Synthetic, pH-responsive glycopolypeptides and their interactions with lectins

Abstract

Event Abstract Back to Event Synthetic, pH-responsive glycopolypeptides and their interactions with lectins Katharina Haak1*, Robert Mildner1* and Henning Menzel1* 1 Braunschweig University of Technology, Institute of Technical Chemistry, Germany Introduction: Many biological processes depend on specific interactions between glycoproteins and lectins, including cell-cell communication and pathogenic infection. Synthetic glycopolypeptides are a useful tool to investigate those processes, since they can mimic properties of glycoproteins (e.g. form secondary structures) and are relatively easy of access compared to natural glycoproteins. Glycopolypeptides can be prepared by the polymerization of glycosylated monomers or by post-polymerization glycosylation in organic solvents[1],[2]. These methods are often restricted to monosaccharides but in most biological interactions more complex sugars are involved. DMT-MM is a suitable coupling agent for monosaccharides with mild reaction conditions in aqueous environment[3]. Using this approach glucose containing oligosaccharides were coupled to poly-(α,L-glutamic acid) scaffolds and compared with the corresponding monosaccharide. Since aromatic spacers can enhance the helicity of glycopolypeptides[4], the influence of spacers on the lectin interaction was also tested. Materials and Methods: Poly-(γ-benzyl-L-glutamate) (PBLG) was prepared by ring-opening polymerization of benzyl-L-glutamate N-carboxyanhydride. Poly-(α,L-glutamic acid) precursors (PGA) were obtained by acidic hydrolysis of PBLG. 1-deoxy-1-amino saccharides were synthesized by Kochetkov-amination and coupled to the PGA-scaffold via amide coupling with DMT-MM. Secondary structures were determined via CD-spectroscopy. Biological activity was tested via isothermal titration calorimetry (ITC) as well as via quantitative precipitation assays with the lectin ConA. The amount of precipitated lectin per glycopolypeptide was determined via UV/Vis spectroscopy. Results and Discussion: Figure 1 Aqueous amide coupling of 1-amino-1-deoxymaltobiose to PGA-Na The sugar density on the PGA-scaffold is adjustable via the amount of DMT-MM used. For glucose-based sugars DS values up to 80 % are possible. CD-spectroscopy revealed that the PGA scaffold shows a pH-dependent helix-coil transition close to pH 5 with a maximum helicity of 80 % at pH 3. The glycosylated PGAs also show helix-coil transitions up to sugar densities of approximately 65 %. However, the transition points shift with increasing amount of sugar to more acidic pH values along with a decrease in helicity. Aromatic spacers can enhance the helicity of glycopolypeptides. Quantitative precipitation assays with the lectin ConA indicate, that glycopolypeptides exhibit stronger interaction with the lectin with increasing degree of substitution as well as with increasing size of the saccharide (Figure 2). ITC measurements yield equilibrium binding constants in nanomolar range, indicating affinity enhancement due to glycoside clustering. Figure 2 Quantitative Precipitation Assay of PGA-Maltotriose (DS 0.8) and PGA-Glucose (DS 0.8) with the lectin ConA Conclusion: Well-defined glycopolypeptides were prepared by post-polymerization glycosylation of poly-(α,L-glutamic acid) with 1-deoxy-1-amino sugars using the amide coupling agent DMT-MM. The glycopolypeptides showed pH-dependent helix-coil transitions. Precipitation assays as well as ITC measurements demonstrate specific interactions with the lectin ConA. This work was supported by the IUPAC Transnational Call in Polymer Chemistry and the German Research Foundation (DFG) (Grant: Me1057/17-1)