We have used the pH-induced self-assembly of a peptide-amphiphile to make a nanostructured fibrous scaffold reminiscent of extracellular matrix. The design of this peptide-amphiphile allows the nanofibers to be reversibly cross-linked to enhance or decrease their structural integrity. After cross-linking, the fibers are able to direct mineralization of hydroxyapatite to form a composite material in which the crystallographic c axes of hydroxyapatite are aligned with the long axes of the fibers. This alignment is the same as that observed between collagen fibrils and hydroxyapatite crystals in bone.

http://science.sciencemag.org/content/sci/294/5547/1684.full.pdf

Self-assembly and mineralization of peptide-amphiphile nanofibers

Bone is a hierarchically structured composite material which, in addition to its obvious biological value, has been well studied by the materials engineering community because of its unique structure and mechanical properties. This article will review the existing bone literature, with emphasis on the prevailing theories regarding bone formation and structure, which lay the groundwork for proposing a new model to explain how intrafibrillar mineralization of collagen can be achieved during bone formation. Intrafibrillar refers to the fact that growth of the mineral phase is somehow directed by the collagen matrix, which leads to a nanostructured architecture consisting of uniaxially oriented nanocrystals of hydroxyapatite embedded within and roughly [0 0 1] aligned parallel to the long collagen fibril axes. Secondary (osteonal) bone, the focus of this review, is a laminated organic–inorganic composite composed primarily of collagen, hydroxyapatite, and water; but minor constituents, such as non-collagenous proteins (NCPs), are also present and are thought to play an important role in bone formation. To date, there has been no clear understanding of the role of these NCPs, although it has been generally assumed that the NCPs regulate solution crystal growth via some type of ‘epitaxial’ relationship between specific crystallographic faces and specific protein conformers. Indeed, ‘epitaxial’ relationships have been calculated; but in practice, it has not been demonstrated that intrafibrillar mineralization can be accomplished via this route. Because of the difficulty in examining biomineralization processes in vivo , the authors of this article have turned to using in vitro model systems to investigate the possible physicochemical mechanisms that may be involved in biomineralization. In the case of bone biomineral, we have now been able to duplicate the most fundamental level of bone structure, the interpenetrating nanostructured architecture, using relatively simple anionic polypeptides that mimic the polyanionic character of the NCPs. We propose that the charged polymer acts as a process-directing agent, by which the conventional solution crystallization is converted into a precursor process. This polymer-induced liquid-precursor (PILP) process generates an amorphous liquid-phase mineral precursor to hydroxyapatite which facilitates intrafibrillar mineralization of type-I collagen because the fluidic character of the amorphous precursor phase enables it to be drawn into the nanoscopic gaps and grooves of collagen fibrils by capillary action. The precursor then solidifies and crystallizes upon loss of hydration waters into the more thermodynamically stable phase, leaving the collagen fibrils embedded with nanoscopic hydroxyapatite (HA) crystals. Electron diffraction patterns of the highly mineralized collagen fibrils are nearly identical to those of natural bone, indicating that the HA crystallites are preferentially aligned with [0 0 1] orientation along the collagen fibril axes. In addition, studies of etched samples of natural bone and our mineralized collagen suggest that the long accepted “deck of cards” model of bone's nanostructured architecture is not entirely accurate. Most importantly, this in vitro model demonstrates that a highly specific, epitaxial-type interaction with NCPs is not needed to stimulate crystal nucleation and regulate crystal orientation, as has long been assumed. Instead, we propose that collagen is the primary template for crystal organization, but with the important caveat that this templating occurs only for crystals formed from an infiltrated amorphous precursor. These results suggest that the 25-year-old debate regarding bone formation via an amorphous precursor phase needs to be revisited. From a biomedical perspective, in addition to providing possible insight into the role of NCPs in bone formation, this in vitro system may pave the way toward the ultimate goal of fabricating a synthetic bone substitute that not only has a composition similar to bone, but has comparable mechanical properties and bioresorptive potential as natural bone. From a materials chemistry perspective, the non-specificity of the PILP process and capillary infiltration mechanism suggests that non-biological materials could also be fabricated into nanostructured composites using this “biomimetic” strategy.

