Why is chitosan sometimes chemically modified?

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Chitosans offer ideal opportunities for chemical modification, especially because of the presence of multiple reactive amine groups of the glucosamine residues, but also through the many hydroxyl groups in both glucosamine and N-acetylglucosamine. Chemical modification of these groups can confer additional properties or improve existing properties of chitosans, to make them more appropriate for certain applications. As an example, chitosans can be chemically sulfated, adding negative charges; sulfated chitosans are reported to possess antiviral activities and are thought to act as bio-mimics for human glycosaminoglycans (which are sulfated aminopolysaccharides present in the extracellular matrix surrounding human cells in a tissue). As another example, the amine group of chitosans can be quaternized (i.e. carrying e.g. three methyl groups instead of two hydrogen atoms) which leads to a positive charge independent of the pH of the medium, increasing the antimicrobial activities and possible the ability to act as a drug delivery system. By chemically modifying chitosans, an unlimited number of chitosan derivatives can be produced with many different properties. However, chitosan derivatives are no longer fully natural biopolymers, possibly compromising their biodegradability and biocompatibility.