Studies of β-turn opening with model peptides containing non-coded α-amino isobutyric acid

Single crystal X-ray diffraction studies show that among the three terminally protected model tripeptides I -III, Boc-Ile-Aib-Xx OMe (Xx in peptide I: Val; II: Leu; III: Phe) with a centrally placed non-coded amino acid Aib (Aib: alpha-amino isobutyric acid), peptide I displays a conformational preference for beta-turn, peptide II forms a hydrated b-turn representing the solvent mediated intermediate for the interconversion between beta-turn and beta-strand and peptide III adopts a completely unfolded beta-strand like structure. By varying the steric bulk of the third residue, Xx(3), various conformations related to the structural interconversion between the beta-turn and beta-strand have been isolated. The peptide conformations in the solution phase have been probed by solvent dependent NMR titration and CD spectroscopy. Morphological studies with scanning electron microscopy (SEM) reveal that among the three peptides only peptide III can form filamentous fibrils in the solid state. (C) 2007 Elsevier Ltd. All rights reserved.