Bone structure and formation: A new perspective

Note: This article was originally published with an incorrect version of the Acknowledgments, which appeared on p. 218 of the print version. The correct version of the Acknowledgments appeared on pp. 1–2. The corrected article is available below.

/pdf/human-health-risk-assessment-for-aluminium-aluminium-oxide-5dfetd69xm.pdf

Human Health Risk Assessment for Aluminium, Aluminium Oxide, and Aluminium Hydroxide

Calcium orthophosphates are the main mineral constituents of bones and teeth, and there is great interest in understanding the physical mechanisms that underlie their growth, dissolution, and phase stability. By definition, all calcium orthophosphates consist of three major chemical elements: calcium (oxidation state +2), phosphorus (oxidation state +5), and oxygen (oxidation state −2).1 The orthophosphate group (PO43−) is structurally different from meta (PO3−), pyro (P2O74−), and poly (PO3)nn−. In this review, only calcium orthophosphates will be discussed. The chemical composition of many calcium orthophosphates includes hydrogen, either as an acidic orthophosphate anion such as HPO42− or H2PO4−, and/or incorporated water as in dicalcium phosphate dihydrate (CaHPO4 · 2H2O).1 Most calcium orthophosphates are sparingly soluble in water, but all dissolve in acids; the calcium to phosphate molar ratios (Ca/P) and the solubilities are important parameters to distinguish between the phases (Table 1) with crystallographic data summarized in Table 2. In general, the lower the Ca/P ratio, the more acidic and soluble the calcium phosphate phase.2 It is now generally recognized that the crystallization of many calcium phosphates involves the formation of metastable precursor phases that subsequently dissolve as the precipitation reactions proceed. Thus, complex intermediate phases can participate in the crystallization process. Moreover, the in vivo presence of small peptides, proteins, and inorganic additives other than calcium and phosphate has a considerable influence on crystallization, making it difficult to predict the possible phases that may form.3 Studies of apatite mineral formation are complicated by the possibility of forming several calcium phosphate phases. The least soluble, hydroxyapatite (HAP), is preferentially formed under neutral or basic conditions. In more acidic solutions, phases such as brushite (DCPD) and octacalcium phosphate (OCP) are often encountered. Even under ideal HAP precipitation conditions, the precipitates are generally nonstoichiometric, suggesting the formation of calcium-deficient apatites. Both DCPD and OCP have been implicated as possible precursors to the formation of apatite. This may occur by the initial precipitation of DCPD and/or OCP followed by transformation to a more apatitic phase. Although DCPD and OCP are often detected during in vitro crystallization, in vivo studies of bone formation rarely show the presence of these acidic calcium phosphate phases. In the latter case, the situation is more complicated, since a large number of ions and molecules are present that can be incorporated into the crystal lattice or adsorbed at the crystallite surfaces. In biological apatite, DCPD and OCP are usually detected only during pathological calcification, where the pH is often relatively low. In normal in vivo calcifications, these phases have not been found, suggesting the involvement of other precursors or the formation of an initial amorphous calcium phosphate phase (ACP) followed by transformation to apatite.



Table 1

Ca/P Molar Ratios, Chemical Formulas, and Solubilitiesa of Some Calcium Orthophosphate Minerals1,3,4





Table 2

Crystallographic Data of Calcium Orthophosphates1,4,5



In this review, we will discuss some important parameters related to crystal nucleation and growth/dissolution including the supersaturation/undersaturation, pH, ionic strength and the ratio of calcium to phosphate activities (Table 3). We then focus on the dynamics of crystallization/dissolution in the presence of additive molecules pertinent to biogenic calcium phosphate minerals.



