• cis-amide;
  • proline;
  • azaproline;
  • 5-alkylproline;
  • oxaproline;
  • thioproline;
  • density functional theory (DFT)


The β-turn is a common motif in both proteins and peptides and often a recognition site in protein interactions. A β-turn of four sequential residues reverses the direction of the peptide chain and is classified by the Φ and Ψ backbone torsional angles of residues i + 1 and i + 2. The type VI turn usually contains a proline with a cis-amide bond at residue i + 2. Cis-proline analogs that constrain the peptide to adopt a type VI turn led to peptidomimetics with enhanced activity or metabolic stability. To compare the impact of different analogs on amide cistrans isomerism and peptide conformation, the conformational preference for the cis-amide bond and the type VI turn was investigated at the MP2/6-31+G** level of theory in water (polarizable continuum water model). Analogs stabilize the cis-amide conformations through different mechanisms: (1) 5-alkylproline, with bulky hydrocarbon substituent on the Cδ of proline, increases the cis-amide population through steric hindrance between the alkyl substituent and the N-terminal residues; (2) oxaproline or thioproline, the oxazolidine- or thiazolidine-derived proline analog, favors interactions between the dipole of the heterocyclic ring and the preceding carbonyl oxygen; and (3) azaproline, containing a nitrogen atom in place of the Cα of proline, prefers the cis-amide bond by lone-pair repulsion between the α-nitrogen and the preceding carbonyl oxygen. Preference for the cis conformation was augmented by combining different modifications within a single proline. Azaproline and its derivatives are most effective in stabilizing cis-amide bonds without introducing additional steric bulk to compromise receptor interactions. © 2005 Wiley Periodicals, Inc. Biopolymers 81:392–406, 2006

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