Predictable Conformational Diversity in Foldamers of Sugar Amino Acids.

A systematic conformational search was carried out for monomers and homohexamers of furanoid beta-amino, acids: cis-(S,R) and trans-(S,S) stereoisomers of aminocyclopentane carboxylic acid (ACPC), two different.aniinofuranuronic acids (AFU(alpha) and AFU(beta)), their isopropylidene derivatives (AFU(ip)), and the key intermediate beta-aminotetrahydrofurancarboxylic acid (ATFC). The stereochemistry of the building blocks was chosen to match that of the natural sugar amino acid (xylose and ribose) precursors (XylAFU and RibAFU). The results show that hexamera of cis-furanoid beta-amino acids show great variability: while hydrophobic.cyclopentane (cis-ACPC)(6) and hydrophilic (XylAFU(alpha/beta))(6) foldamers favor two different zigzagged conformation as hexaMers, the backbone fold turns into a helix in the case of (cis-ATFC)(6) (10 -helix) and (XylAFU(ip))(6) (14 -helix). Trans stereochemistry resulted in hexamers exclusively with the right-handed, helix conformation, (H-12(P))(6), regardless of their polarity. We found that the preferred.oligomeric structure of XylAFU(alpha/beta) is conformationally compatible 'with beta-pleated sheets, while that of the trans/(S,S) units matches with alpha-helices of proteins.