Table 3

Crystal Growth Controls and Their Effect on the Bulk Solution and the Crystal Surfaces6




2. Biologically Related Calcium Phosphate Phases
2.1. Structure, Composition, and Phase Stability


2.1.1. Amorphous Calcium Phosphate (ACP) 
During the synthesis of HAP crystals through the interaction of calcium and phosphate ions in neutral to basic solution, a precursor amorphous phase is formed that is structurally and chemically distinct from HAP.7 However, calculations have shown that the phase consisted of individual or groups of HAP unit cells.8 Chemical analysis of the precursor phase indicated this noncrystalline phase to be a hydrated calcium phosphate (Ca3(PO4)2 · xH2O) with a Ca/P ratio 1.50,8 consisting of roughly spherical Ca9(PO4)6 “Posner’s clusters” (PC) close-packed to form larger spherical particles with water in the interstices.9 PCs appeared to be energetically favored in comparison to alternative candidates including Ca3(PO4)2 and Ca6(PO4)4 clusters.10 The structure of PCs in isolated form is notably different from that in a HAP environment.11 In particular, the chirality feature of PCs found in the HAP environment is suggested to disappear in an isolated form and in aqueous solution. The reconsideration of PCs as possible components in the actual structural model of ACP resulted from the cluster growth model of the HAP crystal.12 Ab initio calculations confirmed that stable isomers exist on the [Ca3(PO4)2]3 potential energy surface (PES).12,13 These isomers correspond to compact arrangements, i.e., arrangements in which the Ca and PO4 are disposed closely together. Their geometries are compatible with the terms “roughly spherical” used in Posner’s hypothesis. The calculations performed on the monomer and dimer PES revealed that the relative energies of the different isomers are governed by a specific bonding pattern in which a calcium atom interacts with two PO43− groups, forming four CaO bonds.12,13 The compact isomers on the trimer PES are energetically favored in comparison to monomer or dimer isomers. This is rationalized by the appearance of a specific bonding pattern for the trimer case in which a calcium forms six CaO bonds with six different PO4 groups. This type of bonding in encountered in HAP.13

It is now generally agreed that, both in vitro and in vivo, precipitation reactions at sufficiently high supersaturation and pH result in the initial formation of an amorphous calcium phosphate with a molar calcium/phosphate ratio of about 1.18–2.50. The chemical composition of ACP is strongly dependent on the solution pH: ACP phases with Ca/P ratios in the range of 1.18:1 precipitated at pH 6.6 to 1.53:1 at pH 11.7 and even as high as 2.5:1.4 Two amorphous calcium phosphates, ACP1 and ACP2, have been reported with the same composition, but differing in morphology and solubility.14,15 The formation of ACP precipitate with little long-range order tends to consist of aggregates of primary nuclei (roughly spherical clusters) with composition Ca9(PO4)65 dependent on the conditions of formation. It hydrolyzes almost instantaneously to more stable phases. These amorphous clusters served as seeds during HAP crystallization via a stepwise assembly process12 and were presumed to pack randomly with respect to each other,16 forming large 300–800 A spheres. Recent experimental studies found that ACP has definite local atomic microcrystalline order rather than a random network structure. NMR of thoroughly dried ACP suggests that the tightly held water resides in the interstices between clusters,17 but these are probably not of intrinsic importance in the structure of ACP. It is well-known that ACP contains 10–20% by weight of tightly bound water, which is removed by vacuum drying at elevated temperature.9 However, drying does not alter the calcium and phosphorus atomic arrangement. The side band intensities of dried ACP suggest that its chemical shift anisotropy is similar to or identical with that of normal ACP.17 ACP has an apatitic short-range structure, but with a crystal size so small that it appears to be amorphous by X-ray analysis. This is supported by extended X-ray absorption fine structure (EXAFS) on biogenic and synthetic ACP samples.18–20

The CaP amorphous phase transforms to HAP microcrystalline in the presence of water. The lifetime of the metastable amorphous precursor in aqueous solution was reported to be a function of the presence of additive molecules and ions, pH, ionic strength, and temperature.21 The transformation kinetics from ACP to HAP, which can be described by a ”first-order” rate law, is a function only of the pH of the mediating solution at constant temperature. The solution-mediated transformation depends upon the conditions which regulate both the dissolution of ACP and the formation of the early HAP nuclei.22 Tropp et al. used 31P NMR to demonstrate that the strength of ACP side bands is due to a characteristic structural distortion of unprotonated phosphate and not to a mixture of protonated and unprotonated phosphates,17 suggesting that ACP could contain substantial amounts of protonated phosphate not in the form of any known phase of calcium phosphate crystals. Yin and Stott suggested that, in the transformation from ACP to HAP, ACP need only dissociate into clusters rather than undergo complete ionic solvation. The cluster with C1 symmetry is the most stable isomer in vacuum. The interaction of Posner’s cluster with sodium ions and especially with protons leads to a considerable stability increase, and surprisingly, the cluster with six protons and six OH− recovers the C3 symmetry and similar atomic arrangement that it has as a structural unit in the HAP crystal. This may be a key factor in the transformation from ACP to HAP crystal.23

