Site-specific modification is a great
challenge for polysaccharide
scientists. Chemo- and regioselective modification of polysaccharide
chains can provide many useful natural-based materials and help us
illuminate fundamental structure–property relationships of
polysaccharide derivatives. The hemiacetal reducing end of a polysaccharide
is in equilibrium with its ring-opened aldehyde form, making it the
most uniquely reactive site on the polysaccharide molecule, ideal
for regioselective decoration such as imine formation. However, all
natural polysaccharides, whether they are branched or not, have only
one reducing end per chain, which means that only one aldehyde-reactive
substituent can be added. We introduce a new approach to selective
functionalization of polysaccharides as an entrée to useful
materials, appending multiple reducing ends to each polysaccharide
molecule. Herein, we reduce the approach to practice using amide formation.
Amine groups on monosaccharides such as glucosamine or galactosamine
can react with carboxyl groups of polysaccharides, whether natural
uronic acids like alginates, or derivatives with carboxyl-containing
substituents such as carboxymethyl cellulose (CMC) or carboxymethyl
dextran (CMD). Amide formation is assisted using the coupling agent
4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride
(DMTMM). By linking the C2 amines of monosaccharides to polysaccharides
in this way, a new class of polysaccharide derivatives possessing
many reducing ends can be obtained. We refer to this class of derivatives
as multi-reducing-end polysaccharides (MREPs). This new family of
derivatives creates the potential for designing polysaccharide-based
materials with many potential applications, including in hydrogels,
block copolymers, prodrugs, and as reactive intermediates for other
derivatives.
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