nonlinear (C—O···H ∼ 100°), the nonlinearity at the H atom is less pronounced (N—H···O ∼ 130–150°) (Baker & Hubbard, 1984). γ-turns are again of several types, depending on the configuration of the central residue. The classic γ-turn, first recognised by Matthews (1972) and Nemethy & Printz (1972), has a central residue with (φ, ψ) angles around (70°, −60°), which puts it in the normally disallowed region of the Ramachandran plot. More common, however, are structures in which an hydrogen bond is associated with a central residue with a configuration around (90°, −70°) (Baker & Hubbard, 1984); these structures are not necessarily true turns in the sense of bringing about a sharp chain reversal, however.