A Reverse Turn Structure Induced by a d,l-α-Aminoxy Acid Dimer

Our previous work revealed that two adjacent d-α-aminoxy acids could form two homochiral
N−O turns, with the backbone folding into an extended helical structure (1.88-helix). Here, we report the
conformational studies of linear peptides 3−6, which contain a d,l-α-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.
1H 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 γ-turn.