Posner's Cluster Research Articles

Molecular and biochemical approach for understanding the transition of amorphous to crystalline calcium phosphate deposits in human teeth.

Calcium phosphate (CaP) deposition during bone mineralization starts with the aggregation of Posner's clusters Ca9(PO4)6 into amorphous Ca-phosphate (ACP), which then transforms into crystalline CaP and finally maturates to hydroxyapatite (HA). Using dentin/enamel of human teeth as a model system, we show that the physiological inorganic polymer polyphosphate (polyP), a phosphate donor in mineralization, prevents the transition from amorphous to crystalline CaP at concentrations>15wt%. Stabilization of the amorphous phase of CaP by polyP is reversed by hydrolysis of the polymer by alkaline phosphatase (ALP), an enzyme that releases phosphate for mineralization. It is still present in calcified enamel and dentin, as shown here by immunostaining and enzyme activity measurements. The phase transfer into crystalline CaP can be prevented by the ALP inhibitor levamisole. Besides TEM and SEM, the modulating effects of polyP and ALP on the kinetics of the phase transition from amorphous to crystalline CaP are demonstrated and confirmed by XRD and FTIR analyses. Molecular modeling studies show that the polyP chains, due to their dimensions, are able to penetrate into the channels between the Posner molecules, preventing cluster association to ACP and impairing HA crystal formation.

Structures and infrared spectra of calcium phosphate clusters by ab initio methods with implicit solvation models.

Since the first detection of pre-nucleation clusters during the formation of calcium phosphate minerals, determining such clusters' compositions and structures has become crucial for understanding the early-stage nucleation of these minerals in solutions. In previous experimental studies, the composition and sizes of pre-nucleation clusters have been calculated, but their structural information has been difficult to determine because they are very small (<1 nm). In this study, we examined the structures and infrared spectra of small- and medium-sized calcium phosphate clusters using ab initio calculations combined with implicit solvation models. Adding solvent effects increased the possibility of the existence of alternative configurations of calcium phosphate clusters other than their compact configurations. The calcium atoms had a tendency to be located outside of the clusters to coordinate with water molecules in the aqueous environment. The computed infrared spectra of extended small calcium phosphate clusters captured some of the features measured in the in situ infrared spectra, which supports the network structures proposed by large-scale molecular dynamics studies and X-ray adsorption near-edge spectra. The relative stabilities of medium-sized Ca9(PO4)6 clusters with respect to the stability of Posner's cluster in water were also reviewed. We found that in water, alternative structures with low symmetry or large dipole moments had lower energies than Posner's cluster.

Detection of Posner's clusters during calcium phosphate nucleation: a molecular dynamics study.

Hydroxyapatite (HA), the main mineral phase of mammalian tooth enamel and bone, originates in body fluids from amorphous calcium phosphate (ACP). ACP presents short-range order in the form of small domains with size of 0.9 nm and chemical formula Ca9(PO4)6, known as Posner's clusters. In this study, the aggregation and clustering of calcium and phosphate ions in water has been investigated by means of shell-model molecular dynamics simulations. Calcium phosphate aggregates form in solution with compositions and Ca coordination that are similar to those found in Posner's cluster, but the stoichiometry of these species is dependent on the ionic composition of the solution: calcium-deficient clusters in solutions with low Ca : P ratio; cluster containing protonated phosphate groups in neutral solutions; sodium ions partially substituting calcium in solutions containing a mixture of sodium and calcium ions. These Posner-like clusters can be connected by phosphate groups, which act as a bridge between their central calcium ions. The simulations of the aggregation in solution of calcium phosphate clusters are an unbiased and unequivocal validation of Posner's model, and reveal for the first time the structure and composition of the species that form during the early stages of ACP nucleation at a scale still inaccessible to experiment.

