Biomolecular interactions at model interfaces : a self-assembled monolayer approach

Abstract: The overall aim of this thesis has been to explore the use of self-assembled monolayers (SAMs) in model studies of biomolecular binding at interfaces. Molecular details of interfaces are difficult to address experimentally, and in spite of the fact that interactions in the living system commonly occur at interfaces, e.g. in the vicinity of a cell or a cell compartment, biomolecular interactions are generally studied in bulk solution. In special cases. such as in experiments with SAMs on metal surfaces, structural and functional information related to interfacial phenomena can be accessed more easily.A strategy was developed for the synthesis of a series of oligo( ethylene glycol)-terminated alkanethiols (HS-(CH2)m-CONH-(CH2CH20)n-H) with the objective of studying structure and stability of the corresponding SAMs on gold. Oligo(ethylene glycol) chains were coupled to alkyl chains via amide or ester linkages. Structural characterisation by infrared reflection-absorption spectroscopy (IRAS) revealed highly ordered SAMs. where the confonnation of the oligo(ethylene glycol) chains varied as a result of the intermolecular interactions in the monolayer. The SAMs were moderately hydrophilic (water contact angles ~30°). and protein adsorption studies indicated reasonable resistance toward simple protein systems. However. substantial amounts of proteins adsorbed from a more complex biofluid such as serum.The trisaccharide globotriose is a recurrent structural motif of glycosphingolipids and is involved in various biological functions. According to IRAS studies, the SAMs of globotriose-terminated alkanethiols were found to form well-ordered structures. By surface plasmon resonance (SPR) biosensor experiment interactions of specific monoclonal antibodies with globotriose-containing SAMs proved sensitive to small fractions of carbohydrate (- 1%) in mixed SAMs. Different antibodies also responded distinctly different to a series of SAMs with increasing globotriose fractions. Thus. the SAM approach has great potential for studying of antibody binding to tailor-made interfaces, where molecular details such as spacer length and antigen distribution can be systematically varied.Different immobilisations of globotriose-derived haptens to dextran-coated sensor chips in SPR biosensor experiments were investigated with the results being in qualitative agreement with solidphase immunoassays (ELISA). However. antibody interactions may be overlooked in the direct immobilisation of haptens to dextran. The work in this thesis suggests that these can be more easily detected using neoglycoproteins.An SPR kinetic analysis of the interactions between carbonic anhydrase (CA) mutants and immobilised benzenesulphonamide inhibitors was performed. Dissociation rates were found to increase for a series of increasingly destabilised. point-mutated CA variants. The mutation was located far away from the binding site. suggesting that the observed effect on dissociation rate might be due to increased molecular dynamics.