Carbohydrates on Surfaces: Lectin Binding and Biosensing Applications

Abstract

Carbohydrates are recognized as information-rich biomolecules that play a major role in the human body. The surface of all mammalian cells is covered with carbohydrates that are attached to proteins and lipids embedded on cell membrane. The interaction with the extracellular world is achieved through interaction between these carbohydrates and carbohydrate-binding proteins (lectins) which are present on the surfaces of other mammalian cells, viruses, bacteria and bacterial toxins. To enhance the strength of cell surface binding, nature often assembles multiple protein–carbohydrate complexes to provide the necessary avidity. The effect of multivalency concerning sugars present on the surfaces compared to monovalancy has been described by several investigators and was found to be of critical importance in the field of protein–carbohydrate interactions. Therefore, it is important to design a multivalent scaffold which is facile and presents non-covalent interactions. Inspired by a large number of the supramolecular assembly of adamantly or ferrocene/β-cyclodextrin associated complexes, I have investigated the role of host-guest interaction on interfaces and its potential biosensor applications. Chapter 1 describes different multivalent scaffolds and surface techniques adopted to study carbohydrate-protein interactions. We highlighted the current efforts made in the synthesis of multivalent glycoprobes and the role of spatial arrangements, chirality and symmetry of these multivalent probes in carbohydrate-protein interactions. Finally, we also highlighted the recent label free techniques adopted to characterize carbohydrate-protein interactions. Chapter 2 summarizes non-covalent host–guest strategy to immobilize heptavalent glyco-β-cyclodextrin on gold-coated glass slides to study multivalent carbohydrate–protein interactions. We have found that the localization of sugar entities on surfaces using β-cyclodextrin (β-CD) chemistry increased the avidity of carbohydrate–protein and carbohydrate–macrophage interactions compared to monovalent-β-CD sugar coated surfaces. Furthermore, the sugar functionalized β-Cyclodextrin-ferrocene glass slides were used to develop fully reversible bacterial biosensors. The prototype D-mannose - E. coli ORN 178, L-fucose - P. Aeruginosa- D-galactose interactions serve as a model to illustrate the new approach. Chapter 3 deals with the synthesis of homo and heteromultivalent mono and oligomannose glycodendrons and their binding to a series of plant and animal lectins to understand specific factors influencing CPIs. Multivalency, heterogenity and oligosaccharide have been successfully xiv adopted to increase the avidity of CPIs. However, there is still a lot of unanswered quenstions about whether these factors, are optional or obligatory to design the glycoclustuers. Our microarray results clearly showed that each lectins displayed its own set of binding preferences. In case of ConA and PNA lectins, oligosaccharides multivalency is more important than heterogeneity. While GNA and galaectin lectins displayed heterogeneity of the glycodendrons as critical factor for better CPIs. Overall these results demonstrate that the each lectin possess its one set of rules for CPIs and difficult be rationalized. Chapter 4 demonstrates the inherent chirality of the sugar as one of the promising factor to generate bacterial recognizing. We have shown that while D/L-enantiomers of α-mannose and β -galactose reveal significant differences in the recognition of bacteria (E. coli) and pathogen (toxoplasmosis gondii), cell-adhesion and cell-proliferation were barely influenced by the two configurations. Finally, we have used bioorthogonal conjugation techniques to bind D/L-mannose enantiomers on HeLa cell surfaces and exploited the difference in cellular and bacterial binding recognition of the two molecules to prevent bacterial-cell infection.