Towards Dextran-Based Block Polysaccharides
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Carbohydrates covalently linked with other chemical species are termed glycoconjugates. They are important compounds in biology, and are involved in many processes, such as cell-cell interactions. Glycoconjugates can be prepared by conjugating a compound to the reducing end of a carbohydrate directly or by modification of the reducing end by a multifunctional compound which in a second step can be conjugated to other molecules. Chemically modified glycoconjugates with carbohydrates conjugated to multifunctional compounds, could provide a vast amount of possibilities for further conjugation, opening up for constructing novel biopolymer hybrid molecules, for example polysaccharide block copolymers. The aim of this work was to investigate the suitability of two types of bifunctional molecules in ligations to the carbonyl group at the reducing end of dextran oligosaccharides, and thereby activating the dextran for further conjugation. Dextran was first partially degraded by acid hydrolysis to obtain oligosaccharides (oligomers) of varying degree of polymerization (DP). Time-series samples were collected throughout degradation of two different dextran concentrations, with subsequent analysis of samples to obtain data DPn throughout the degradations. Optimized degradation conditions determined from the degradation study were used, and a scaled up degradation to obtain oligomers of DP 2-10 was performed. The oligomers were separated and purified. The DP of the purified oligomers was determined using reducing end assays and proton nuclear magnetic resonance (1H-NMR) spectroscopy. Dextran was also partially degraded to obtain oligomer mixtures with a number average DP (DPn) of 34 and 49. To study the degree of branching, dextran T-2000 (undegraded and partially degraded) and dextran from Leuconostoc spp (undegraded) were characterized by 1H-NMR and compared. Thus, information of the degree of branching at certain degradation times and in different types of dextrans was obtained. The degree of branching was found to be lowest in the most degraded dextran, and approximately the same in the two different types of undegraded dextrans. Dextran oligomers of varying DPs were successfully conjugated to the oxyamine O,O′-1,3-Propanediylbishydroxylamine (PDHA) and the dihydrazide, Adipic acid dihydrazide (ADH), providing decent yields of conjugates. The conjugation reactions were studied using real-time NMR. The rates of product formation were found to be faster for the conjugation reactions involving PDHA, compared to ADH. Increased reaction rates were also observed for oligomer mixtures with DPn = 34, compared to purified oligomers of DP 2-10. The yields of oligomers conjugated to PDHA ranged from 83 95 %, using 2 molar equivalents of PDHA. The yields of oligomers conjugated to ADH ranged from 34 - 63 % using 2 molar equivalents of ADH. Using 10 molar equivalents of linker molecules provided yields up to 97 % for PDHA conjugates, and up to 80 % for ADH conjugates. Real-time NMR proved to be a viable experimental approach to study the yields and kinetics of the reactions.