Cyclotetrapeptides with alternating d-Ala residues: synthesis and spectroscopic studies

Abstract

Three cyclotetrapeptides, c[Leu- d-Ala-Xaa- d-Ala], where Xaa is Leu ( P1), Lys ( P2) and Glu ( P3) were synthesized and studied by 1H and 13C NMR and CD spectroscopy. These cyclotetrapeptides exhibit similar coupling constants, 3 J HNHα, in the range of 8.56–9.93 Hz, commonly observed for β-turn structures. All amide proton chemical shifts for P1, P2 and P3 exhibited linear dependence on temperature with moderate temperature coefficients ranging from −3.1 to −9.8 ppb/K. Amide proton signal broadening was observed for all residues in P1, P2 and P3, indicating that they are solvent accessible. The number of resonance observed for P1 was half of the total counts, indicating a C2 symmetric conformation. P2 and P3 exhibit similar CD in solvents of varying dielectric constants and dilutions, with characteristic positive CD bands at ca. 210 and 222 nm, which correspond to a β-turn type structure. Small CD/temperature effect was also observed with isodichroic points, consistent with conformational stability and a well-populated cyclotetrapeptide energy state. These heterochiral cyclotetrapeptides consisting of alternating d-Ala residues adopt stabilized open β-turn conformations and may be useful as a ligand template for further functionalization.