Structural analyses of mannose pentasaccharide of high mannose type oligosaccharides by 1D and 2D NMR spectroscopy

Complete assignment of 1H and 13C NMR signals of monoglucosylated high-mannose type glycan attached to asparagine

Glc1Man9GlcNAc2 (G1M9) glycan and other high mannose-type glycans play key roles in the quality control mechanisms of glycoprotein synthesis. The lectin-like proteins calnexin (CNX) and calreticulin (CRT) specifically recognize G1M9 glycan and assist newly synthesized glycoproteins to achieving correct folding. Nuclear magnetic resonance (NMR) spectroscopy is a unique method for analyzing the conformation, dynamics and interactions of glycans like G1M9 glycan and CNX/CRT. Accurate assignment of 1H and 13C signals is a prerequisite for such analyses. Here, we present the complete assignment of 1H and 13C signals for the Asn-linked G1M9 glycan, modified at its N-terminus with a 9-fluorenylmethyloxycarbonyl (Fmoc) group (Fmoc-Asn-G1M9). Using conventional two-dimensional NMR techniques including 1H-1H COSY, 1H-1H NOESY, 1H-13C HSQC, 1H-13C HMBC and 1H-13C HSQC-TOCSY, we achieved a comprehensive spectral assignment. Our results are consistent with previously reported assignments of the partial pentasaccharide structure of G1M9 glycan. This complete assessment of G1M9 glycan signals provides a foundation for detailed studies of its interactions with CNX/CRT, which will advance our understanding of the molecular mechanisms underlying glycoprotein quality control.

Molecular Iodine-Mediated β-Glycosylation of Thiomannosides with 1,6-Anhydrosugars

β-Mannosides play a crucial role in cellular processes and immune responses, and their synthesis remains one of the most challenging tasks in carbohydrate chemistry. Glycosyl donors, such as thiomannosides, are stable and compatible with a range of protection and deprotection conditions. In this study, we demonstrate that molecular iodine efficiently induces the activation and coupling of thiomannosides with various 1,6-anhydrosugars as acceptors. This method provides mild activation conditions and high β-stereoselectivity for the synthesis of multiple β-mannosides.

Characterization of therapeutic protein AvidinOX by an integrated analytical approach

AvidinOX, the oxidized derivative of Avidin, is a chemically modified glycoprotein, being currently under clinical investigation for targeted delivery of radioactive biotin to inoperable tumors. AvidinOX is produced by 4-hydroxyazobenzene-2-carboxylic acid (HABA)-assisted sodium periodate oxidation of Avidin. The peculiar property of the periodate-generated glycol-split carbohydrate moieties to form Schiff's bases with amino groups of the tissue proteins allows to achieve a tissue half-life of 2 weeks compared to 2 h of native Avidin. Carbohydrate oxidation, along with possible minor amino acid modifications, introduces additional microheterogeneity in the glycoprotein structure, making its characterization even more demanding than for native glycoproteins. Aiming at the elucidation of the effects of oxidation conditions on the AvidinOX protein backbone and sugars, this microheterogeneous glycoprotein derivative was characterized for the first time using a combination of different analytical methods, including colorimetric methods, mass spectrometry, hollow-fiber flow field-flow fractionation with UV and multi-angle laser scattering detection (HF5-UV-MALS), and NMR. The proposed integrated approach reveals structural features of AvidinOX relevant for its biological activity, e.g., oxidized sites within both carbohydrate moieties and protein backbone and conformational stability, and will be considered as an analytical tool for AvidinOX industrial preparations. It is worth noting that this study enriches also the structural data of native Avidin published up-to-date (e.g., glycan structure and distribution, peptide fingerprint, etc.). Graphical abstract Scheme of phenylacetic hydrazide/MALDI-TOF approach for quantification of aldehydes in AvidinOX based on the determination of the number of hydrazone adducts between hydrazide reagent and aldehyde groups of protein.

Interactions of Glycopolymers with Assemblies of Peptide Amphiphiles via Dynamic Covalent Bonding

In this work, peptide amphiphile (PA) with benzoboroxole (BOB) group at the hydrophilic end was prepared and assembled into fibers (PAA) with BOB group on the fiber surface. Then glycopolymer with mannopyranoside as pendent group interacted with the PAA via dynamic covalent bond between sugar and BOB. By combining the results from 2D ¹H NMR spectroscopy, the exact binding mode of mannopyranoside pendent group and BOB, i.e., mannopyranoside participated by its diol on 2,3-position instead of that on 4,6-position, which was clearly observed on the fiber surface. The success in determining this binding mode in macroscopic material was due to the high density of BOB on PAA and the multivalent effect between the multiple BOB moieties on fiber surface and repeating mannopyranoside groups of the glycopolymer.

Conformation and dynamics at a flexible glycosidic linkage revealed by NMR spectroscopy and molecular dynamics simulations: analysis of β-L-Fucp-(1→6)-α-D-Glcp-OMe in water solution

The intrinsic flexibility of carbohydrates facilitates different 3D structures in response to altered environments. At glycosidic (1→6)-linkages, three torsion angles are variable, and herein the conformation and dynamics of β-L-Fucp-(1→6)-α-D-Glcp-OMe are investigated using a combination of NMR spectroscopy and molecular dynamics (MD) simulations. The disaccharide shows evidence of conformational averaging for the ψ and ω torsion angles, best explained by a four-state conformational distribution. Notably, there is a significant population of conformations having ψ = 85° (clinal) in addition to those having ψ = 180° (antiperiplanar). Moderate differences in (13)C R1 relaxation rates are found to be best explained by axially symmetric tumbling in combination with minor differences in librational motion for the two residues, whereas the isomerization motions are occurring too slowly to be contributing significantly to the observed relaxation rates. The MD simulation was found to give a reasonably good agreement with experiment, especially with respect to diffusive properties, among which the rotational anisotropy, D∥/D⊥, is found to be 2.35. The force field employed showed too narrow ω torsion angles in the gauche-trans and gauche-gauche states as well as overestimating the population of the gauche-trans conformer. This information can subsequently be used in directing parameter developments and emphasizes the need for refinement of force fields for (1→6)-linked carbohydrates.