Crystal structures of peptidic catalysts of the H-dPro-Pro-Xaa type.

Crystal structures of catalytically active tripeptides of the general type H-dPro-Pro-Xaa and related N-acetylated analogs were compared. The influence of acylation at the N-terminus, the nature of the C-terminal residue, coordinating groups, and intramolecular hydrogen bonds on the conformation of the tripeptides was examined. Regardless of the presence or absence of stabilizing intramolecular H-bonds or n → π* interactions, all of the analyzed peptides share a ß-turn-like conformation, which highlights the structural rigidity of the dPro-Pro motif and its value for conformational preorganization. The C-terminal residues and coordinating moieties were found to affect the turn-conformation, which suggests that H-dPro-Pro-Xaa type peptides are sufficiently flexible to adopt distinctly different but related conformations.

Peptide-catalyzed 1,4-addition reactions of aldehydes to nitroolefins.

Conjugate addition reactions of aldehydes to nitroolefins provide synthetically useful gamma-nitroaldehydes. Here we summarize our research on peptide-catalyzed conjugate addition reactions of aldehydes to differently substituted nitroolefins. We show that peptides of the general type Pro-Pro-Xaa (Xaa = acidic amino acid) are not only highly active, robust and stereoselective catalysts but have also remarkable chemoselectivities.