Investigation of folding patterns in homo-oligomers of (R)-β2,2-amino acids with carbohydrate side chain

Design and synthesis of peptides with hybrid helix-turn-helix (HTH) motif and their conformational study

The present study is aimed at the design and synthesis of peptides with hybrid helix-turn-helix (HTH) motif and their conformational analysis (NMR, MD, and CD studies). The requisite peptides with heterogeneous backbones were prepared from β-, γ-, and δ-amino acids with carbohydrate side chains and α-amino acid, L-Ala. The α/β-peptides were prepared from (S)-β-Caa(l) (C-linked carbo-β-amino acid with D-lyxo furanoside side chain) and L-Ala with a 1:1 alternation. The α/β-peptides with "helix-turn" motif displayed a 11/9-helix nucleating a 13-atom H-bonding turn. The α/β-octapeptides showed the presence of HTH structures with bifurcated 11/15-H-bonded turn. Further, the α/β-hexapeptide with HT motif, independently on coupling with γ/α/γ/α- and δ/α/δ/α-tetrapeptides at the C-terminus provided access to the decapeptides with "hybrid HTH" motifs. The decapeptide ("α-β-α-β-α-β-γ-α-γ-α") showed a hybrid HTH with "11/9/11/9/11/16/9/12/10" H-bonding, while the decapeptide ("α-β-α-β-α-β-δ-α-δ-α") revealed the presence of a "11/9/11/9/11/17/9/13/11" helical pattern. The above peptides thus have shown compatibility between different types of helices and serendipitous bifurcated 11/16- and 11/17-turns. The present study thus provided the first opportunity for the design and study of "hybrid HTH" motifs with more than one kind of helical structures in them.

Synthesis and conformational analysis of cyclic homooligomers from pyranoid ε-sugar amino acids

New pyranoid ε-sugar amino acids were designed as building blocks, in which the carboxylic acid and the amine groups were placed in positions C2 and C3 with respect to the tetrahydropyran oxygen atom. By using standard solution-phase coupling procedures, cyclic homooligomers containing pyranoid ε-sugar amino acids were synthesized. Conformation analysis was performed by using NMR spectroscopic experiments, FTIR spectroscopic studies, X-ray analysis, and a theoretical conformation search. These studies reveal that the presence of a methoxy group in the position C4 of the pyran ring produces an important structural change in the cyclodipeptides. When the methoxy groups are present, the structure collapses through interresidue hydrogen bonds between the oxygen atoms of the pyran ring and the amide protons. However, when the cyclodipeptide lacks the methoxy groups, a U-shape structure is adopted, in which there is a hydrophilic concave face with four oxygen atoms and two amide protons directed toward the center of the cavity. Additionally, we found important evidence of the key role played by weak electrostatic interactions, such as the five-membered hydrogen-bonded pseudocycles (C5) between the amide protons and the ether oxygen atoms, in the conformation equilibrium of the macrocycles and in the cyclization step of the cyclic tetrapeptides.