In general, ACP is a highly unstable phase that hydrolyzes almost instantaneously to more stable phases. In the presence of other ions and macromolecules or under in vivo conditions, ACP may persist for appreciable periods3 and retain the amporphous state under some specific experimental conditions.24

/pdf/calcium-orthophosphates-crystallization-and-dissolution-1c1riyx5dd.pdf

Calcium orthophosphates: crystallization and dissolution.

Organic molecules can alter inorganic microstructures, offering a very powerful tool for the design of novel materials. In biological systems, this tool is often used to create microstructures in which the organic manipulators are a minority component. Three groups of materials-biomaterials, ceramics, and semiconductors-have been selected to illustrate this concept as used by nature and by synthetic laboratories exploring its potential in materials technology. In some of nature's biomaterials, macromolecules such as proteins, glycoproteins, and polysaccharides are used to control nucleation and growth of mineral phases and thus manipulate microstructure and physical properties. This concept has been used synthetically to generate apatite-based materials that can function as artificial bone in humans. Synthetic polymers and surfactants can also drastically change the morphology of ceramic particles, impart new functional properties, and provide new processing methods for the formation of useful objects. Interesting opportunities also exist in creating semiconducting materials in which molecular manipulators connect quantum dots or template cavities, which change their electronic properties and functionality.

https://science.sciencemag.org/content/sci/277/5330/1242.full.pdf

Molecular Manipulation of Microstructures: Biomaterials, Ceramics, and Semiconductors

A synergistic effect has been demonstrated when magnesium and adenosine triphosphate (ATP) are used together in solution to delay the conversion of a slurry of amorphous calcium phosphate (ACP) to crystalline hydroxyapatite (HA). Conversion is delayed in some instances more than 10 times as long as with either ATP or Mg alone. In all experiments conversion did not begin until ATP in solution had decreased through hydrolysis to an undetectable level. The effect of Mg is to decrease substantially the rate at which ATP hydrolysis occurs. Once conversion began it proceeded more slowly in the presence of both Mg and ATP than with Mg or ATP alone. ATP was also found to prevent the formation of HA from metastable solutions of calcium and phosphate which did not contain any solid phase. Over the time period of these experiments, ATP hydrolyzed to a negligible extent in Tris-HCl buffer and in solutions containing Ca, PO4, and Ca plus PO4 ions. Hydrolysis of ATP does occur in the presence of ACP or HA, presumably by transphosphorylation on the surface of the solid calcium phosphate phase. It was concluded that ATP stabilized ACP, not by affecting its dissolution, but either by poisoning heteronuclear growth sites, or by poisoning the growth of embryonic HA nuclei (formed heterogeneously or homogeneously) before their critical size is reached, or by poisoning both. In the case of embryonic HA nuclei, the poisoned nuclei would go back into solution preventing HA crystal formation. In addition, it was found that the neutral Ca9(PO4)6 clusters, which are believed to be the basic structural unit of ACP, break down into individual Ca and PO4 ions when ACP dissolves in aqueous medium.

Stabilization of amorphous calcium phosphate by Mg and ATP.

Well-characterized bovine nasal proteoglycan A1 fraction (aggregate) and proteoglycan D1 fraction (subunit) have been shown to be effective inhibitors of hydroxyapatite (HA) formation in two in vitro test systems: (a) the transformation of amorphous calcium phosphate (ACP) to crystalline HA, and, (b) the direct precipitation of HA from low-concentration calcium phosphate solutions. A1 or D1 in solution slowed the transformation kinetics in system (a) without affecting the time to the onset of conversion. In system (b), A1 or D1 in solution increased the time to the onset of HA formation without affecting the HA formation kinetics. In both test systems A1 was a more effective inhibitor than D1, although the difference was not great. In both systems the inhibitory effect was proportional to the A1 or D1 solution concentration. The action of solutions of low and high molecular weight neutral dextrans on both test systems showed that high molecular weight and/or extended spatial molecular conformation has a much stronger correlation with inhibitory ability than solution viscosity. Proteoglycans have been implicated as playing a role in regulating biological mineralization particularly in the epiphyseal growth plate. Our study suggests that just enzymatic cleavage of aggregate into subunit is not sufficient to allow mineralization to occur, since we find that D1 itself is a potent inhibitor of HA formation. Further degradation and/or removal of D1 appears to be necessary for calcification to take place.

