Preparation and characterization of branched chitosans

Fully de-N-acetylated chito-oligomers were prepared by de-amination of chitosan using nitrous acid (HNO2). The dimer to hexamer fractions of chitosan oligomers were fractionated with preparative size-exclusion chromatography (SEC) and characterized using H-1 NMR spectroscopy. The purified oligomers were polymerized by reductive N-alkylation by the action of NaCNBH3, in a reaction between the terminal 2,5-anhydro-D-mannofuranose (M-unit) and the primary amines of the repeating units (1 -> 4) bound beta-D-glucosamine (D-unit) resulting in a covalent bond and formation of secondary amines. The polymerized products were studied by SEC-RI and SEC-MALLS-RI and were confirmed to be branched reaching molecular weights of up to 10 000 g/mol. Analysis by H-1 NMR spectroscopy confirmed that the chitosan oligomers had been branched by formation of secondary amines. Fully de-N-acetylated chitosan (F-A < 0.001) was degraded to various extents using nitrous acid resulting in degrees of scission (alpha(s)) in the range 0.01-0.10. The resulting chitosans, with the reactive M-unit on the reducing end, were branched by reductive N-alkylation using NaCNBH3. Analysis by SEC-MALLS-RI-visc showed that the weight-average molecular weight (M-w) and polydispersity (M-w/M-n) had increased. with a degree of branching in the range of 1.2-2.1. The structure in solution had become more compact compared to linear chitosan of the same molecular weight as demonstrated by a significant reduction of the radius of gyration and the intrinsic viscosity, with a corresponding change in the calculated viscosity contraction factor (g') and the geometric contraction factor (g), respectively. (C) 2010 Elsevier Ltd. All rights reserved.