Synthesis and Conformational Analysis of Linear and Cyclic Peptides Containing Sugar Amino Acids

Sugar amino acids (SAAs) were designed and synthesized as new non-peptide peptidomimetics utilizing carbohydrates as peptide building blocks. They represent sugar-like ring structures that carry an amino and a carboxylic functional group and have a specific conformational influence on the backbone of peptides due to their distinct substitution patterns in rigid pyranose sugar rings. Five different SAAs (SAA1α, SAA1β, SAA2, SAA3, and SAA4) have been synthesized that show the ability to constrain linear backbone conformations or distinct turn structures. Linear and cyclic peptides involving SAAs have been prepared in solution as well as by solid phase synthesis. SAA1α and SAA2 were incorporated into two linear Leu-enkephalin analogs, replacing the natural Gly-Gly dipeptide. NMR studies provide evidence for the conformation-inducing effect of the carbohydrate moiety. SAA2 and SAA3 have been placed in cyclic hexapeptide analogs of somatostatin; SAA4 was incorporated in a model peptide. The conformation of the cyclic peptides cyclo(-SAA2-Phe-d-Trp-Lys-Thr-), cyclo(-SAA3-Phe-d-Trp-Lys(Boc)Thr(tBu)-), and cyclo(-SAA4-Ala-d-Pro-Ala-Ala-) have been analyzed by various NMR techniques in combination with distance geometry calculations and subsequent molecular dynamic simulations. The determined solution conformations were compared to representative idealized peptide backbones. SAA2 and SAA3 induce a β-turn structure while SAA4 mimics a γ-turn. Both enkephalin analogs were not active in the guinea pig ileum assay. The somatostatin analog containing SAA2 has an inhibition constant (IC50) of 0.15 μM for the inhibition of the release of growth hormone.