NMR evidence for a type I β‐turn in (Pro²)‐tetrapeptides and interdependence of <i>cis</i>:<i>Trans</i> isomerism, ring flexibility, and backbone conformation

Abstract

Abstract Tetrapeptides with proline in position 2, asparagine or leucine in position 3, and glycine in positions 1 and 4, with end groups free or blocked on the N ‐terminal side, were studied in their various ionic states in 2 H 2 O and in Me 2 SO‐ d 6 by 1 H‐ and 13 C‐nmr. In order to clarify or refine some details, successive substitutions of the residues in these peptides with amino acids enriched to 85% in 13 C, or to 85% 13 C plus 97% 2 H were carried out. The 1 H and 13 C chemical shifts as well as the 1 H‐ 1 H, 13 C‐ 13 C, and 13 C‐ 1 H coupling constants and the signal intensities show strong similarity of behavior between the tetrapeptides of asparagine and leucine. The main conformational characteristics are (1) the almost total stabilization of the trans conformer in the type I β‐turn structure when the peptide is in the zwitterion state dissolved in Me 2 SO. This is deduced from the 3 J and the 3 J coupling constants, which both furnish a dihedral angle of ϕ 3 = −90°, and from the positive value of the temperature coefficient of the glycine‐4 amide protons, which suggests a type 4 → 1 hydrogen bond; (2) the evolution of cis and trans isomer fractions which change with the ionic state of the peptides in Me 2 SO, whereas they remain constant in aqueous solution; and (3) the conformation of the pyrrolidine ring as it follows the variations in cis : trans isomer populations together with the side‐chain rotamer fractions of the residue in position 3. In the β‐turn conformation the isomer cis is less abundant and the pyrrolidine ring is more flexible; this explains the perfect accommodation of the proline residue in position 2 of a bend. The interdependence of these phenomena where interactive forces play a predominant role underlines the importance of cooperative effects in the molecule. The results also suggest that the cis isomer of proline can adapt itself just as well as the trans isomer to position 2 of a type I β‐turn.