High molecular weight chitosan 6‐O‐sulfate. Synthesis, ESR and NMR characterization

Abstract

AbstractSelective sulfation of the primary hydroxyl group of chitosan was performed by means of the pyridine‐SO3 complex after the positions 2 and 3 of the aminoglucopyranosidic ring were protected by coordination with copper ions. The requirements for complexing copper ions were investigated as a function of the pH and Cu(II)/chitosan mole ratio of the starting solution. Cu(II)‐chitosan complexes were characterized by ESR and 13C NMR CP‐MAS investigations; two different paramagnetic Cu(II) species were observed, whose relative quantity changes by changing the pH of the starting solution. The g tensor values (species I: g1‖ = 2,28, g1‖ = 2,06, A2‖ = 160 G; species II; g2‖ = 2,24, g2‖ = 2,06, A2‖ = 160 G) are consistent with a CuN2O2 chromophore. The sulfation reaction was studied in the range of 25 to 75°C using 2 : 1 to 6 : 1 pyridine‐SO3/Cu(II)‐chitosan mole ratios. Under optimized reaction conditions, a selective 6‐O sulfation was obtained in more than 90% yield.