π‐Turns in Peptides: A Crystal‐State Literature Survey

The results of an analysis on the presence of π‐turns, characterized by an i ← i + 5 C=O···H–N intramolecular hydrogen bond, in the X‐ray diffraction structures of peptides are discussed. The survey returned a total of 55 π‐turn occurrences in linear and cyclic peptides. π‐Turns characterized by a helical conformation for residue i + 4, but with a screw sense opposite to that of the three preceding residues, are largely prevailing. They are often found at the C‐end of incipient or fully developed α‐helices, 3 10 ‐helices, and mixed α‐/3 10 ‐helices, thus acting as a C‐capping motif. However, the structures of two linear peptides and 15 cyclopeptides indicate that these types of π‐turns can exist in isolation, without the support of a preceding helix. The frequent presence of additional intramolecular hydrogen bonds internal to the π‐turn is also investigated. Cyclopeptides offered examples of two types of π‐turns that have no parallel in the structures of proteins. Differently from proteins, π‐turns characterized by helical ϕ, ψ sets of the same screw sense for all internal residues are hitherto unreported in the X‐ray diffraction structures of peptides. A suggestion for the rational design in peptides/peptidomimetics of a π‐turn featuring the screw‐sense reversal of residue i + 4 is proposed.