Catch the twist: The cis Piv-Pro conformer (Piv=pivaloyl) of peptides is no longer inaccessible. Any cis X-Pro tertiary-amide-bond conformer can be stabilized in crystals of peptides by accommodating the unavoidable distortion of the dihedral angle of the peptide bond to the carbonyl group of the Pro residue (see picture), in this case through ni−1→πi* interactions. Steric clashes were not observed in the cis Piv-Pro rotamers studied.
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