Adsorption study of lac dyes with chitosan coated on silk fibroin using molecular dynamics simulations

Silk is a protein polymer composed of the polypeptide chains including the repeating units of glycine and alanine. Lac dye is composed mainly of two major anthraquinone based components: laccaic acids A and B. Previously, chitosan was reported and used to coat silk in the lac dyeing process for enhancing the uptake of lac dye on silk. Therefore, this work aims to explain why chitosan can help in lac dyeing process on silk by using molecular docking and molecular dynamics (MD) simulations. The molecular mechanics Poisson-Boltzmann surface area (MM-PBSA) analysis was also applied to calculate the binding free energy. The results revealed the attractive interaction between silk and chitosan. Moreover, an increasing unit of acetylglucosamine in the chitosan structure increases the binding interaction between silk and chitosan. After that, the binding between silk complexed with chitosan and laccaic acids A and B were reported. It was clearly found that the average of binding free energies between lac dyes and silk complexed with chitosan (-257.7 and -230.9 kJ/mol) was lower than those between lac dyes and silk without chitosan (-13.4 and -108.5 kJ/mol) indicating the better binding of lac dyes when chitosan is added on the silk surface. The obtained results can be explained why the existence of chitosan on silk surface could increase the binding of lac dyes. Therefore, this study can be used as a guideline in order to understand and improve the fastness properties of textiles.