Isolated alpha-turns in peptides: a selected literature survey

The results of classifying into various types the 68 examples of isolated alpha-turns in the X-ray diffraction crystal structures of peptides documented in the literature are presented and discussed in this review article. alpha-Turns characterized by the trans disposition of all omega torsion angles are common for the backbone linear peptides investigated. In contrast, the cis arrangement of the N-terminal (omega(i + 1)) torsion angle, among those generated by the three residues internal to the alpha-turn, is a peculiar feature of 65% of the cyclic peptides. Among linear and cyclic peptides featuring the all-trans disposition of the omega torsion angles, only one third of the alpha-turns display phi,psi values not too far from those characterizing regular alpha-helices. In general, our findings, taken together, suggest that a significant conformational diversity is compatible with the formation of an intramolecularly H-bonded C-13-member pseudocycle (alpha-turn) in linear and cyclic peptides.