Synthesis and O-Glycosidic Linkage Conformational Analysis of ¹³C-Labeled Oligosaccharide Fragments of an Antifreeze Glycolipid

Abstract

NMR studies of two 13C-labeled disaccharides and a tetrasaccharide were undertaken that comprise the backbone of a novel thermal hysteresis glycolipid containing a linear glycan sequence of alternating [βXylp-(1→4)-βManp-(1→4)] n dimers. Experimental trans-glycoside NMR J-couplings, parameterized equations obtained from density functional theory (DFT) calculations, and an in-house circular statistics package (MA’AT) were used to derive conformational models of linkage torsion angles ϕ and ψ in solution, which were compared to those obtained from molecular dynamics simulations. Modeling using different probability distribution functions showed that MA’AT models of ϕ in βMan(1→4)­βXyl and βXyl(1→4)­βMan linkages are very similar in the disaccharide building blocks, whereas MA’AT models of ψ differ. This pattern is conserved in the tetrasaccharide, showing that linkage context does not influence linkage geometry in this linear system. Good agreement was observed between the MA’AT and MD models of ψ with respect to mean values and circular standard deviations. Significant differences were observed for ϕ, indicating that revision of the force-field employed by GLYCAM is probably needed. Incorporation of the experimental models of ϕ and ψ into the backbone of an octasaccharide fragment leads to a helical amphipathic topography that may affect the thermal hysteresis properties of the glycolipid.

Use of Circular Statistics To Model αMan-(1→2)-αMan and αMan-(1→3)-α/βMan O-Glycosidic Linkage Conformation in 13C-Labeled Disaccharides and High-Mannose Oligosaccharides

“…Similar observations were made recently in similar comparisons of NMR-and MD-determined behaviors of ϕ in β-(1→4) linkages. 15,71 In contrast to ϕ, mean values of ψ obtained by MD simulation compare favorably with values determined from MA'AT analysis (Table S11; Supporting Information). For example, mean values of ψ for the αMan-(1→2)-αMan linkages in 2 and 7 were found by MD to be 21°and 22°, respectively, compared to 20.2°and 19.5°, respectively, by NMR J-coupling analysis.…”

“…Additional J -coupling constraints may also improve the modeling; for example, for the αMan-(1→3)-α/βMan linkage, 2 J H1′,C2′ may serve as an additional constraint on ϕ, while 2 J C2,H3 , 2 J C4,H3 , and 2 J C2,C4 may serve as constraints on ψ (Scheme ). , …”

Helical polysaccharides

“…However, generating a 3D structure solely from NMR data is rather difficult. The combination of NMR experiments with theoretical MD simulations is often employed and provided important structural information …”

Computational1H and13C NMR in structural and stereochemical studies

“…Very recently, Lin, et al [376] prepared and studied by the solid-state 13 C NMR spectroscopy and DFT calculations seven doubly 13 In this direction, NMR studies of 13 C-labeled disaccharides 98-101 and a tetrasaccharide 102 shown in Scheme 17 were undertaken. [377] The experimental transglycoside NMR 1 H-1 H spin-spin coupling constants together with the related parameterized equations obtained from the related DFT calculations were used to derive conformational models of linkage torsion angles in solution, which were compared to those obtained from the molecular dynamics simulations.…”

“…Reproduced with minor editing privilege from Lin, et al [376] with the permission of the Royal Society of Chemistry (102). Reproduced with minor editing privilege from Zhang, et al [377] with the permission of the American Chemical Society…”