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

Area of Science:

• Structural biology
• Peptide chemistry
• Biophysics

Background:

• Peptides feature diverse secondary structures beyond standard helices and sheets.
• Intramolecular hydrogen bonds stabilize peptide conformations.
• π-turns are a specific structural motif characterized by an i→i+5 C=O···H-N hydrogen bond.

Purpose of the Study:

• To analyze the occurrence and characteristics of π-turns in peptide X-ray diffraction structures.
• To investigate the role of π-turns as capping motifs and their independent existence.
• To explore novel π-turn structures in cyclopeptides compared to proteins.

Main Methods:

• Analysis of X-ray diffraction data for known peptide structures.
• Identification and classification of π-turn occurrences based on hydrogen bonding and conformational parameters.
• Comparison of π-turn characteristics in linear versus cyclic peptides and proteins.

Main Results:

• 55 π-turn occurrences were identified in linear and cyclic peptides.
• π-turns with a reversed screw sense at residue i+4 were prevalent, often acting as C-capping motifs for α- and 3₁₀-helices.
• π-turns were observed to exist independently of preceding helices, particularly in cyclopeptides, some exhibiting novel structures not seen in proteins.

Conclusions:

• π-turns are significant structural elements in peptides, frequently involved in helix capping.
• Cyclic peptides present unique π-turn architectures, expanding the known structural diversity.
• The findings suggest possibilities for rational design of peptides and peptidomimetics incorporating specific π-turn features.