Effect of proteoglycans on in vitro hydroxyapatite formation

Amorphous calcium phosphate (ACP) was transformed at 25 degrees to hydroxyapatite (HA) in horse and bovine serum; solutions of serum-protein fractions in tris-HC1 buffer (pH 7.4), and pH 7.4 buffers containing from 0.1 to 10 times physiological CO3(2-) concentration. The ACP-to-HA transformation was slower in whole serum and serum fractions than in control buffer solution. The observed adsorption of serum proteins on ACP and HA probably inhibits both the dissolution of the ACP particles and the growth of HA crystals. After 72 h all transformations were complete as determined by X-ray diffraction. The HA crystal dimensions decreased with increasing C03(2-) but the shape, as shown by X-ray linewidths, was relatively constant up to about 4% CO3(2-). At 15% CO3(2-) the crystals were more equiaxial and less needle-like in habit. The radial distribution function (RDF) of HA with 3.7% CO3(2-) is less well resolved than the RDF of HA with ambient CO3(2-) (1.1%). The peaks are less sharp and their amplitude falls more rapidly with increasing atomic separation than for low CO3(2-)-HA. These effects show that CO3(2-) decreases the regularity of the atomic arrangement when incorporated in HA. The rapid decrease, with increasing CO3(2-) content, of the IR splitting of the P-O bending mode of CO3(2-)-HA is attributed to reduced crystal size and possibly to a perturbation of the crystal field due to CO3(2-)-induced lattice distortion. Finally, for bone mineral, it is probable that the poor resolution of the X-ray and IR patterns is due, in large part, to small crystal size and internal disorder caused by CO3(2-).

Effect of carbonate and biological macromolecules on formation and properties of hydroxyapatite.

Introduction: 0steomalacia renal osteodystrophy in patients on long term hemodialysis has often been associated with aluminum accumulation in bone (i). Aluminum has been shown to be localized at the mineralization front in dialysis osteomalacia, suggesting it as a possible etiologic agent in this disorder. Animal models of aluminum-induced osteomalacia have been defined which accurately reflect the clinical disease (2,3).

In vitro model of aluminum-induced osteomalacia: inhibition of hydroxyapatite formation and growth.

The radial distribution function calculated from x-ray diffraction of mineralized cytoplasmic structures isolated from the hepatopancreas of the blue crab (Callinectes sapidus) is very similar to that previously found for synthetic amorphous calcium phosphate. Both types of mineral apparently have only short-range atomic order, represented as a neutral ion cluster of about 10 A in longest dimension, whose probable composition is expressed by the formula Ca9(PO4)6. The minor differences observed are attributed to the presence in the biological mineral of significant amounts of Mg-2+ and ATP. Synthetic amorphous calcium phosphate in contact with a solution containing an amount of ATP equivalent to that of the biological mineral failed to undergo conversion to the thermodynamically more stable hydroxyapatite. The amorphous calcium phosphate of the cytoplasmic mineral granules is similarly stable, and does not undergo conversion to hydroxyapatite, presumably owing to the presence of ATP and Mg-2+, known in inhibitors of the conversion process. The physiological implications of mineral deposits consisting of stabilized calcium phosphate ion clusters are discussed.

N. C. Blumenthal

Papers

Stabilization of amorphous calcium phosphate by Mg and ATP.

Effect of proteoglycans on in vitro hydroxyapatite formation

Effect of carbonate and biological macromolecules on formation and properties of hydroxyapatite.

In vitro model of aluminum-induced osteomalacia: inhibition of hydroxyapatite formation and growth.

Atomic structure of intracellular amorphous calcium phosphate deposits