Effect of sodium polyacrylate on the hydrolysis of octacalcium phosphate
Introduction
The inorganic phase deposited in mineralized tissues, such as bone and dentine, is a poor crystalline carbonated apatite. However, it has been proposed that the mechanism of biological apatite formation involves octacalcium phosphate (Ca8H2(PO4)6·5H2O, OCP) as a precursor phase [1], [2]. OCP triclinic crystals consist of alternating hydrated and apatitic layers, the latter closely resembling hydroxyapatite (Ca10(PO4)6(OH)2, HA) [3]. OCP has a greater solubility than the thermodynamically more stable HA, and it undergoes a spontaneous and topotactic transformation into HA [4], [5]. The involvement of OCP in the first stages of crystal growth of bone and tooth tissues has been invoked to explain the lack of HA crystals having hexagonal symmetry in the initial stages of biomineralization [6], as well as the crystal morphology of bone and dentine, which is plate or ribbon like, with the thin plates elongated along the crystallographic c-axis [7], [8]. Furthermore, OCP has been found in dental calculi [9], [10], as well as in other pathological calcifications [11]. However, just recently a high resolution transmission electron microscopy study of calcifying dentine reported the first observation of OCP in biological crystals [12]. The presence of OCP in the central part and of HA at the extremities of the same crystal was interpreted as suggesting that the hydrolysis of OCP in the formation of biological crystals develops along the (100) crystal planes. The matrix macromolecules of the vertebrate mineralized tissues contain acidic proteins and glycoproteins, rich in acidic amino acids with charged carboxylate groups [13]. Other negatively charged groups, such as phosphates and sulphates, are often present. These proteins may affect crystal induction and crystal growth regulation, possibly through interactions with the charged crystal surfaces [14]. The results of in vitro experiments carried out on OCP crystallization suggest a specific interaction of the highly phosphorylated acidic protein, phosphophoryn, with the (010) face, whereas carboxylate-rich proteins seem to interact preferentially with the hydrated layer of the (100) face [14]. We have recently found that sodium polyacrylate (NaPA), a carboxylate-rich polyelectrolyte, inhibits the nucleation and growth of synthetic OCP crystals [15]. The results of the structural and morphological investigation suggest a non-specific interaction with the polyelectrolyte, which is not significantly adsorbed on OCP crystals. On the other hand, preliminary data indicate that NaPA can be adsorbed on OCP from aqueous solutions and prevent its hydrolysis into HA [15]. In order to investigate the structural interaction of the polyelectrolyte with OCP, we have carried out a structural and morphological investigation on OCP submitted to hydrolysis in aqueous solutions at different NaPA concentrations.
Section snippets
Materials and methods
Octacalcium phosphate was synthesized by dropwise addition of 250 mL of 0.04 M Ca(CH3COO)2 (Carlo Erba) into 750 mL of a phosphate solution containing 5 mmol of Na2HPO4 (Carlo Erba) and 5 mmol of NaH2PO4 (Carlo Erba) maintained at 60 °C, starting pH 5 [16]. The precipitate was stored in contact with the mother solution for 10 min, filtered, repeatedly washed with bidistilled water and dried at 37 °C.
Hydrolysis of OCP was carried out in distilled water and in solutions at different NaPA
Results
The most frequent morphology exhibited by the crystal aggregates present in the SEM micrographs of the synthesized OCP is that of long blades radiating from a common origin to form spherulitic growths, as that reported in Fig. 1(a). The TEM micrographs of the starting OCP show long plate-like crystals with straight edges, whose mean dimensions are 2×8 μm (Fig. 1(b)). The thickness of the crystals, which can only be measured in a few cases, is around 0.3–0.4 μm. Before treatment in aqueous
Discussion
The hydrolysis of OCP to HA in aqueous solutions is a well-known transformation, which depends on several parameters, such as temperature and ionic composition of the solution [2], [21], [22].
The results of this paper indicate that a concentration of NaPA of 10−1 mM prevents the hydrolysis of OCP to HA in aqueous solution at 60 °C. OCP is partially converted into HA after 48 h of storage in 10−2 mM polyelectrolyte solution, whereas complete transformation can be obtained when the concentration
Abbreviations
OCP octacalcium phosphate HA hydroxyapatite NaPA sodium polyacrylate SEM scanning electron microscopy TEM transmission electron microscopy ED electron diffraction
Acknowledgements
This research was carried out with the financial support of MURST, CNR (PF MSTA II) and the University of Bologna (Funds for Selected Research Topics).
References (27)
- et al.
Nature
(1962) - et al.
J. Cryst. Growth
(1996) - J.C. Elliott (Ed.), Structure and Chemistry of the Apatites and Other Calcium Orthophosphates, Elsevier Science,...
- et al.
J. Phys. Chem.
(1992) - et al.
J. Cryst. Growth
(1997) - et al.
Anat. Rec.
(1989) - et al.
Calcif. Tissue Res.
(1973) - et al.
J. Cryst. Growth
(1987) J. Dent. Res.
(1974)- et al.
Scanning Microsc.
(1992)
Calcif. Tissue Int.
Acta Crystallogr., Sect. D
Cited by (34)
In-situ high temperature XRD and TEM study of the thermal stability and sintering behavior of octacalcium phosphate
2019, Journal of Alloys and CompoundsIn situ silk fibroin-mediated crystal formation of octacalcium phosphate and its application in bone repair
2019, Materials Science and Engineering CCitation Excerpt :One potential mechanism is that such proteins, when bound to the surface of OCP where the hydrated layers were exposed, lead to the elongating or thickening of crystals in that axis [18,47]. Similarly, carboxylate-rich polymers could also affect the synthesis and hydrolysis of OCP [50–52]. In our study, SF appeared to play an important role in regulating OCP crystal formation.
Functionalization of octacalcium phosphate for bone replacement
2019, Octacalcium Phosphate Biomaterials: Understanding of Bioactive Properties and ApplicationNovel scaffold composites containing octacalcium phosphate and their role in bone repair
2019, Octacalcium Phosphate Biomaterials: Understanding of Bioactive Properties and ApplicationBiomimetic fabrication of antibacterial calcium phosphates mediated by polydopamine
2018, Journal of Inorganic BiochemistryCitation Excerpt :Enhancement of bone formation has been demonstrated both when OCP granules are used to repair bone defects and when OCP is applied as a coating on metallic surfaces [2–6]. OCP can hydrolyze into HA in aqueous solution [7,8] However, the process is relatively slow at physiological pH and temperature, whereas it is accelerated at higher temperatures and in the presence of small amount of fluoride [9–12]. Also other CaPs, in particular α-tricalcium phosphate (αTCP), hydrolyze into HA [13].