Identifying Efficient Clostridium difficile Toxin A Binders with a Multivalent Neo-Glycoprotein Glycan Library

Clostridium difficile infections cause gastrointestinal disorders and can lead to life-threatening conditions. The symptoms can vary from severe diarrhea to the formation of pseudomembranous colitis and therefore trigger a need for new therapies. The initial step of disease is the binding of the bacterial enterotoxins toxin A and B to the cell surface of epithelial intestinal cells. Scavenging of the toxins is crucial to inhibit their fatal effect in the human body and circumvent the administration of antibiotics. Cell surface glycans are common as ligands for TcdA. Although crucial for carbohydrate-protein interactions, a multivalent presentation of glycans for binding has been hardly considered. Here, we establish a neo-glycoprotein-based glycan library to identify an effective multivalent glycan ligand for TcdA. It comprises 40 different glycan epitopes based on N-acetyllactosamine precursors. Nine structures exhibit strong binding of the receptor domain. Among them, the Lewis^y-Lewis^x-epitope shows the best performance for binding both the receptor domain and the holotoxin. Therefore, the glycan was synthesized de novo and coupled to BSA as a scaffold for multivalent presentation. The corresponding neo-glycoprotein facilitates the proper scavenging of TcdA in vitro and effectively protects HT29 cells from TcdA-induced cell damage.

Synthesis of rationally designed tetrasaccharides for crystallographic and binding studies with Clostridium difficile toxins and unexpected partial N-methylations during catalytic hydrogenation of azides in methanol

A β-GlcNAc-LeX tetrasaccharides and another α-Gal-LeX analog have been synthesized for studying interaction with toxins produced by the human pathogen Clostridium difficile. LeX-based trisaccharides bearing either a 2-azidoethyl or a 6-azidohexyl aglycone have been employed in the total synthesis to afford the desired tetrasaccharides. Interestingly, during the final catalytic hydrogenation step to remove benzyl and benzylidene protecting groups and simultaneously reduce azide functionality, partial N-methylations were observed. The N-methylations appear to be a general issue with catalytic hydrogenation of azides in methanol.

Synthesis of Tumor-Associated Le(a)Le(x) Hexasaccharides: Instability of a Thiol-Containing Oligosaccharide in Mass Spectrometry and Hypermetalation Detected by ESI FAIMS

We report the efficient synthesis of three analogues of the tumor-associated carbohydrate antigen Le(a)Le(x). This hexasaccharide was prepared as a soluble inhibitor hexyl glycoside, as a 6-aminohexyl glycoside for conjugation to proteins, and as a 6-thiohexyl glycoside for immobilization to a gold surface. These three analogues were obtained from a common hexasaccharide intermediate and isolated pure following efficient deprotection reactions that involved metal-dissolving conditions. While all other intermediates and analogues gave the expected molecular ions in ESI HRMS, the 6-thiohexyl glycoside final compound gave a complex spectrum in which no signal matched the molecular ion. Using ESI FAIMS HRMS, we were able to prevent ion dissociation reactions and obtained high quality spectral data. The ions detected could be characterized unambiguously from their accurate masses and gave insight into the behavior of the thiohexyl analogue in the gas phase. These results indicate that the 6-thiohexyl glycoside lost water and led to the formation of "hypermetalated" species which we propose are cyclic.

Aggregation of a Tetrasaccharide Acceptor Observed by NMR: Synthesis of Pentasaccharide Fragments of the Le(a)Le(x) Tumor-Associated Hexasaccharide Antigen

We report the synthesis of a tetrasaccharide and two pentasaccharide fragments of the Le(a)Le(x) tumor-associated carbohydrate antigen α-L-Fuc-(1→4)-[β-D-Gal-(1→3)]-β-D-GlcNAc-(1→3)-β-D-Gal-(1→4)-[α-L-Fuc-(1→3)]-β-D-GlcNAc-(1→OR). The choice of protecting groups permitted a one-step global deprotection (Na/NH3(l)). The protected chlorohexyl glycoside pentasaccharide was the precursor to the hexyl glycoside, to be used as a soluble inhibitor, and the aminohexyl glycoside analogue, to be conjugated to proteins for surface immobilization and immunization experiments. We observed that a linear tetrasaccharide that contained two N-acetylglucosamine residues and a free OH group gave two distinct sets of (1)H NMR signals when the data were acquired in deuterated chloroform. Data acquisition at variable concentrations and variable temperatures suggests that the second set of NMR signals results from aggregation of the tetrasaccharide driven by the formation of intermolecular H-bonds involving the NHAc. While the formation of intra- and intermolecular H-bonds involving N-acetylgucosamine residues has been reported in non-H-bonding solvents, this is, to our knowledge, the first time that these have lead to the appearance of two distinct sets of signals in the NMR spectra. This aggregation may explain the lack of reactivity observed when an attempt is made to glycosylate such an acceptor using non-H-bonding solvents such as dichloromethane.