Hydroxyapatite (HAP) is one of the most important inorganic components in biological minerals such as bones and teeth. More than 90% of the total citrate is accumulated in human bones and other biomineralized tissues. In addition, mineralizing proteins are enriched in glutamate and aspartate residues, which are important for their mineral-regulating properties. However, how citrate ions (CITs) and/or acidic amino acids regulate the formation of HAP is still unclear. In this work, molecular dynamics simulations were performed to study how CIT regulates the adsorption behavior of polyaspartic acid (PASP) on the HAP surface in the calcium phosphate solution. The simulation results indicate that PASP can be used as an ion chelator to complex Ca2+ and can serve as templates for HAP mineralization by templating the distribution of Ca2+ on its surface, which are attributed to the −COO– and α-helix structure. Most importantly, the orientation distributions of PASP in all systems are narrower with the help of CIT, thereby PASP can be adsorbed on the HAP surface stably with a “lying-down” orientation. This indicates that CIT can be used as a bridging agent to bond the acidic peptide to the HAP surface in biomineralization. Thus, the synergic role of CIT and the acidic peptide on the HAP surface were revealed in this work, which can provide new insights into the interfacial phenomena during the biomineralization.

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Because of the high prevalence and ability to rapidly break the enamel, [6] it is important to develop the effective treatment and prevention methods. Unfortunately, up to now, there is no mature standard method for the treatment of enamel erosion. Although the use of fluorinated dental products can inhibit the occurrence of enamel erosion to a certain extent, [7] yet it does not solve the problem from the root.There are several natural protective mechanisms for tooth demineralization in human saliva and the formation of acquired pellicle (AP) is one of them. AP is a membrane formed onto the surface of teeth in saliva, including some lipids and glycoproteins. [8] As a natural barrier, AP can prevent the direct contact between enamel and food acid. Under acidic condition, the diffusion rate of phosphate and calcium ions (Ca 2+ ) into surrounding saliva decreases significantly, thus preventing enamel demineralization. [9] Statherin (SN) is an important protein for AP, which can selectively adsorb on enamel surface. [10] It is composed of 43 amino acids and has a high degree of structural and charge asymmetry. SN can effectively inhibit the excessive precipitation of calcium phosphate and maintain saliva as a supersaturated solution of Ca 2+ and phosphorus ions. It can also promote enamel remineralization when enamel is demineralized. [11] Early results showed that the high negatively charged density at N-terminus of SN and the α-helix structure composed of the first 15 residues were the key factors for its adsorption on the HAP surface. [11a] In addition, its N-terminus pre-hexapeptide D-pS-pS-EEK (pS denotes the phosphoserine) could also form α-helix structure and adsorb on HAP, and its variant DDDEEK also had similar adsorption capacity. [12] However, a molecular understanding of adsorption process of SN on HAP surface is lacking. Currently, some researchers have attempted to utilize the molecular-level simulation method to study the adsorption mechanism of protein on HAP surface. Hoff et al. [13] used molecular dynamics (MD) method to quantify the binding free energies of 20 natural amino acids and citrate ions on HAP surface, and found that citrate has the highest affinity to HAP, followed by charged residues, while amino acids without ionic side groups exhibit weak binding to HAP. Basu et al. [14] investigated the influence of electrical stimuli on fibronectin (FN) adsorption on HAP surface using the MD simulation method, and found that the charged residues contributed majorly to FN adsorption, and the adsorption was significantly affected by electric field strength. Moreover, Statherin (SN) is an important protein that can adsorb on enamel to maintain calcium ions balance and promote biomineralization in the human oral cavity. In this work, parallel tempering Monte Carlo simulation and all-atom molecular dynamics simulation are combined to study the behavior of SN adsorbed on the hydroxyapatite (HAP) (001) surface. The results indicate that SN can "anchor" on HAP (001) surface by its Asp1 and Sep2 with "head...

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