Crystal structures of peptides and modified peptides.

The X-ray diffraction experiments on peptides and related molecules which have been carried out in Western Europe, except Italy, in the last eight years are reviewed. The crystal structures of some bioactive peptides such as Leu-enkephalin (a neurotransmitter), cyclosporin A (an immunomodulator in both the free and protein-bound state), balhimycin (an antibiotic) and octreotide (a somatostatin analogue) are briefly presented. Crystallized N- and C-protected model peptides have given an insight into the folding tendency and folding modes depending on the peptide sequences. The crystal structures of various pseudopeptide molecules reveal how the three-dimensional structure of peptide analogues can be modulated by substituting non-peptide groups for the peptide bond. A few examples of structural mimetics of the beta- and gamma-turns, and of templates for alpha-helix induction are also presented.