Conformational analysis of small disulfide loops. Spectroscopic and theoretical studies on a synthetic cyclic tetrapeptide containing cystine

Abstract

The conformational analysis of the synthetic peptide Boc-Cys-Pro-Val-Cys-NHMe has been carried out, as a model for small disulfide loops, in biologically active polypeptides. ¹H NMR studies (270 MHz) establish that the Val(3) and Cys(4) NH groups are solvent shielded, while ¹³C studies establish an all-trans peptide backbone. Circular dichroism and Raman spectroscopy provide evidence for a right-handed twist of the disulfide bond. Analysis of the vicinal (J_αβ) coupling constants for the two Cys residues establishes that χ^I˜ ± 60° for Cys(4), while some flexibility is suggested at Cys( 1). Conformational energy calculations, imposing intramolecular hydrogen bonding constraints, favor a β-turn (type I) structure with Pro(2)-Va1(3) as the corner residues. Theoretical and spectroscopic results are consistent with the presence of a transannular 4 → 1 hydrogen bond between Cys(1) CO and Cys(4) NH groups, with the Val NH being sterically shielded from the solvent environment.