Phosphates as Assisting Groups in Glycan Synthesis

In nature, phosphates are added to and cleaved from molecules to direct biological pathways. The concept was adapted to overcome limitations in the chemical synthesis of complex oligosaccharides. Phosphates were chemically placed on synthetic glycans to ensure site-specific enzymatic elongation by sialylation. In addition, the deliberate placement of phosphates helped to solubilize and isolate aggregating glycans. Upon traceless removal of the phosphates by enzymatic treatment with alkaline phosphatase, the native glycan structure was revealed, and the assembly of glycan nanostructures was triggered.

Stereoselective synthesis of photoactivatable Man(β1,4)GlcNAc-based bioorthogonal probes

We report an operationally facile protocol to prepare photoactivatable probes of the bioactive mammalian disaccharide, Man(β1,4)GlcNAc. Using conformationally restricted mannosyl hemi-acetal donors in a one-pot chlorination, iodination and glycosylation sequence, β-mannosides were generated in excellent diastereoselectivities and yields. Upon accessing the disaccharide, we generated the corresponding photoactivatable probes by appending a diazirine-alkyne equipped linker via a condensation reaction between a diazirine-containing linker and C-1 and C-2 derivatized mannosylamines to furnish the desired C-1 and C-2 modified Man(β1,4)GlcNAc-based probes. This new synthetic protocol greatly simplifies the preparation of this important bioactive disaccharide to enable future work to identify its protein binding partners in cells.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2019-2020

This review is the tenth update of the original article published in 1999 on the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2020. Also included are papers that describe methods appropriate to analysis by MALDI, such as sample preparation techniques, even though the ionization method is not MALDI. The review is basically divided into three sections: (1) general aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, fragmentation, quantification and the use of arrays. (2) Applications to various structural types such as oligo- and polysaccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals, and (3) other areas such as medicine, industrial processes and glycan synthesis where MALDI is extensively used. Much of the material relating to applications is presented in tabular form. The reported work shows increasing use of incorporation of new techniques such as ion mobility and the enormous impact that MALDI imaging is having. MALDI, although invented nearly 40 years ago is still an ideal technique for carbohydrate analysis and advancements in the technique and range of applications show little sign of diminishing.

Expedited synthesis of mannose-6-phosphate containing oligosaccharides

Currently, the reaction toolbox for the functionalization of glycans assembled on solid-phase is quite limited. Automated (1 h) and manual (overnight) phosphorylation protocols that enable the solid-phase synthesis of oligosaccharides containing up to two mannose-6-phosphates are presented. Automated glycan assembly expedited access to substrates and facilitated the screening of experimental conditions.

Facile anomer-oriented syntheses of 4-methylumbelliferyl sialic acid glycosides

As part of a program to find new sialidases and determine their enzymatic specificity and catalytic activity, a library of 4-methylumbelliferyl sialic acid glycosides derivatised at the C-5 position were prepared from N-acetylneuraminic acid. Both α- and β-4-methylumbelliferyl sialic acid glycosides were prepared in high yields and stereoselectivity. α-Anomers were accessed via reagent control by utilising additive CH3CN and TBAI, whereas the β-anomers were synthesised through a diastereoselective addition reaction of iodine and the aglycone to the corresponding glycal followed by reduction of the resulting 3-iodo compounds. Both anomer-oriented synthetic pathways allow for gram-scale stereoselective syntheses of the desired C-5 modified neuraminic acid derivatives for use as tools to quantify the enzymatic activity and substrate specificity of known sialidases, and potential detection and investigation of novel sialidases.