Screening of Neu5Acα(2-6)gal isomer preferences of siglecs with a sialic acid microarray

Syntheses of α(2,8) Sialosides Containing NeuAc and NeuGc by Using Double Carbonyl-Protected N-Acyl Sialyl Donors

We report on the syntheses of NeuAc and NeuGc-containing glycosides via the use of double carbonyl-protected N-acetyl sialyl donors. The 7-O,9-O-carbonyl protection of an N-acyl-5-N,4-O-carbonyl-protected sialyl donor markedly increased the α-selectivity during glycosylation, particularly when glycosylating the C-8 hydroxyl group of sialic acids. The N-acyl carbamates were selectively opened with ethanethiol under basic conditions to provide N-acyl amines. It is noteworthy that N-glycolyl carbamate was more reactive to nucleophiles by comparison with the N-acetyl carbamate due to the electron-withdrawing oxygen in the N-acyl group and however, allowed selective opening of the carbamates without the loss of N-glycolyl groups. To demonstrate the utility of the approach, we began by synthesizing α(2,3) and α(2,6) sialyl galactosides. Glycosylation of the hydroxy groups of galactosides at the C-6 position with the NeuAc and NeuGc donors provided the corresponding sialyl galactoses in good yields with excellent α-selectivity. However, glycosylation of the 2,3-diol galactosyl acceptor selectively provided Siaα(2,2)Gal. Next, we prepared a series of α(2,8) disialosides composed of NeuAc and NeuGc. Glycosylation of NeuGc and NeuAc acceptors at the C-8 hydroxyl group with NeuGc and NeuAc sialyl donors provided the corresponding α(2,8) disialosides, and no significant differences were detected in the reactivities of these acceptors.

Parametric Analysis of Donor Activation for Glycosylation Reactions

The chemical synthesis of complex oligosaccharides relies on efficient and highly reproducible glycosylation reactions. The outcome of a glycosylation is contingent upon several environmental factors, such as temperature, acidity, the presence of residual moisture, as well as the steric, electronic, and conformational aspects of the reactants. Each glycosylation proceeds rapidly and with a high yield within a rather narrow temperature range. For better control over glycosylations and to ensure fast and reliable reactions, a systematic analysis of 18 glycosyl donors revealed the effect of reagent concentration, water content, protecting groups, and structure of the glycosyl donors on the activation temperature. With these insights, we parametrize the first step of the glycosylation reaction to be executed reliably and efficiently.

Biocatalysis versus Molecular Recognition in Sialoside-Selective Neuraminidase Biosensing

Sialic acid recognition and hydrolysis are essential parts of cellular function and pathogen infectivity. Neuraminidases are enzymes that detach sialic acid from sialosides, and their inhibition is a prime target for viral infection treatment. The connectivity and type of sialic acid influence the recognition and hydrolysis activity of the many different neuraminidases. The common strategies to evaluate neuraminidase activity, recognition, and inhibition rely on extensive labeling and require a large amount of sialylated glycans. The above limitations make the effort of finding viral inhibitors extremely difficult. We used synthetic sialylated glycans and developed a label-free electrochemical method to show that sialoside structural features lead to selective neuraminidase biosensing. We compared Neu5Ac to Neu5Gc sialosides to evaluate the organism-dependent neuraminidase selectivity-sensitivity relationship. We demonstrated that the type of surface and the glycan monolayer density direct the response to either binding or enzymatic activity. We proved that while the hydrophobic glassy carbon surface increases the interaction with the enzyme hydrophobic interface, the negatively charged interface of the lipoic acid monolayer on gold repels the protein and enables biocatalysis. We showed that the sialoside monolayers can serve as tools to evaluate the inhibition of neuraminidases both by biocatalysis and molecular recognition.

