Functionalisation of the non-reducing end of chitin by selective periodate oxidation: A new approach to form complex block polysaccharides and water-soluble chitin-based block polymers

Abstract

Most polysaccharides used in polysaccharide-based block copolymers are attached to the second block through the reducing end, due to the few and highly polysaccharide specific non-reducing end (NRE) functionalisation methods available. Chitin oligomers, prepared by nitrous acid degradation of chitosan (A_nM) can, however, be selectively oxidised by periodate since they only possess a single vicinal diol in the NRE residue. Here, we show that both aldehydes formed after oxidation are highly reactive towards bifunctional oxyamines and hydrazide linkers. Sub-stochiometric amounts of linkers resulted in conjugation of A_nM oligomers through both chain termini to yield a discrete distribution of 'polymerised' oligomers. Such chitin-based block polymers were, in contrast to chitins of the same chain lengths, water-soluble. Oxidised A_nM oligomers, functionalised at both termini can also enable the preparation of more complex block polysaccharides such as ABA- or ABC-type.

Keywords: Chitin oligosaccharides; Dihydrazides; Dioxyamine; Non-reducing end functionalization; Periodate oxidation; Vicinal diol.