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The earliest hard exoskeleton-forming fossils are from the Vendian period (~ 650 – 543 Mya), also known as the Ediacaran period, in the latest Proterozoic Era, and just predating the Cambrian Explosion which is itself marked by a rapid increase in the occurrence of preserved mineralized fossils in the rock record. The earliest mineralized exoskeletons from the Vendian show a preference for calcium phosphate (apatite) mineralization but the dominant biomineral shifts to calcium carbonate in the Cambrain Explosion (Lowenstam and Weiner, 1989). Interestingly, the inarticulate brachiopods, including Lingula, are among the earliest to appear in the Cambrian record and are the only brachiopds to show calcium phosphate (francollite, which is fluorinated hydroxyapatite) mineralization, whereas all other and later brachiopods show calcium carbonate mineralization. Thus, studying the mechanisms of calcium phosphate mineralization may help understand the earliest evolution of exoskeletons and the preservation potential of fossils in the rock record. Vertebrates are characterized by internal skeletons made up of bone, which is a composite material of organic matrix and the calcium phosphate mineral, hydroxyapatite (Ca5(PO₄)₃OH, or dahlite whichis carbonated hydroxyapatite). Bone is well-studied, so we chose to use it as a model system for understanding the mechanisms of organic-mediated Ca-PO₄ mineralization.

Bone sialoproteisn (BSP) is a highly phosphorylated, acidic, noncollagenous protein in bone matrix. Although BSP has been proposed to be a nucleator of hydroxyapatite (Ca₅(PO₄)₃OH), the major mineral component of bone, no detailed mechanism for the nucleation process has been elucidated at the atomic level to date. In the present work, using a peptide model, we apply molecular dynamics (MD) simulations to study the conformational effect of a proposed nucleating motif of BSP (a phosphorylated, acidic, 10 amino-acid residue sequence) on controlling the distributions of Ca²⁺ and inorganic phosphate (Pi) ions in solution, and specifically, we explore whether a nucleating template for orientated hydroxyapatite could be formed in different peptide conformations. Both the α-helical conformation and the random coil structure have been studied, and inorganic solutions without the peptide are simulated as reference. Ca²⁺ distributions around the peptide surface and interactions between Ca²⁺ and Pi in the presence of the peptide are examined in detail. From the MD simulations, in some cases for the α-helical conformation, we observe that a Ca²⁺ equilateral triangle forms around the surface of peptide which matches the distribution of Ca²⁺ ions on the (001) face of the hydroxyapatite crystal (Figure 1 a). However, the template structure does not form consistently for various different starting ion distributions around the α-helix peptide, and in some simulations, even where the template does form, it is unstable over longer simulation periods greater than 5 ns up to 35 ns. The random coil conformation of the peptide helps to stabilize an interconnected Ca-PO₄ network around the peptide which remains interconnected for the duration of the simulations (up to 35 ns) (Figure 2b), and this network more stable that the Ca-PO₄ clusters formed in the inorganic, reference solution which tend to form and fall apart (Figure 2). Therefore, independent of conformations, the BSP nucleating motif is more likely to help nucleate an amorphous calcium phosphate cluster, which ultimately converts to crystalline hydroxyapatite.

These results are consistent with recent experimental evidence for amorphous Ca-PO₄ precursor, which recrystallizes to francollite in Lingula anatina shell, and for amorphous CaCO₃ precursors to calcite and aragonite biominerals. The consistency between our Ca-PO₄ and previous Ca-CO₃ studies indicates that universal principles underly biomineralization processes and the understanding gained from our computational study of peptide-promoted calcium phosphate nucleation can be extended to CaCO₃ and other biominerals. The apparent switch in major biomineral from Ca-PO₄ phases to Ca-CO₃ phases at the end of the Vendian period may be related to the fact that the phosphate phases are more insoluble than the carbonates, so possibly, their formation may require a lower degree of regulation by organisms which were less evolved in the Vendian compared to the more evolved organisms of the Cambrian (Lowenstam and Weiner, 1989).

Future work will focus on the potential role of inorganic and organic ligands in promoting the formation of mixed cation carbonate such as dolomite ((Ca, Mg) CO₃), and (Ca, Mg, Fe)CO₃, which are known to form on Earth surface or near-surface conditions primarily through bacterial mediation.