The nano- and meso-scale structure of amorphous calcium carbonate

Abstract

Understanding the underlying processes of biomineralization is crucial to a range of disciplines allowing us to quantify the effects of climate change on marine organisms, decipher the details of paleoclimate records and advance the development of biomimetic materials. Many biological minerals form via intermediate amorphous phases, which are hard to characterize due to their transient nature and a lack of long-range order. Here, using Monte Carlo simulations constrained by X-ray and neutron scattering data together with model building, we demonstrate a method for determining the structure of these intermediates with a study of amorphous calcium carbonate (ACC) which is a precursor in the bio-formation of crystalline calcium carbonates. We find that ACC consists of highly ordered anhydrous nano-domains of approx. 2 nm that can be described as nanocrystalline. These nano-domains are held together by an interstitial net-like matrix of water molecules which generate, on the mesoscale, a heterogeneous and gel-like structure of ACC. We probed the structural stability and dynamics of our model on the nanosecond timescale by molecular dynamics simulations. These simulations revealed a gel-like and glassy nature of ACC due to the water molecules and carbonate ions in the interstitial matrix featuring pronounced orientational and translational flexibility. This allows for viscous mobility with diffusion constants four to five orders of magnitude lower than those observed in solutions. Small and ultra-small angle neutron scattering indicates a hierarchically-ordered organization of ACC across length scales that allow us, based on our nano-domain model, to build a comprehensive picture of ACC formation by cluster assembly from solution. This contribution provides a new atomic-scale understanding of ACC and provides a framework for the general exploration of biomineralization and biomimetic processes.

Pyrophosphate-stabilised amorphous calcium carbonate for bone substitution: toward a doping-dependent cluster-based model

“…Indeed, we did not detect any evidence of the presence of Ca–Py entities (clusters) at a nanoscale. A recent study of Clark et al 86 revealed the same kind of model for undoped ACC based on neutrons and X-ray scattering combined to reverse Monte Carlo modelling, with 55 nm-ACC particles containing crystalline anhydrous nanodomains of 2 nm whose cohesion relied on interstitial strongly bound water molecules. Moreover, molecular dynamics simulation assessed the stability of such organisation, in particular when in contact with a water interface.…”

Structural water in amorphous carbonate minerals: ab initio molecular dynamics simulations of X-ray pair distribution experiments

“…While it is now generally accepted that amorphous carbonates can be formed with a wide range of hydration levels, the nature of the hydrating water molecules is not well understood, particularly the role of hydrogen bonding in stabilizing amorphous structures. Several previous works have presented atomistic models of hydrated ACC 11,12,16,17,59,62,67,76,77 and AMC. 66,70 We are not aware of any similar reports for ASC.…”

“…The nanopores were not found in subsequent CMD 12 and EPSR 62 models, suggesting that homogenous disordered structures could account for all data, although recent work using EPSR and CMD has shown evidence for some degree of nanoporosity. 76,77 To our knowledge, whether ACC (or other amorphous carbonates) feature connected channels is still an open question. But, like many other works, 55,79 we find that open channels do not need to be invoked to reproduce PDF data.…”

Acceleration of Wound Healing through Amorphous Calcium Carbonate, Stabilized with High-Energy Polyphosphate

“…In human bone formation, ACC can also be converted to amorphous calcium phosphate, depending on the availability of exogenous phosphate [ 27 ]. Based on model simulations, it was recently proposed that ACC consists of a hierarchically organized gel-like structure of anhydrous CaCO 3 nanodomains embedded in a matrix of network-like arranged water molecules [ 63 ]. This could explain the stabilization of ACC by polyelectrolytes, both protein and polyP as shown here, which could enter the aqueous channels of this structure.…”