WATER-SOLUBILITY OF PARTIALLY N-ACETYLATED CHITOSANS AS A FUNCTION OF PH - EFFECT OF CHEMICAL-COMPOSITION AND DEPOLYMERIZATION

The solubility of four partially N-acetylated chitosans with fraction of acetylated units (F-A) of 0.01, 0.17, 0.37 and 0.60 as a function of pH was investigated. The chitosan with F-A = 0.60 was soluble at all pH-values between 4 and 9. The solubility versus pH curve of the other chitosans, showed that all chitosans precipitated between pH 6 and 7.5, but with increasing solubility at higher pH-values with increasing F-A. Such solubility differences may have profound effects on enzyme accessability and biological effects of chitosans. The three chitosans with the lowest F-A values were depolymerised by treatment with nitrous acid, and the fraction of water-soluble material at pH 7.5 was determined. The almost fully deacetylated chitosan was completely insoluble at pH 7.5 in the depolymerisation range investigated, while the most acetylated chitosan (F-A = 0.60) was fully soluble at all pH-values. However, the two chitosans with F-A = 0.17 and 0.37 could be fractionated into a neutral-soluble and a neutral-insoluble fraction. The amount of neutral-soluble material increased with decreasing depolymerisation. The neutral-soluble and the neutral-insoluble fraction differed in both chemical composition and degree of polymerisation. Generally, the neutral-soluble fraction had a higher fraction of acetylated units and a lower degree of depolymerisation than the neutral-insoluble fraction. This compositional heterogeneity of the degraded chitosans was shown to be consistent with what is expected from the theoretical random degradation of chitosans with a Bernoullian (random) distribution of acetylated and deacetylated units.