Photoswitchable cluster glycosides as tools to probe carbohydrate–protein interactions: synthesis and lectin-binding studies of azobenzene containing multivalent sugar ligands

Abstract

Synthetic cluster glycosides have often been used to unravel mechanisms of carbohydrate-protein interactions. Although synthetic cluster glycosides are constituted on scaffolds to achieve high avidities in lectin binding, there have been no known attempts to modulate the orientations of the sugar clusters with the aid of a functional scaffold onto which the sugar units are linked. Herein, we describe synthesis, physical, and lectin-binding studies of a series of alpha-D-mannopyranoside and beta-D-galactopyranosyl-(1-->4)-beta-D-glucopyranoside glycoclusters that are attached to a photoswitchable azobenzenoid core. These glycoclusters were synthesized by the amidation of amine-tethered glycopyranosides with azobenzene carbonyl chlorides. From kinetic studies, the cis forms of the azobenzene-glycopyranoside derivative were found to be more stable in aqueous solutions than in organic solvents. Molecular modeling studies were performed to estimate the relative geometries of the photoswitchable glycoclusters in the trans- and cis-isomeric forms. Isothermal titration calorimetry (ITC) was employed to assess the binding of these glycoclusters to lectins peanut agglutinin (PNA) and concanavalin A (Con A). Although binding affinities were enhanced several orders higher as the valency of the sugar was increased, a biphasic-binding profile in ITC plots was observed during few glycoclusters lectin-binding processes. The biphasic-binding profile indicates a "cooperativity" in the binding process. An important outcome of this study is that in addition to inherent clustering of the sugar units as a molecular feature, an induced clustering emanates because of the isomerization of the trans form of the azobenzene scaffold to the cis-isomeric form.

Azobenzenes—synthesis and carbohydrate applications

“…17). 107 Similarly, azobenzene 3,3 0 ,5,5 0 -tetracarbonyl chloride (190) led to the azobenzene derivatives 191-193 (Fig. 18).…”

“…Selective amide bond formation between an amine-tethered mannopyranoside derivative 146 and 4,4 0 -azobenzene dicarbonyl chloride (149) was performed in the presence of triethylamine to obtain the dissymmetrical compound 147 in 76% yield (Scheme 53). 107 Classical deprotection of the glycone moiety furnished the targeted azobenzene 148 in quantitative yield.…”

“…18). 107 Recently, dimers of cyclodextrin linked by an azobenzene moiety were reported in the literature. Liu …”

Rapid Screening of Lectins for Multivalency Effects with a Glycodendrimer Microarray

“…Generally multivalency effects with small multivalent sugars binding to PNA have been small, [31] but in selected cases, effects of around one order of magnitude have been reported. [32] Cholera toxin (CT) [24] is a member of the AB 5 toxins that contain a disease-causing A-subunit surrounded by five carbohydrate-binding B-subunits. These B-subunits attach the toxins to cell surfaces as a first step towards disease.…”

SPR and ITC determination of the kinetics and the thermodynamics of bivalent versus monovalent sugar ligand–lectin interactions

“…It is seen, however, that the studies of the kinetics of the ligand-lectin interactions are much less known [18,19]. In our continuing studies on multivalent interactions [20][21][22][23], we undertook a detailed study of the kinetics of the bivalent vs monovalent sugar ligands during the lectin binding, by employing the SPR technique. The results were correlated with thermodynamic parameters, secured from the ITC measurements.…”