On the Complexation of Bombyx mori Protein with Chitosan and Their Thermal Properties

Abstract

The experimental results confirmed the possibility of forming complexes between Bombyx mori protein and chitosan. It was found that changes in the pH of the medium significantly influence the complexation process. In particular, in 2% acetic acid solution, the formation of protein–chitosan complexes was most intensive in the pH range of 4.8–6.7, as observed by the turbidimetric method. This phenomenon is associated with the charge state of macromolecules and can be explained by electrostatic charge compensation and increased intermolecular association between protein and chitosan molecules within this pH range. Infrared spectroscopic analysis revealed shifts in the absorption bands at 1641, 1538, and 1068 cm^–1. These changes indicate the formation of complex structures through hydrogen bonding and electrostatic interactions involving amide, amino, and hydroxyl groups. In particular, the shift at 1641 cm^–1 is associated with conformational rearrangement of amide I vibrations, indicating local changes in the secondary structure of the protein. Thermogravimetric analysis made it possible to evaluate changes in the thermo-oxidative degradation behavior resulting from complex formation. Comparative analysis showed that the studied biopolymers exhibit general degradation patterns along with sample-specific characteristics during thermal oxidation. Thermograms demonstrated that the protein exhibits higher thermal stability compared to chitosan. In addition, a decrease in medium acidity led to a reduction in the thermal stability of protein–chitosan complexes.