Protein-free formation of bone-like apatite: New insights into the key role of carbonation

Abstract

The nanometer-sized plate-like morphology of bone mineral is necessary for proper bone mechanics and physiology. However, mechanisms regulating the morphology of these mineral nanocrystals remain unclear. The dominant hypothesis attributes the size and shape regulation to organic-mineral interactions. Here, we present data supporting the hypothesis that physicochemical effects of carbonate integration within the apatite lattice control the morphology, size, and mechanics of bioapatite mineral crystals. Carbonated apatites synthesized in the absence of organic molecules presented plate-like morphologies and nanoscale crystallite dimensions. Experimentally-determined crystallite size, lattice spacing, solubility and atomic order were modified by carbonate concentration. Molecular dynamics (MD) simulations and density functional theory (DFT) calculations predicted changes in surface energy and elastic moduli with carbonate concentration. Combining these results with a scaling law predicted the experimentally observed scaling of size and energetics with carbonate concentration. The experiments and models describe a clear mechanism by which crystal dimensions are controlled by carbonate substitution. Furthermore, the results demonstrate that carbonate substitution is sufficient to drive the formation of bone-like crystallites. This new understanding points to pathways for biomimetic synthesis of novel, nanostructured biomaterials.

Carbonate substitution in the mineral component of bone: Discriminating the structural changes, simultaneously imposed by carbonate in A and B sites of apatite

“…This trend is consistent with a smaller trigonal planar carbonate substituting for a larger tetrahedral phosphate [45, 60]. The contraction in the a -axis length also correlates with decreasing A-site substitution (Table 3).…”

“…Independent studies of Vignoles and Baxter et al [43, 44] on synthetic B-type carbonated apatites and bone, suggest that in B- CAp the ν 3 antisymmetric stretching vibration splits into ν 3a and ν 3b peaks at 1423 cm −1 and 1456 cm −1 , respectively (Figure 2a). An additional broad shoulder at ~1480 cm −1 can be attributed to the presence of surface labile carbonate [45]. HAp (Figure 2c) shows traces of ν 3a B-type carbonate incorporated as evidenced from a very weak peak at 1410 cm −1 .…”

“…The best peak fit (standard error SE = (0.0004) was obtained by deconvoluting into three totally Gaussian bands (Figure 4 – I). The peak at 866 cm −1 (labeled 3 in Fig, 4I) has been attributed to either A- or B – type carbonate with slightly different orientation, denoted as A2 and B2 by Fleet [45, 49]. This assignment varies depending on the presence of other substituents in the apatitic structure as well as the heating and annealing conditions applied.…”

Carbonate Apatite Precipitation from Synthetic Municipal Wastewater

“…It is possible that the carbonate ions have impacted the restructuring of the initial, spherical amorphous precursor solids as they transform to a poorly crystalline carbonate apatite, without significantly changing morphology. Carbonate apatite precipitated at 60-85 • C temperatures exhibited plate morphology with a c-axis length of 291.8 ± 222.0 at lower carbonate weight percent (3.63 wt %), and a reduced c-axis length of 36.0 ± 28.4 nm for 17.8 weight percent carbonate, as measured by TEM [34]. This result also indicates a carbonate effect on apatite crystal habit, although the carbonate apatite precipitation conditions for this work were different than presented here.…”

“…Inorganic carbon coulometry confirmed the carbonate content of the samples with 5 and 7 mM initial [Pi] to be within reported carbonate bounds of PR. These samples would have had a higher dissolved carbonate to Pi ratio, and this could have contributed to their higher precipitate carbonate content [34].…”