Nanocrystals of apatitic calcium phosphate impart the organic-inorganic nanocomposite in bone with favorable mechanical properties. So far, the factors preventing crystal growth beyond the favorable thickness of ca. 3 nm have not been identified. Here we show that the apatite surfaces are studded with strongly bound citrate molecules, whose signals have been identified unambiguously by multinuclear magnetic resonance (NMR) analysis. NMR reveals that bound citrate accounts for 5.5 wt% of the organic matter in bone and covers apatite at a density of about 1 molecule per (2 nm)2, with its three carboxylate groups at distances of 0.3 to 0.45 nm from the apatite surface. Bound citrate is highly conserved, being found in fish, avian, and mammalian bone, which indicates its critical role in interfering with crystal thickening and stabilizing the apatite nanocrystals in bone.

https://www.pnas.org/content/pnas/107/52/22425.full.pdf

Strongly bound citrate stabilizes the apatite nanocrystals in bone

The relationship between crystal morphology and XRD peak intensity on CaSO4·2H2O

Molecular dynamics simulations of α-alumina and γ-alumina surfaces

A new computer code (MARVIN), employing two-dimensional (2D) periodic boundary conditions, has been developed for the simulation of surfaces and interfaces. The models and methodologies incorporated within the program are discussed. The utility of the program in calculating crystal morphologies is explored using α-Al2O3 and zircon as examples. The important aspects of these calculations are that they include the use of covalent-type force fields in the latter potential model and that the effects of surface relaxation on the growth morphology are calculated for the first time. It is demonstrated that relaxation has a much larger effect on the equilibrium morphology than the growth morphology, but it can still be significant on the latter. A previously derived relationship between the growth and equilibrium morphologies is shown not to hold for relaxed systems. The growth morphologies are found to be in better agreement with experiment than the equilibrium morphologies since the latter overestimates the importance of high-index faces, especially after relaxation. Finally, the calculated surface relaxation for the basal plane of α-Al2O3 is found to be in complete agreement with Hartree–Fock ab initio calculations, verifying that the bulk potentials transfer to this surface.

MARVIN: a new computer code for studying surfaces and interfaces and its application to calculating the crystal morphologies of corundum and zircon

Simultaneous inner- and outer-sphere arsenate adsorption on corundum and hematite

The habit (external morphology) of a crystal is controlled by both the external (environmental) conditions of crystallization and the internal (structural) factors of the crystal. In order to separate the effects of the crystal structure and of the solvent and other external factors on the experimentally observed growth habit, the theoretical habit can be derived from the crystal structure using the periodic bond chain (PBC) theory and attachment energy considerations. According to the PBC theory the crystal habit is governed by a set of uninterrupted chains of strong bonds formed in the crystal lattice. In addition, the attachment energy (E(att)) is defined as the energy released per mole when a new layer is deposited on a crystal face. Since the habit of a crystal is determined by the relative growth rate (R) of the various faces, by taking R proportional to E(att), the theoretical habit can thus be derived from E(att). Using this approach, we obtained the theoretical crystal habit of an antitumor drug, hexamethylmelamine (HMM). The possible effect of solvents on the habit modification of HMM is discussed. This technique, based purely on the knowledge of the crystal structure, is directly applicable to other pharmaceuticals in deriving their theoretical crystal habit.

P. Hartman

Papers

Application of the periodic bond chain (PBC) theory and attachment energy consideration to derive the crystal morphology of hexamethylmelamine.

The effect of surface relaxation on crystal habit: Cases of corundum (α-Al2O3) and Hematite (α-Fe2O3)

Structural morphology of gypsum (CaSO4·2H2O), brushite (CaHPO4·2H2O) and pharmacolite (CaHAsO4·2H2O)

Structural morphology of YBa2Cu3O7-x

The Habit of gypsum and solvent interaction