Carbohydrates

Dynamic Multivalent Neoglycoconjugates for Binding Lectins

Multivalent interactions between multimeric or membrane-bound carbohydrate binding proteins (lectins) with their corresponding carbohydrate ligands mediate biological processes throughout Nature. Chemists (Professor Kiessling, Professor Cloninger) have built a variety of neoglycoconjugates using dendritic and polymeric scaffolds to mimic the complex multivalent scaffolds found in biological systems. There is considerable interest in how the architectural features of synthetic multivalent structures, including rigidity, spacing, topology, and density of saccharides, influences their avidity. We are interested in the synthesis of pseudopolyrotaxanes and polyrotaxanes comprised of saccharide-displaying cyclodextrins (CDs) threaded onto linear polymer chains as dynamic multivalent neoglycoconjugates for interacting with lectins. In addition to the convenience of synthesis via self-assembly, these intriguing “beads on a string” offer a flexible and dynamic platform for the multivalent display of ligands, as the cyclodextrins are free to rotate around the polymer, and move along the polymer backbone. We are currently working with Professor Jim Paulson at the Scripps Research Institute to target membrane-bound proteins known as siglecs, and Professor Linda Baum in the UCLA Department of Pathology to target multimeric galectins.

Chemically Defined Glycodendrimers for Binding Siglecs

The carbohydrates responsible for mediating biological processes, such as cellular adhesion and communication are often complex oligosaccharides attached to proteins or lipids at the cell surface. The large structural and conformational diversity of these complex oligosaccharides allows nature to create a variety of scaffolds onto which the terminal saccharide ligands, recognized by lectins, are attached. Carbohydrates provide unique recognition units and scaffolds for mediating both physiological and pathological processes ranging from fertilization to angiogenesis. To overcome the inherently weak affinity of a protein receptor (lectin) for its saccharide ligand, nature uses multivalency to enhance the avidity. Glycodendrimers, which have a dendritic backbone with biologically active saccharides attached at the periphery, are able to imitate the multivalency found in Nature. We have taken Nature’s lead to construct carbohydrate-based glycodendrimers for binding lectins. Using an AB-2 trisaccharide as a monomer, our aim is to synthesize well-defined, water-soluble carbohydrate-based glycodendrons. The trisaccharide monomer consists of one masked aldehyde (A) and two protected methyl amino groups (B), which allows the monomer to be coupled together by reductive amination in a stepwise fashion. In collaboration with Jim Paulson’s Group at the Scripps Research Institute, sialic acid is attached enzymatically, to form an a-2,3 or a-2,6 linkage with the galactosyl residues at the periphery of the dendrons. Alternative dendritic oligosaccharide scaffolds are synthesized by changing an anomeric linkage within the trisaccharide monomer, thus changing the overall conformation of the dendron. Evaluation of the dendrons using a competitve ELISA assay determined their affinity for sialoadhesion. Notably, we have thus far found that conformational differences between divalent sialoside dendrons effected its binding to the lectin (J. Org. Chem. 2003, 68, 8485–8493). With these initial results in hand, we aim to find strongly binding dendritic probes specific for each of the eleven known sialic acid-binding proteins known as siglecs.

Carbohydrates-Based Dynamic Combinatorial Libraries

Dynamic covalent chemistry relates to the study of chemical reactions carried out under thermodynamic control. To date, labile coordinative bonds associated with certain metal-ligand interactions, ring-opening/ring-closing metathesis reactions, and imine and disulfide formation protocols, have all been exploited in the self-assembly of catenanes and rotaxanes under thermodynamically controlled conditions. A wide range of other functionalities, including acetals, esters, hydrazones, anhydrides of hydroxyborazaaromatics and oximes have been explored recently in the creation of dynamic combinatorial libraries (DCLs). Carbohydrates command a unique status in the realm of dynamic covalent chemistry. The aldohexoses, for example, display multiple equilibria between constitutional (furanoses and pyranoses) and configurational (a- and b-glycoses), as well as conformational (e.g., 4C1 and 1C4 for the pyranoses), isomers in aqueous solution. Cyclic acetal formation between alditols and aldehydes or ketones, under conditions of acid catalysis, provides yet another example of a well-known reaction where covalent (C–O) bonds are made and broken with varying degrees of ease under thermodynamic control. The constitutions of the configurationally isomeric erythro- and threo-1,2,3,4-butane- tetraols result in their undergoing, with aldehydes and ketones, three different kinds of acetal ring closures – (i) 1,3:2,4-diacetal formation yields ‘6/6’ bicycles, (ii) 1,2:3,4-diacetal formation affords ‘5/5’ bicycles, and (iii) 1,4:2,3-diacetal formation yields ‘5/7’ bicycles. With aldehydes, ‘6/6’ or 1,3,5,7-tetraoxadecalin (TOD) formation usually predominates at equilibrium, whereas with ketones, ‘5/5’ bicycles are invariably the major ones. The TOD-forming reactions involving aldehydes (RCHO) are completely diastereospecific, viz., erythritol gives trans-TOD and threitol affords cis-TOD, which, with its highly distinctive stereoelectronic properties, has attracted the interest of numerous stereochemists for over half a century now. Beyond any doubt is the fact that the O-inside conformations of cis-TOD with equatorial substituents (R) prevail at equilibrium. We aim to synthesize carbohydrate-containing macrocycles from dynamic combinatorial libraries using reversible acetal formation. These chiral receptors are currently being investigated for their ability to selectively bind amino acids.