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

Abstract

SynopsisTetrapeptides 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 2H20 and in MeZSO-ds by 'H-and '%nmr.In order to clarify or refine some details, successive substitutions of the residues in these peptides with amino acids enriched to 85% in I3C, or to 85% 13C plus 97% *H were carried out. The 'H and 13C chemical shifts as well as the 'H-'H, 13C-W, and 13C-lH 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 P-turn structure when the peptide is in the zwitterion state dissolved in MeZSO. This is deduced from the and the 3 J~; .~3 9 .coupling constants, which both furnish a dihedral angle of = -go", 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 Me2.90, 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 @-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 @-turn.

Nmr studies of a free and protected tetrapeptide glycyl‐L‐prolylglycylglycine in β‐turn‐supporting environment

“…peptides.lg However, it is interesting to note that for the zwitterionic deprotected tetrapeptide +H3N-Gly-Pro-Gly-Gly-COO-, the NH(Gly,) proton exhibits a weak but significant positive temperature dependence that is considered by Toma et al 4 as characteristic of a proton engaged in a strong hydrogen bond. This observation suggests that the free tetrapeptide adopts a P-turn structure in Me2SO.…”

13C‐Nmr studies of ACTH: Assignment of resonances and conformational features

“…Peptide content: 94.5%. Natural abundance and (85% 13C-Tyr23)-ACTH (22)(23)(24)(25)(26) were obtained by solid-phase synthesis, as will be described in a subsequent paper. The 85% 13C uniformly enriched amino acids were produced biosynthetically in our laboratory.…”

Comparative studies on peptides representing the so-called tachykinin-like region of the Alzheimer Aβ peptide [Aβ(25–35)]

“…For peptides dissolved in DMSO, values of the amide temperature coefficient greater than k3 p.p.b.\K are usually taken to indicate fairly extensive shielding from exposure to solvent, whereas values less than k5 p.p.b.\K are usually taken as indicating full exposure to solvent [51]. The values that could be determined for Aβ[Phe$"](25-35) are all less than k5 p.p.b.\K, indicating that in this peptide these amide protons are fully exposed to solvent, i.e.…”