Impact of azaproline on amide cis-trans isomerism: Conformational analyses and NMR studies of model peptides including TRH analogues

The β-turn is a well-studied motif in both proteins and peptides. Four residues, making almost a complete 180°-turn in the direction of the peptide chain, define the β-turn. Several types of the β-turn are defined according to Φ and Ψ torsional angles of the backbone for residues i + 1 and i + 2. One special type of β-turn, the type VI-turn, usually contains a proline with a cis-amide bond at residue i + 2. In an aza-amino acid, the α-carbon of the amino acid is changed to nitrogen. Peptides containing azaproline (azPro) have been shown to prefer the type VI β-turn both in crystals and in organic solvents by NMR studies. MC/MD simulations using the GB/SA solvation model for water explored the conformational preferences of azPro-containing peptides in aqueous systems. An increase in the conformational preference for the cis-amide conformer of azPro was clearly seen, but the increased stability was relatively minor with respect to the trans-conformer as compared to previous suggestions. To test the validity of the calculations in view of the experimental data from crystal structures and NMR in organic solvents, [azPro³]-TRH and [Phe², azPro³]-TRH were synthesized, and their conformational preferences were determined by NMR in polar solvents as well as the impact of the azPro substitution on their biological activities.