The majority of studies of fungal utilization of chitosan are associated with the production of a specific enzyme, chitosanase, which catalyzes the hydrolytic cleavage of the macrochain. In our opinion, the development of approaches to obtaining materials with new functional properties based on non-destructive chitosan transformation by living organisms and their enzyme systems is promising. This study was conducted using a wide range of classical and modern methods of microbiology, biochemistry, and physical chemistry. The ability of the ascomycete Fusarium oxysporum Schltdl. to modify films of chitosan with average-viscosity molecular weights of 200, 450, and 530 kDa was discovered. F. oxysporum was shown to use chitosan as the sole source of carbon/energy and actively overgrew films without deformations and signs of integrity loss. Scanning electron microscopy (SEM) recorded an increase in the porosity of film substrates. An analysis of the FTIR spectra revealed the occurrence of oxidation processes and crosslinking of macrochains without breaking β-(1,4)-glycosidic bonds. After F. oxysporum growth, the resistance of the films to mechanical dispersion and the degree of ordering of the polymer structure increased, while their solubility in the acetate buffer with pH 4.4 and sorption capacity for Fe²⁺ and Cu²⁺ decreased. Elemental analysis revealed a decrease in the nitrogen content in chitosan, which may indicate its inclusion into the fungal metabolism. The film transformation was accompanied by the production of extracellular hydrolase (different from chitosanase) and peroxidase, as well as biosurfactants. The results obtained indicate a specific mechanism of aminopolysaccharide transformation by F. oxysporum. Although the biochemical mechanisms of action remain to be analyzed in detail, the results obtained create new ways of using fungi and show the potential for the use of Fusarium and/or its extracellular enzymes for the formation of chitosan-containing materials with the required range of functional properties and qualities for biotechnological applications.

In the traditional method of removing excess acid used to obtain the salt form of chitosan, not only a reaction with neutralizing bases proceeds, but also partial deprotonation of macrochains. Since charged amino groups are responsible for the biological activity of the aminopolysaccharide, it seems important to develop methods for optimizing the acid–base composition of a chitosan-containing system without proton elimination. The paper presents the results of our study on the removal of non-stoichiometric aspartic acid from an aqueous dispersion of chitosan aspartate nanoparticles obtained by in situ self-assembly during counterion association on charged polymer chains and stabilized by a polysiloxane shell coating using isohydric and isothermal crystallization approaches. It is shown that a temperature decrease in the range of 50–4°C in combination with water distillation by 50–85% leads to supersaturation of the nanodispersion and crystallization of the poorly soluble component of the dispersion medium with precipitation. The influence of the acid/polymer molar ratio (1.3–1.7 mol/mol of NH2) in the target substance of the dispersed phase, the cooling rate (10 and 30°C/h) and the dispersion concentration degree (50–85%), as well as the temperature (22±2 and 4°C) and holding time (1–200 days) on the morphostructure, chemical composition, crystalline ordering and quantitative yield of the solid phase has been estimated. The correspondence of the air-dry form of the isolated precipitate to crystals of the zwitterionic form of aspartic acid has been proven by IR spectroscopy, X-ray structural analysis, dialysis, potentiometry and gravimetry. It has been found that the most optimal option for preparative removal of non-stoichiometric double bipolar ions is the concentration of nanodispersion by 85% followed by supercooling from 50 down to 4°C at a rate of 10°C/h. The developed approach to combining isohydric and isothermal crystallization of non-stoichiometric aspartic acid has been successfully tested on moderately concentrated solutions of the complex salt of chitosan hydrochloride–aspartate.