Journal
JOURNAL OF ORGANIC CHEMISTRY
Volume 75, Issue 14, Pages 4796-4805Publisher
AMER CHEMICAL SOC
DOI: 10.1021/jo100810m
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Funding
- Fudan University
- National Natural Science Foundation of China [20202001]
- Fok Ying Tung Education Foundation [94023]
- HKU-Fudan Joint Laboratory on Molecular Design and Synthesis
- Research Grants Council of Hong Kong [HKU 7654/06M, HKU 2/06C]
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The monomer 1 derived from achiral 1-(aminoxy)cyclopropanecarboxylic acid (OAcc) and oligopeptides 2-9 consisting of a chiral alpha-aminoxy acid and an achiral alpha-aminoxy acid such as OAcc were synthesized and their structures characterized. The eight-membered-ring intramolecular hydrogen bond, namely the alpha N-O turn, was formed between adjacent residues independent of their chirality. However, the helix formation was sequence-dependent. Dipeptide 2 bearing chiral alpha-aminoxy acid (D-OAA) at the N-terminus and achiral OAcc at the C-terminus preferentially adopted a right-handed 1.8(8) helical structure, but dipeptide 3 (OAcc-D-OAA) did not. Theoretical calculation results, in good agreement with experimental ones, revealed that the biased handedness of alpha N-O turn found in OAcc residue depends on its preceding chiral residue. It was then found that the helical conformation was destroyed in the case of oligopeptides 6 and 7 [OAA-(OAcc)(n), n = 2, 3]. The crystal structure of tripeptide 8 ((PrCO)-Pr-i-D-OVal-OAcc-D-OVal-(NHBu)-Bu-i) further disclosed the helical structure formed by three consecutive homochiral alpha N-O turns. This study has uncovered achiral aminoxy acid residues such as the OAcc unit as a useful building block to be incorporated into chiral aminoxy peptides to mimic chiral helix structure.
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