Synthesis and Azobenzene Isomerization Investigation of Photoswitchable Glycomacrocycles

Macrocyclic glycoazobenzenes, as an emerging class of photoswitchable chiral macrocyclic compounds, have shown interesting properties since their discovery in 2017. We have recently employed the azobenzene-ester-linked glycosyl donor-acceptor pairs to study the influence of photoisomerization on intramolecular glycosylation. To continue the investigation on the stereoselectivity aspect of glycosylation and also to enlarge the diversity of photoswitchable glycomacrocycles, we have chosen azobenzene-triazole linkers in the present study and shown that the stereoselectivity of the glycosylation is dependent on the linker length, the configuration of the azobenzene template, as well as the reaction concentration. We have optimized the reaction conditions to prepare in good yields new glycomacrocycles, which displayed excellent photochromic properties. The influence of glycosylation reagents and acidity on the stability of the Z-azobenzene substrates and cyclic glycoazobenzenes has also been investigated, demonstrating that isomerization of macrocyclic azobenzene can be tuned by photo-, thermo-, and acid stimulus.

Regioselective Reductive Opening of Benzylidene Acetals with Dichlorophenylborane/Triethylsilane: Previously Unreported Side Reactions and How to Prevent Them

Arylidene acetals are widely used protecting groups, because of not only the high regioselectivity of their introduction but also the possibility of performing further regioselective reductive opening in the presence of a hydride donor and an acid catalyst. In this context, the Et₃SiH/PhBCl₂ system presents several advantages: silanes are efficient, environmentally benign, and user-friendly hydride donors, while PhBCl₂ opens the way to unique regioselectivity with regard to all other Brønsted or Lewis acids used with silanes. This system has been extensively used by several groups, and we have demonstrated its high regioselectivity in the reductive opening of 4,6- and 2,4-O-p-methoxybenzylidene moieties in protected disaccharides. Surprisingly, its use on 4,6-O-benzylidene-containing substrates 1 and 2 led to unreproducible yields due to the unexpected formation of several side products. Their characterizations allowed us to identify different pitfalls potentially affecting the outcome of reductive opening of arylidenes with the Et₃SiH/PhBCl₂ reagent system: alkene hydroboration, azide reduction, and/or Lewis acid-promoted cleavage of the arylidene. With this knowledge, we optimized reproducible and high-yielding reaction conditions that secure and extend the scope of the Et₃SiH/PhBCl₂ system as a reagent for the regioselective opening of arylidenes in complex and multifunctional molecules.

Expedient assembly of Oligo-LacNAcs by a sugar nucleotide regeneration system: Finding the role of tandem LacNAc and sialic acid position towards siglec binding

Sialosides containing (oligo-)N-acetyllactosamine (LacNAc, Galβ(1,4)GlcNAc) as core structure are known to serve as ligands for Siglecs. However, the role of tandem inner epitope for Siglec interaction has never been reported. Herein, we report the effect of internal glycan (by length and type) on the binding affinity and describe a simple and efficient chemo-enzymatic sugar nucleotide regeneration protocol for the preparative-scale synthesis of oligo-LacNAcs by the sequential use of β1,4-galactosyltransferase (β4GalT) and β1,3-N-acetylglucosyl transferase (β3GlcNAcT). Further modification of these oligo-LacNAcs was performed in one-pot enzymatic synthesis to yield sialylated and/or fucosylated analogs. A glycan library of 23 different sialosides containing various LacNAc lengths or Lac core with natural/unnatural sialylation and/or fucosylation was synthesized. These glycans were used to fabricate a glycan microarray that was utilized to screen glycan binding preferences against five different Siglecs (2, 7, 9, 14 and 15).

ABO Antigens Active Tri- and Disaccharides Microarray to Evaluate C-type Lectin Receptor Binding Preferences

Understanding blood group antigen binding preferences for C-type lectin receptors holds promise for modulating immune responses, since several Gram-negative bacteria express blood group antigens as molecular mimicry to evade immune responses. Herein, we report the synthesis of ABO blood group antigen active tri and disaccharides to investigate the binding specificity with various C-type lectin receptors using glycan microarray. The results of binding preferences show that distinct glycosylation on the galactose and fucose motifs are key for C-type lectin receptor binding and that these interactions occur in a Ca²⁺-dependent fashion.