Mechanism of Randall’s Plugs Development

Mechanism of formation and development of intraluminal concretion, also called Randall's plug, extracted from a female patient forming calcium oxalate dihydrate (COD) calculi was examined. Some of these calculi were connected to the papillary tip, and had connections with the interior of the papilla with finger-like extensions in the collecting duct (CD). The intraluminal concretion consisted of inter-grown COD crystals of irregular size (30–100 μm), approximately 5% of biological hydroxyapatite (BHAP) and an organic matter. Urine of the patient was moderately supersaturated with respect to COD and amorphous calcium phosphate (ACP). Model of kidney, recently refined by Robertson, was used in calculations. Calculated Reynolds number indicated that the flow of liquid through tubules was purely laminar with parabolic velocity profile. COD crystals formed at the beginning of ascending loop of Henle by heterogeneous nucleation. Concentration of COD crystals in urine was limited and considered equal to concentration of crystals during crystaluria. The free particle and the fixed particle mechanisms were considered. The free particle mechanism assumes formation of a single crystal or agglomerate of crystals blocking the CD by virtue of size. The growth of COD crystals at concrete urinary supersaturation was too slow for a single crystal to attain size with settling velocity faster than the translation flow rate of liquid. Hydrodynamic shear caused aggregation of COD solid particles dispersed in a liquid flowing in the nephron. Number of COD crystals present in urine was not sufficient for formation of fractal agglomerate blocking the Bellini duct. Similarly, a fractal agglomerate of urinary phosphate present in the form of Posner's clusters was not large enough to obstruct the Bellini duct. The opening of the CD could not be obstructed by a single crystal of COD or fractal agglomerate composed of either COD crystals or calcium phosphate clusters, formed in urine by virtue of size. Solid objects not immobilised inside the CD were always washed out by urine flow from the CD of any orientation (also upward-draining CD). The formation and development of plug of our patient was explained by the fixed particle mechanism assuming that Randall's plug developes from crystal(s) attached directly to the tubule wall. The plug was modelled as concretion composed of successive layers of COD crystals originating on the top of underlying layer. When growth of a layer stopped, its surface was covered by organic matter that served as a substrate for nucleation of a new layer. The time of plug development was estimated as the time a COD crystal needed to reach the opposite side of the duct plus duration of interruptions of crystalline growth when plug surface was covered by a layer of organic matter and phosphatic particles were incorporated into concretion. The flux of Posner's clusters arriving to the concretion surface was estimated from theory of Brownian motion. These calculations suggested that obstruction of the Bellini duct of our patient by the Randall's plug occurred over a period of approximately 4 months after nucleus of concretion became attached to the duct wall or on the papillary tip.

Assembly of Amorphous Clusters under Floating Monolayers: A Comparison ofin Situandex SituTechniques

We report synchrotron X-ray scattering studies of biomimetic crystallization of hydroxyapatite (the primary constituent of bone), using monolayers of fatty acid molecules floating on simulated body fluid (SBF) as well as aqueous solutions of calcium phosphate. A ∼10 Å thick film of amorphous material is observed to form immediately at the molecular monolayer, consistent with the proposed formation of "Posner clusters". This layer becomes denser but not significantly thicker as the subphase concentration and the temperature approach physiological conditions. The amorphous films do not crystallize within 24 h, in contrast to prior reports of more rapid crystallization using electron microscopy on ex situ samples. However, crystallization occurs almost immediately after our films are transferred onto solid substrates. These results illustrate the importance of in situ measurements for model biomineralization experiments.

Structure and formation mechanism of calcium phosphate concretions formed in simulated body fluid

Synthetic calcium phosphate (CaP) concretions in the form of hemispheres formed under carefully controlled conditions (pH 7.4, 37 °C, no evaporation) from stagnant simulated body fluid on anionic polymeric substrate (laminin) were observed by scanning and transmission electron microscopy and atomic force microscopy. The hemispheres with diameter between approximately 50 and 200 μm were composed of closely connected round spherical and elliptical objects of diameter varying from 70 to 120 nm with surface layer composed of tightly packed spherical objects of diameter 25-30 nm. The phase composition of concretions consisting of amorphous material was uniform. The concretions were formed by aggregation of CaP clusters (Posner's clusters Ca9(PO4)3 or [Ca3(PO4)2] n ) generated in the solution by perikinetic coagulation, their settling onto a substrate and subsequent accumulation through surface migration (surface nucleation) followed by accretion of nanoparticles arriving from surrounding solution. The similarity of ultrafine structures of these synthetic CaP concretions and the compact phosphatic phase appearing in some phosphate calculi indicates that analogous mechanisms could be active at the formation of the latter in the kidneys.

Amorphous Calcium Orthophosphates: Nature, Chemistry and Biomedical Applications

Amorphous calcium orthophosphates (ACPs) represent a unique class of biomedically relevant calcium or- thophosphate salts, having variable chemical but rather identical glass-like physical properties, in which there are neither translational nor orientational long-range orders of the atomic positions. Normally, ACPs are the first solid phases, precipi- tated after a rapid mixing of aqueous solutions containing ions of Ca 2+ and PO4 3- ; however, other production techniques are known. Interestingly, but ACPs prepared by wet-chemical techniques were found to possess a relative constancy in their chemical composition over a relatively wide range of preparation conditions, which suggested the presence of a well-defined local structural unit, presumably, with the structure of Ca9(PO4)6 - so-called a Posner's cluster. However, the presence of similar clusters in ACPs produced by other techniques remains uncertain. All ACPs are thermodynamically unstable com- pounds and, unless stored in dry conditions or doped by stabilizers, spontaneously they tend to transform to crystalline cal- cium orthophosphates, mainly to calcium apatites. This solution instability of ACPs and their easy transformation to crys- talline phases are of a great biological relevance. Namely, the initiating role ACPs play in matrix vesicle biomineralization raises the importance of ACPs from a mere laboratory curiosity to that of a reasonable key intermediate in skeletal calcifi- cation. In addition, due to great chemical and structural similarities to the calcified tissues of mammals, as well as excellent biocompatibility and bioresorbability, all types of ACPs are very promising candidates to manufacture artificial bone grafts. In this review, current knowledge on occurrence, preparation, composition, structure, major properties and biomedical ap- plications of ACPs have been summarized. To assist the readers in looking for the specific details on ACPs, a great number of references have been collected and systematized.

