Convergent Synthesis and Conformational Analysis of the Hexasaccharide Repeating Unit of the O‐Antigen of Shigella flexneri Serotype 1d

Abstract

A hexasaccharide repeating unit of the O‐antigen of the cell wall of Shigella flexneri type 1d has been synthesized using a stereoselective [3+3] block glycosylation approach. Recently developed glycosylation conditions were used in the synthesis. A thioglycoside was used as an orthogonal glycosyl donor during the synthesis. The synthesized hexasaccharide was subjected to detailed NMR spectroscopic and molecular modeling studies to determine its conformational behavior in water. The NOE‐based two‐dimensional NOSEY experiment and all‐atom explicit molecular dynamics (MD) simulation studies suggest that the oligosaccharide is not very flexible, that it remains rigid with respect to the glycosidic linkages between the sugar moieties.

Linear synthesis of the hexasaccharide related to the repeating unit of the O-antigen from Shigella flexneri serotype 1d (I: 7,8)

“…It is important to note that the more reactive glucosyl and rhamnosyl donors used by Misra et. al 6 failed to provide satisfactory stereochemical output in our hand and the glycosyl donors having chloroacetate functionality showed optimum reactivity and absolute stereoselectivity. The 4-methoxyphenyl glycoside was purposefully chosen at the reducing end so that its selective cleavage from the per-acetate of the target hexasaccharide followed by formation of trichloroacetimidate derivative can pave the path for further conjugation with suitable aglycon.…”

“…al 6 resulted in α/β mixture of disaccharide in 3:1 ratio. Formation of the desired 1,2-cis glycoside was evident from the 1 H signal at  5.02 (d, 1H, J 1, 2 3.2 Hz, H-1) and 13 C signal at  96.4 assigned to the newly formed glycosidic linkage.…”

Computer-Aided Engineering of a Transglycosylase for the Glucosylation of an Unnatural Disaccharide of Relevance for Bacterial Antigen Synthesis

“…Obviously, although improvements are still necessary, these new findings open the way to alternatives to ongoing developments toward chemically synthesized oligosaccharides representing fragments from the O-SP of various S. flexneri type 1 strains. 51,52 However, the low hit rate emphasizes the extreme complexity of redesigning amylosucrase, given its double-step mechanism that involves the sequential binding of a donor (sucrose) and an acceptor substrate along the catalytic reaction, and the constrained active site topology which needs to be adapted to D′A′ disaccharide which presents considerable structural differences with natural acceptors. Indeed, the novel substrate specificity of the mutant was obtained upon introduction of 7 combined amino acid mutations in the first shell of the active site, enabling a drastic reshaping of the catalytic binding site to accommodate non-natural acceptor D′A′ in a productive manner without perturbing too much its original specificity for donor sucrose.…”

Synthesis of the pentasaccharide repeating unit of the O-antigen of Escherichia coli O175 using one-pot glycosylations and its conformational analysis

“…29 The protocol used for the theoretical computing was similar to that of our previous study. 30 Briefly, the molecular dynamics was performed using parameters like isothermaleisobaric periodic boundary conditions, M-SHAKE algorithm 31 for restraining hydrogen bond and cutoff distance of 8 A for accounting nonbonded interactions. Integration time step of 2 fs was used with trajectory saved at an interval of 4 ps.…”