Block polysaccharides may be prepared by terminal conjugation of oligosaccharides. When blocks are conjugated terminally, as opposed to laterally, the intrinsic physiochemical properties of the blocks will be retained. Consequently, properties such as sensitivity to pH, ionic strength and reactivity towards divalent cations can be used as switches to manipulate chain-chain interactions and assembly of the blocks.
Block polysaccharides are conjugated using a small spacer molecule, selectively reactive towards the reducing end. The reducing end of a series of biopolymers (alginate, chitosan, chitin, dextran and amylose) has been modified with bifunctional oxyamines (NH₂-O-) and hydrazides (NH₂-NH-CO-). Reductive amination with α-picoline borane was used for all conjugation reactions. The amination and the reduction were studies by NMR in real time, revealing both the product distribution and the reaction kinetics.
A range of alginate containing block polysaccharides have been prepared. The reactivity of the block polysaccharides towards calcium ions was studied by light scattering over time using a dialysis setup. The properties of the blocks are expected to depend both on the properties of the biopolymers and their chain length. Mono- and bifunctional PEGs have also been conjugated to alginate according to the same method. Conjugation to bifunctional PEGs represents an alternative application of the chemistry. Many systems rely on reducing end modifications of polysaccharides to form effective precursors, for example for applications in bioorthogonal chemistry.