Synthesis and nmr analysis of cyclo-[85% 13C-asp]-pro: 13C- 13C vicinal coupling constants and conformation

Abstract

The dipeptide cyclo(Asp-Pro) where the aspartic acid residue was 85% 13C enriched, was synthesized with the aim of analyzing its conformation in solution by using 1C- 1H, 13H- 1H and 13C- 13C coupling constant parameters. The values of these couplings agree well with each other and show that the side chain of the aspartic acid residue adopts privileged conformations the proportions of which vary somewhat with pH, and more weakly, with a change in solvent. The 13C- 13C interresidue coupling constants 3J C l' – C 2 β and 3J C l' – C 2 γ obtained after long accumulation of the signals of unenriched carbons, have different values; they show puckering in the pyrrolidine ring similar to that found in cyclo(Leu-Pro) in the solid state. It was concluded that 13C- 13C coupling constants represent an excellent means of determining the side chain conformation (whenever the incorporation of an enriched amino acid into the peptide is possible) that will find applications particularly in the case of peptides with complicated proton spectra.