Conformational Heterogeneity of Pro–Pro Containing 23-Membered Ring Conopeptides by NMR

Conformational diversity in conopeptides, Be828 and Li833, for which sequences are identified from <i>Conus betulinus</i> and <i>Conus lividus</i> has been explored in the present study. Be828 and Li833 have 23-membered rings with two consecutive prolines at positions 2/3 and 4/5, respectively, an unusual phenomenon observed in contryphans. These peptides provided an excellent scope to probe distinct conformational states, i.e., <i>cis</i>-<i>trans</i> isomerization around Xaa-Pro and Pro-Pro bonds. NMR analysis revealed that Be828 exists as a single trans (C1-P2)/trans (P2-P3) conformation, whereas Li833 exists in four distinct conformers across the consecutive proline bonds. Studies on synthetic analogues disclosed that the conformational heterogeneity in Li833 is influenced by the constraints of the disulfide loop and preceding aromatic and charged residues, i.e., Y2 and R3, unperceived so far in 23-membered rings. Be828 is stabilized by a type IV₄ β-turn, while Li833 predominantly adopts a type VIa1 β-turn. The investigation of conformational heterogeneity of consecutive prolines in the 23-membered may aid in designing peptide-based ligands.