The role of collagen in bone apatite formation in the presence of hydroxyapatite nucleation inhibitors

Bone is a composite material in which collagen fibrils form a scaffold for a highly organized arrangement of uniaxially oriented apatite crystals. In the periodic 67¿nm cross-striated pattern of the collagen fibril, the less dense 40-nm-long gap zone has been implicated as the place where apatite crystals nucleate from an amorphous phase, and subsequently grow. This process is believed to be directed by highly acidic non-collagenous proteins, however, the role of the collagen matrix during bone apatite mineralization remains unknown. Here, combining nanometre-scale resolution cryogenic transmission electron microscopy and cryogenic electron tomography with molecular modelling, we show that collagen functions in synergy with inhibitors of hydroxyapatite nucleation to actively control mineralization. The positive net charge close to the C-terminal end of the collagen molecules promotes the infiltration of the fibrils with amorphous calcium phosphate (ACP). Furthermore, the clusters of charged amino acids, both in gap and overlap regions, form nucleation sites controlling the conversion of ACP into a parallel array of oriented apatite crystals. We developed a model describing the mechanisms through which the structure, supramolecular assembly and charge distribution of collagen can control mineralization in the presence of inhibitors of hydroxyapatite nucleatio

From Amorphous Aggregates to Crystallites: Modelling Studies of Crystal Growth in Vacuum

The growth of NaCl and CaF2 aggregates is studied with the framework of an atomistic simulation approach. Our method combines a Monte-Carlo type scheme for the identification of ion adsorption sites and structural optimization of the ion cluster after individual growth steps. This allows investigating putative conformers of ion aggregates as a function of size. The structure of clusters counting only a small number of ions was found to be quite amorphous. During the early stage of aggregate growth, the addition of further ions leads to considerable structural changes. Once the clusters exceed a critical size, a disorder[RIGHTWARDS ARROW]order transition is observed, leading to large domains exhibiting motifs of the crystalline structure. Such crystalline regions were found to be stable and hence change only marginally during further aggregate growth