Tuning the Properties of High Molecular Weight Chitosans to Develop Full Water Solubility Within a Wide pH Range
The preparation of chitosans soluble in physiological conditions has been sought for years, but so far solubility in non-acidic aqueous media has only been achieved at the expense of lowering chitosan molecular weight. In this work, we applied the multistep ultrasound-assisted deacetylation process (USAD process) to β-chitin and obtained extensively deacetylated chitosans with high molecular weights (Mw ≥ 1,000,000 g mol-1). The homogeneous N-acetylation of a chitosan sample resulting from three consecutive USAD procedures allowed us to produce chitosans with a high weight average degree of polymerization (DPw ≈ 6,000) and tunable degrees of acetylation (DA from 5 to 80%). N-acetylation was carried out under mild conditions to minimize depolymerization, while preserving a predominantly random distribution of 2-amino-2-deoxy-D-glucopyanose (GlcN) and 2-acetamido-2-deoxy-D-glucopyanose (GlcNAc) units. This close to random distribution, inferred with deconvolution of nuclear magnetic resonance (1H NMR) spectra, is considered as responsible for the solubility within a wide pH range. Two of the highly N-acetylated chitosans (DA ≈ 60 % and ≈ 70 %) exhibited full water solubility even at neutral pH, which can expand the biomedical applications of chitosans.