Overlapping Double Turn Conformations Adopted by Tetrapeptides Containing Non-Coded α-Amino Isobutyric Acid (AIB) and Formation of Tape-Like Structures Through Supramolecular Helix Mediated Self-Assembly

Abstract

Single crystal X-ray diffraction studies and solvent dependent (1)H NMR titrations reveal that a set of four tetrapeptides with general formula Boc-Xx(1)-Aib(2)-Yy(3)-Zz(4)-OMe, where Xx, Yy and Zz are coded L-amino acids, adopt equivalent conformations that can be described as overlapping double turn conformations stabilized by two 4-->1 intramolecular hydrogen bonds between Yy(3)-NH and Boc C=O and Zz(4)-NH and Xx(1)C=O. In the crystalline state, the double turn structures are packed in head-to-tail fashion through intermolecular hydrogen bonds to create supramolecular helical structures. Field emission scanning electron microscopic (FE-SEM) images of the tetrapeptides in the solid state reveal that they can form flat tape-like structures. The results establish that synthetic Aib containing supramolecular helices can form highly ordered self-aggregated amyloid plaque like human amylin.