Ultrafine Structure of the Hydroxyapatite Amorphous Phase in Noninfectious Phosphate Renal Calculi

We aimed to establish detailed morphology of the structureless amorphous hydroxyapatite (HAP) phase to improve our understanding of the formation mechanism of these concretions. Noninfectious phosphate renal calculi composed mainly of HAP consist of inorganic material in the form of spherules, in a seemingly structureless and amorphous phase and organic matter. Several cross-sections of a fraction of phosphate renal stone composed solely of the amorphous HAP phase were examined with atomic force microscope. Both 2- and 3-dimensional images of their structure and nanoscale elastic modulus maps were obtained. The amorphous hap phase consists of 2 distinctly different morphologic forms of hydroxyapatite: separate and/or intergrown columnar crystals, and spherical agglomerates with diameters in the range 150-300 nm consisting of spherulites approximately 10 nm in diameter. The columnar crystals are irregularly disseminated in the stone interior, which is porous because of cavities with depths in excess of 100 nm. Organic matter is almost evenly distributed throughout the stone interior. Based on the observed calculus structure, the following mechanism of formation of the noninfectious phosphate calculi is suggested: Spherulites formed via the perikinetic aggregation of Posner's clusters present in urine supersaturated with respect to hydroxyapatite aggregate into spherical agglomerates that, after reaching a certain size, are retained in cavities with poor urodynamics, gradually settle, and become incorporated into developing concretion. The columnar crystals are probably nucleated on the detritus of organic origin embedded in the hydroxyapatite structureless phase.

Posner's cluster revisited: direct imaging of nucleation and growth of nanoscale calcium phosphate clusters at the calcite-water interface

Although many in vitro studies have looked at calcium phosphate (Ca–P) mineralization, they have not emphasized the earliest events and the pathway of crystallization from solvated ions to the final apatitic mineral phase. Only recently has it become possible to unravel experimentally the processes of Ca–P formation through a cluster-growth model. Here we use mineral replacement reactions by the interaction of phosphate-bearing solutions with calcite surfaces in a fluid cell of an atomic force microscope (AFM) and reveal that the mineral surface-induced formation of an apatitic phase proceeds through the nucleation and aggregation of nanosized clusters with dimensions similar to those of Posner's clusters, which subsequently form stable amorphous calcium phosphate (ACP) plates prior to the transformation to the final crystalline phase. Our direct AFM observations provide evidence for the existence of stable Posner's clusters even though no organic template is applied.

A hierarchical structure for apatite crystals

Based on the experimental results taken from the references, a reasonable hierarchical structure of apatite (both fluorapatite and hydroxyapatite) crystals has been proposed for the first time. The structure consists of four levels of the hierarchy: the smallest level is made of single unit-cells and/or Posner's clusters with the linear dimensions slightly below 1 nm, the second level comprises X-ray coherent scattering blocks of 50-80 nm in size, the third level is represented by dislocation blocks of 0.3-2.0 microm in size and, finally, there are macroblocks of 35-50 microm in size.

Calcium phosphate clusters

The potential energy surfaces associated with [Ca 3(PO 4) 2] n clusters are analyzed in detail using ab initio calculations for n ranging from one to four. Considering separated clusters, energy criteria favor the so-called Posner's cluster Ca 9(PO 4) 6, which is the core of the actual structural model of amorphous calcium phosphate. This is rationalized through the existence of a distinct CaO bonding pattern in this cluster. Considering aggregated clusters as a possible model for amorphous calcium phosphate, the aggregation of Ca 3(PO 4) 2 clusters appears as an alternative to Posner's hypothesis.

Symmetry of Posner's Cluster

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationNEXTSymmetry of Posner's ClusterGabin Treboux, Pierre Layrolle, Noriko Kanzaki, Kazuo Onuma, and Atsuo ItoView Author Information National Institute for Advanced Interdisciplinary Research 1-1-4 Higashi, Tsukuba-shi, Ibaraki 305-8562, Japan Cite this: J. Am. Chem. Soc. 2000, 122, 34, 8323–8324Publication Date (Web):August 10, 2000Publication History Received7 December 1999Revised19 June 2000Published online10 August 2000Published inissue 1 August 2000https://doi.org/10.1021/ja994286nCopyright © 2000 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views780Altmetric-Citations45LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit Read OnlinePDF (56 KB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information SUBJECTS:Cluster chemistry,Cluster structure,Group theory,Mathematical methods,Molecular structure Get e-Alerts

Existence of Posner's Cluster in Vacuum

Using ab initio methods, a single isomer of D3h symmetry was found to be preeminent on the Ca3(PO4)2 cluster potential energy surface. This isomer has the distinct feature that each calcium atom is surrounded by four oxygen atoms. We studied the aggregation of this isomer in the [Ca3(PO4)2]n systemwith n being 2 or 3. This gives us an energy reference for analyzing the stability of the so-called Posner's cluster Ca9(PO4)6 which is the core of the actual structural model of amorphous calcium phosphate. Posner's cluster is shown to be significantly stabilized in comparison to the energy reference.