# Effect of Nanohydroxyapatite on Silk Fibroin-Chitosan Interactions--Molecular Dynamics Study
## 1 Introduction
A closer insight into the less-ordered regions aligned with the fiber axis reveals the presence of the 3\({}_{1}\)-helix motif (which grants elasticity to the fibers) and a greater abundance of \(\beta\)-bends/turns and potentially \(\beta\)-spirals. These \(\beta\)-structures, prevalent in spider silk, might be responsible for the rapid shrinking of the thread in water (super contraction effect) [24; 25; 26]. Silk fibroin consists of two main chains: a heavier H-chain (around 300 kDa in _B. mori_) with a hydrophobic nature and a lighter L-chain (25 kDa) with a hydrophilic nature. These chains are linked by a disulfide bridge. The structure also includes a chaperone protein, P25, which is a fibrohexamer glycoprotein. The structure of the silk thread is supported by strong hydrogen bonds, especially in the crystalline areas, and by Van der Waals interactions. According to Kaplan models, the primary structure's chain is predominantly hydrophobic, interspersed with small repetitive sections of a hydrophilic nature [27]. These alternating hydrophilic and hydrophobic interactions lead to the formation of micelles, where water is bound within the hydrophilic domains (approximately 8%). These micelles, measuring up to 200 nm in size, ultimately form fibroin globulins. When subjected to various physical or chemical stimuli, such as changes in pH, temperature fluctuations, variations in metal ion concentration, increased fibroin concentration, ultrasound exposure, or mechanical shearing, disordered and \(\beta\)-helical regions transform into ordered \(\beta\)-sheet structures. Concentrated globulins transition into a liquid crystal phase, with particles aligning parallel to one another as they flow, solidifying upon exposure to air to form a thread. Due to its hydrophobic nature, and the presence of inter- and intramolecular strong hydrogen bonds in many crystalline regions, natural silk fibers dissolve only in a few solvents, including concentrated acids: H\({}_{3}\)PO\({}_{4}\), HCOOH, H\({}_{3}\)SO\({}_{4}\), and HCl and aqueous solutions of concentrated salts: LiSCN, LiBr, CaCl\({}_{2}\), Ca(SCN)\({}_{2}\), ZnCl\({}_{2}\), NH\({}_{4}\)SCN, CuSO4, Ca(NO\({}_{3}\))\({}_{2}\)[17]. Despite the solubility limitation, it is possible to obtain many different forms of silk material, e.g., films, foams, hydrogels, powders, nanofibers, and microcapsules [18]. Silk is used not only in the clothing industry for producing fabrics, anti-allergic bedding, bulletproof vests, ropes, and parachutes, but also in medical applications. Silk fibers (catgut) have been used as surgical threads for years. There is a lot of interest in an aqueous solution of silk fibroin, which, thanks to its biocompatibility, biodegradability, and similarity to collagen and elastin, is successfully used in tissue engineering. Silk fibroin is a promising scaffold material for cell growth and regeneration in various damaged tissues, including connective, nervous, cartilage, and bone tissues [7]. Furthermore, it has a wide range of other biomedical and pharmaceutical applications, including drug carriers [28; 29; 30] and hydrogels for transparent wound dressings [31; 32]. Notably, additional beneficial properties, such as antibacterial properties, increased strength, resistance to degradation, viscosity suitable for bioprinting, and others, can be achieved by adding other polymers, such as chitosan, hyaluronic acid, cellulose, sodium alginate, starch, carrageenan, collagen, gelatin, PVA, PVP, and poly(lactic acid) [33; 34]. Among polysaccharide analogues, chitosan demonstrates exceptional biocompatibility and biodegradability [35; 36]. It is derived from chitin, an abundant and renewable resource, and is widely acknowledged for its safety profile [36].