Glycans are involved in various life processes and represent critical targets of biomedical developments. Nevertheless, the accessibility to long glycans with precise structures remains challenging. Here we report on the synthesis of glycans consisting of [→4)-α-Rha-(1 → 3)-β-Man-(1 → ] repeating unit, which are relevant to the O-antigen of Bacteroides vulgatus, a common component of gut microbiota. The optimal combination of assembly strategy, protecting group arrangement, and glycosylation reaction has enabled us to synthesize up to a 128-mer glycan. The synthetic glycans are accurately characterized by advanced NMR and MS approaches, the 3D structures are defined, and their potent binding activity with human DC-SIGN, a receptor associated with the gut lymphoid tissue, is disclosed.
Stereoselective β-rhamnopyranosylation remains a challenge, due to the unfavorable anomeric effect and steric hindrance of the C2-substituent; herein, this challenge is addressed with a gold(I)-catalyzed SN2-like glycosylation protocol employing α-rhamnopyranosyl 2-alkynyl-4-nitro-benzoates as donors.
Carbohydrates, such
as oligo- and polysaccharides, are highly abundant
biopolymers that are involved in numerous processes. The study of
their structure and functions is commonly based on a material that
is isolated from complex natural sources. However, a more precise
analysis requires pure compounds with well-defined structures that
can be obtained from chemical or enzymatic syntheses. Novel synthetic
strategies have increased the accessibility of larger monodisperse
polysaccharides, posing a challenge to the analytical methods used
for their molecular characterization. Here, we present wide mass range
ultrahigh-resolution matrix-assisted laser desorption/ionization (MALDI)
Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry
(MS) as a powerful platform for the analysis of synthetic oligo- and
polysaccharides. Synthetic carbohydrates 16-, 64-, 100-, and 151-mers
were mass analyzed and characterized by MALDI in-source decay FT-ICR
MS. Detection of fragment ions generated from glycosidic bond cleavage
(or cross-ring cleavage) provided information of the monosaccharide
content and the linkage type, allowing for the corroboration of the
carbohydrate compositions and structures.
The β-(1→2)-linked 6-deoxy-l-altropyranose di- to pentasaccharides 2-5, relevant to the O-antigen of the infectious Yersinia enterocolitica O:3, were synthesized for the first time. The challenging 1,2-cis-altropyranosyl linkage was assembled effectively via glycosylation with 2-O-benzyl-3,4-di-O-benzoyl-6-deoxy-l-altropyranosyl ortho-hexynylbenzoate (7) under the catalysis of PPhAuNTf. NMR and molecular modeling studies showed that the pentasaccharide (5) adopted a left-handed helical conformation.
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