title: A reverse turn structure induced by a D,L-alpha-aminoxy acid dimer.
abstract: Our previous work revealed that two adjacent D-alpha-aminoxy acids could form two homochiral N-O turns, with the backbone folding into an extended helical structure (1.8(8)-helix). Here, we report the conformational studies of linear peptides 3-6, which contain a D,L-alpha-aminoxy acid dimer segment. The NMR and X-ray analysis of 3 showed that it folded into a loop conformation with two heterochiral N-O turns. This loop segment can be used to constrain tetrapeptides 4 and 6 to form a reverse turn structure. (1)H NMR dilution studies, DMSO-d6 addition studies, and 2D-NOESY data indicated that tetrapeptides 4 and 6 folded into reverse turn conformations featured by a head-to-tail 16-membered-ring intramolecular hydrogen bond. In contrast, tetrapeptide 5 with L-Ala instead of Gly or D-Ala as the N-terminal amino acid could not form the desired reverse turn structure for steric reasons. Quantum mechanics calculations showed that model pentamide 7, with the same substitution pattern of 4, adopted a novel reverse turn conformation featuring two heterochiral N-O turns (each of an 8-membered ring hydrogen bond), a cross-strand 16-membered ring hydrogen bond, and a 7-membered ring gamma-turn.
authors: Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, P.R. China. yangdan@hku.hk
authors: Dan
authors: D
authors: Yang
authors: Jin
authors: J
authors: Qu
authors: Wei
authors: W
authors: Li
authors: De-Ping
authors: DP
authors: Wang
authors: Yi
authors: Y
authors: Ren
authors: Yun-Dong
authors: YD
authors: Wu
mesh_terms: D000596:Amino Acids / Q000737:chemistry*; D019281:Dimerization; D006860:Hydrogen Bonding; D008958:Models, Molecular; D019906:Nuclear Magnetic Resonance, Biomolecular; D009842:Oligopeptides / Q000737:chemistry*; D011487:Protein Conformation; D017433:Protein Structure, Secondary
chemical_list: D000596:Amino Acids; D009842:Oligopeptides