Towards block polysaccharides: Terminal activation of chitin and chitosan oligosaccharides by dioxyamines and dihydrazides and the preparation of block structures

• Main Author: Mo, Ingrid Vikøren
• Format: Dissertation
• Language: English

Polysaccharides are highly abundant and due to the large variation in chemical compositions, they possess a range of intrinsic properties, biological functions, and industrial applications. In the context of block copolymers, polysaccharide-containing structures are attracting increasing interest since they also serve as more sustainable alternatives to copolymers exclusively composed of synthetic polymers. Block polysaccharides represent a new class of engineered block polymers, exclusively composed of terminally linked oligo- or polysaccharides. Terminal coupling of blocks will, in contrast to the traditional lateral substitution, retain the intrinsic polysaccharide properties, as it does not perturbate the chain structure. Such block polysaccharides can be relevant for a wide range of applications in e.g. the biomedical and (bio)material fields. Chitin is the second most abundant polysaccharide found in nature after cellulose and is the major structural component of the exoskeleton of crustaceans and insects. Chitin is a water-insoluble high molecular weight unbranched homopolysaccharide composed of β-1,4-linked N-acetylglucosamine (GlcNAc, A) residues, whereas its de-N-acetylated derivative, chitosan, has high water-solubility at acidic pH due to the positively charged amino groups (pKa approx. 6.5) of the glucosamine (GlcN, D) residues. Chitin and chitosan are particularly interesting in the context of block polysaccharides due to their high abundance, biocompatibility, biodegradability, self-assembling properties (chitin) and positive charge (chitosan). In the work of this thesis the terminal activation of chitin and chitosan oligomers by a dioxyamine (O,O’-1,3-propanediylbishydroxylamine, PDHA) and a dihydrazide (adipic acid dihydrazide, ADH) using reductive amination with α-picoline borane (PB) as the reductant was studied in detail. In Paper I and Paper II the chemistry and kinetics of the reducing end activation of chitooligosaccharides (CHOS) were investigated. A simple pseudo first order model was introduced to obtain kinetic data which enable simulation of reactions under different conditions as a tool to develop preparative protocols. Activated CHOS were also purified and thoroughly characterised. In contrast to other “click” reagents, oxyamines and hydrazides can react directly with the reducing end aldehyde of carbohydrates without an intermediate reaction step. PDHA and ADH can therefore serve as linkers between two polysaccharide blocks f