A promising route to the fabrication of biomimetic coatings for artificial implants is the deposition of organic/inorganic composite materials consisting of polyelectrolyte multilayers alternating with layers of "in situ" grown calcium phosphate crystals. To facilitate understanding of the underlying mechanisms, in this paper we discuss the influence of polyelectrolytes (PEs), polystyrene sulfonate (PSS), poly-L-lysine (PLL), and poly-L-glutamic acid (PGA) on the formation and properties of amorphous calcium phosphate (ACP) and on the nucleation and growth morphology of the crystalline phase. pH vs time curves revealed three distinct precipitation events, i.e., (I) precipitation of ACP, (II) secondary precipitation of a crystalline phase upon the amorphous precursor, and (III) solution-mediated phase transformation and crystal growth. Finally, crystalline mixtures with low Ca/P molar ratios (1.39), consisting of octacalcium phosphate crystals and small amounts of apatite, were obtained. From the pH vs time curves, the induction time, t i , preceding crystal nucleation was determined. All PEs at low concentrations induced and at high concentrations inhibited nucleation. The efficiency of induction increased in the order: LMw PGA = HMw PGA < LMw PLL < HMw PLL < PSS, while the inhibition efficiency increased as LMw PLL = HMw PLL < PSS < LMw PGA < HMw PGA. ACP particles formed in the presence of PE were smaller and less aggregated and had a higher surface charge than in the controls. All investigated PEs also inhibited growth of the crystalline phase in a nonspecific way.
Acidic matrix macromolecules, present in many mineralized tissues, including those of vertebrates, are thought to be involved in controlling crystal formation. Little, however, is known about their in vivo functions, particularly in relation to calcium-phosphate-containing crystals. The manner in which a variety of synthetic and natural acidic macromolecules interact in vitro with crystals of octacalcium phosphate (OCP) has been studied. Interactions were assessed by examining changes in morphology of the crystals resulting from preferential interaction of the additive with some crystal faces and not others. Macromolecules rich in acidic amino acids, with or without polysaccharides, such as polyaspartate and mollusk shell proteins respectively, were shown to interact preferentially with rows of Ca ions exposed on the hydrated plate surface of OCP crystals. In contrast, the phosphorylated proteins, phosphophoryn and phosvitin, interacted specifically with the apatite-like motifs on the OCP side faces. BSP did not interact specifically with OCP, under the experimental conditions used. The observation that these classes of acidic macromolecules recognize different crystal faces should be taken into account when evaluating functions of acidic matrix macromolecules in mineralized tissues.
As a basis for crystallization studies, the solubilization of amino acids (glycine, l-histidine, and l-phenylalanine) in water-in-isooctane microemulsions stabilized by AOT (sodium di-2-ethylhexyl sulfosuccinate) was investigated. The maximum amount of amino acid that could be solubilized was determined by the solid-liquid extraction method, and the effect of the guest molecules (amino acids) on the size and shape of the microemulsion droplets and their thermal properties were determined using SAXS and DSC measurements, respectively. The solubilization of glycine molecules, which primarily dissolve in the water pool, was slightly lower than their solubility in pure water, decreasing with increasing concentration of AOT and increasing with increasing water content in the microemulsion. In contrast, the solubilization of phenylalanine, which is primarily located at the water/oil interface, exceeded several times the solubility in water, the solubilized amount increasing with increasing AOT and/or water concentrations. Histidine had characteristics intermediate between these two extremes. Solubilization of those molecules effected an increase in droplet size. The thermal analysis showed that loading of the microemulsion droplets with glycine has a much stronger effect on the thermal behavior of the emulsified water than has loading with phenylalanine. The low solubilization of glycine as compared to its solubility in pure water can be explained by the state of water within the microemulsion droplets, i.e., part of it is present as free water and part as water bound to the AOT headgroups. The loading of phenylalanine changed the shape of the microemulsion droplets from spherical to ellipsoidal, and with increasing droplet sizes, the [phenylalanine]/[AOT] molar ratio at the interface increased.
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