PH dependent interfacial adhesion between collagen and hydroxyapatite in the mineralized collagen fibril

The objective of this current study is to understand the molecular level mechanism of pH-dependent interface interaction process between collagen molecules (TC) and hydroxyapatite (HAP) crystals in mineralized collagen fibril. It is known that TC and HAP crystals are the two major nano-structural components of mineralized collagen fibrils. These fibrils are the subnanostructural members of the hierarchical organization of trabecular bone. Therefore, hen a trabecular bone breaks, it can be hypothesized that the fracture process initiates from the mineralized fibril level, more specifically from the TC-HAP interface. Studies suggest that bone fracture is a major concern for patients suffering from a disease called osteoporosis. From physiological point of view, osteoporosis occurs when bone-killing cell 'osteoclasts' become more active than bone-forming cell osteoblasts. In order to minimize bone decay, it is imperative that the resorption rate of bone be controlled or slowed down. Studies suggest that bone resorption process begins when potential bone resorption zone is interacted with acidic environment and hydrolytic enzymes. It could be hypothesized that bone resorption rate could be altered if the acidic environment near bone is altered. Since changing acidity is directly related to pH of the environment, this study uncovers the role pH plays on the TC-HAP interface adhesion using a novel constant pH molecular dynamics simulations method. Our results suggest that the interfacial adhesion between TC and HAP can be drastically reduced if the pH of the buffer environment they are submerged are changed from 12 to 2. A pH dependent change in the electrostatic surface potentials of bone mineral crystals and collagen molecules are responsible for such interface energy variation.