Chitosan intermolecular hydrogen bonds

Chitosan is a multi-nucleophilic polymer due to the presence of the NH2 and OH functional groups. The initial sites where substitution occurs are the more nucleophilic amino groups. However, the experimental conditions and protection of the NH2 groups reduces the intermolecular hydrogen bonding and creates space for water molecules to fill in and solvate the hydrophilic groups of the polymer backbone (Sashiwa and Shigemasa 1999). A -alkylated derivatives can be obtained by the treatment of chitosan with aldehydes or ketones via formation of Schiff base intermediates, aldimines (from reactions with aldehydes), or ketimines (from reactions with ketones) followed by reduction of the imine with sodium borohydride. 

FUR and XPS results showed that intermolecular hydrogen bonds existed between two components in the blends, and the hydrogen bonds were mainly between carbonyls of PLLA and amino groups of chitosan. 

One important characteristic related with chemical structure, which affects directly the processability of chitosan is the solubility. Chitosan is insoluble in water and alkaline media. This is due to its rigid and compact crystalline structure and strong intra- and intermolecular hydrogen bonding. However, chitosan is soluble only in few dilute acid solutions (Vroman and Tighzert, 2009]. In fact, solubility is a very difficult parameter to control, and it is related to the degree of acetylation, the ionic concentration, the pH, the nature of the acid used for protonation, and the distribution... 

In spite of the alteration due to deacetylation, chitosan from crab tendon possesses a crystal structure showing an orthorhombic unit cell with dimensions a = 0.828, b = 0.862 and c = 1.043 nm (fiber axis). The unit cell comprises four glucosamine units two chains pass through the unit cell with an antiparallel packing arrangement. The main hydrogen bonds are 03 05 (intramolecular) and N2 06 (intermolecular) [82]. This material has also found medical uses (below). 

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