Isolated α‐turns in peptides: a selected literature survey

Abstract

The results of classifying into various types the 68 examples of isolated α‐turns in the X‐ray diffraction crystal structures of peptides documented in the literature are presented and discussed in this review article. α‐Turns characterized by the trans disposition of all ω torsion angles are common for the backbone linear peptides investigated. In contrast, the cis arrangement of the N‐terminal (ωi + 1) torsion angle, among those generated by the three residues internal to the α‐turn, is a peculiar feature of 65% of the cyclic peptides. Among linear and cyclic peptides featuring the all‐trans disposition of the ω torsion angles, only one third of the α‐turns display φ,ψ values not too far from those characterizing regular α‐helices. In general, our findings, taken together, suggest that a significant conformational diversity is compatible with the formation of an intramolecularly H‐bonded C13‐member pseudocycle (α‐turn) in linear and cyclic peptides.