Kinetics and Template Nucleation of Self-Assembled Hydroxyapatite Nanocrystallites by Chondroitin Sulfate

Abstract

Biomineralization is an important process, which is often assisted by biomolecules. In this paper, the effect of chondroitin sulfate on the crystallization of hydroxyapatite was examined quantitatively based on a generic heterogeneous nucleation model. It is found that chondroitin sulfate can suppress the supersaturation-driven interfacial structure mismatch between the hydroxyapatite crystal and the substrate and promote the formation of ordered hydroxyapatite nanocrystallite assemblies. The nucleation mechanism of selfaligned hydroxyapatite nanocrystallites was examined from the viewpoints of kinetics and interfacial structure and properties, which contributes to an understanding of the fundamentals of biomineralization of self-assembled structures. The results obtained from this study will provide a basic principle to design and fabricate highly orderly organic-inorganic hybrid materials.Natural materials such as bones and teeth consist of biocomposites with well organized and assembled hydroxyapatite (HAP 2 ; Ca 5 (PO 4 ) 3 OH) nanobiominerals, to perform important biological functions. Although such composites in hard tissues are formed under mild conditions, they exhibit unusual mechanical properties that outperform synthetic materials (1, 2). How can biomineral nanocrystallites be assembled to form a synergetic structure? Why are some molecules, in particular biomacromolecules, capable of fabricating different patterns of biominerals? Recently, these questions concerning the biological mechanisms that control mineralized tissue construction have attracted a great deal of attention in fields ranging from biology and chemistry to materials science and bioengineering (3-7).It is well documented that the ordered structures in mineralized tissues appeared to originate from organized assemblies of biomacromolecules, such as proteins, polysaccharides, or proteoglycans, and inorganic compound salts (8 -10). Previous studies indicate that some biomacromolecules are involved in controlling the nucleation, growth, size, and shape of the mineral phases (11), since they can act as templates through self-assembly to facilitate interaction with an insoluble matrix and to induce the desired stereochemistry for the construction of organized structures (12, 13). Normally, these molecules are functionalized with acidic groups such as carboxylic acid, sulfonate, and phosphate moieties, which enable them to become an effective metal ion chelator to combine with the inorganic matrix (14 -16). Chondroitin 4-sulfate (ChS) is just one of these biomolecules. It belongs to the family of glycosaminoglycans, which can be found on cell surfaces and in the extracellular matrix of cartilage and bone. Large amounts of it in the cartilage permit diffusion of substances between blood and vessels. ChS consists of repeated disaccharide units; one of the two monosaccharides is N-acetylgalactosamine sulfate (GalNAc-OSO 3 Ϫ ), and the other is glucuronic acid that contains a carboxylate group (cf. Fig. 1). Previous investigation indicated that ...

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