# Effect of Nanohydroxyapatite on Silk Fibroin-Chitosan Interactions--Molecular Dynamics Study
## 1 Introduction
Its versatility and potential or actual applications have been widely demonstrated in numerous fields, including agriculture [37; 38], pharmacy and medicine [30; 36; 37; 39; 40; 41; 42; 43], food industry [36; 37; 44], textile industry [36; 37; 45; 46], biotechnology [47], cosmetics [37; 48; 49; 50; 51], paper production [37; 52] and various green and sustainable technologies [53; 54; 55]. Chitosan emerges as a highly desirable substance, owing to its non-toxicity, cationic nature, biocompatibility, and antibacterial properties, alongside its unique chemical composition [35]. In general, composites stabilized by strong intermolecular interactions exhibit superior mechanical durability. For instance, there are documented correlations between the denaturation temperature characterizing the thermal stability of protein-containing biomaterials and their mechanical and rheological properties [56; 57; 58]. Importantly, the direct evidence of the relationships between the strength and mechanical properties of intermolecular interactions was demonstrated by studies that explored experimental analysis with molecular dynamics computations [59; 60]. Our previous research employed molecular dynamics simulations to explore complexes of chitosan/chitin with collagen [61; 62; 63] and hyaluron [64]. Understanding these interactions is crucial for developing strategies for cartilage regeneration [65]. Likewise, chitosan-fibroin composites are relevant from biomedical and pharmaceutical perspectives [34; 66; 67; 68; 69; 70]. Notably, chitosan/fibroin composites containing hydroxyapatite exhibit exceptional tissue regeneration potential due to their unique properties [66; 68]. These composites hold promise for various applications. For instance, they could serve as durable, natural fillings and dental implant scaffolds. Notably, all the named substances--chitosan, fibroin, and hydroxyapatite--are either applied or considered as promising materials in dentistry thanks to their biocompatibility and mechanical properties [71; 72; 73; 74; 75]. Their degradation products are non-toxic, and the materials can be engineered to resist wear and tear depending on the number of intermolecular contacts stabilizing the molecular assemblies. Additionally, these composites can be used for bone reconstruction. The intermolecular contacts between silk fibroin and chitosan seems to be a key factor influencing the mechanical behavior of load-bearing applications [34; 76]. A thorough understanding of these interactions will allow for the design of materials with controlled degradation rates. This could be crucial for creating scaffolds that degrade at a pace matching new bone tissue regeneration, thereby preventing the formation of pseudoarthrosis (a common complication in long bone defects characterized by improper mineralization) [77]. This study focuses on unraveling the intermolecular interactions between fibroin and chitosan in the presence of HPO42- and Ca2+ ions using molecular dynamics simulations. The main goal is to elucidate the factors that contribute to the remarkable stability of these composites.