Mechanics of molecular collagen is influenced by hydroxyapatite in natural bone

As often seen in biological structural materials, bone exhibits complex hierarchical structure. The primary constituents of bone are collagen and hydroxyapatite (HAP). HAP mineralizes at specific locations at collagen, in such a way that the c-axis of HAP aligns parallel to collagen molecule. The collagen molecule is helical overall with non-helical ends that are N- or C-telopeptides. The collagen molecule with telopeptides interacts with specific surfaces of mineralized HAP. When subjected to load, the interactions at the interface between HAP and collagen may significantly affect the overall mechanics of the collagen molecule. Here, we have performed molecular dynamics (MD) and steered MD (SMD) simulations in order to understand the load carrying behavior of collagen in the proximity of HAP. Our simulations indicate that the load-deformation response of collagen is different when it interacts with HAP as compared to its response in the absence of HAP. The interface between HAP and collagen affects the overall load-deformation response of collagen. Further, bone also has considerable amount of water and we have observed that water significantly influences the loaddeformation response of collagen due to collagen-water- HAP interactions.