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Introduction to Biomineralization

Although the phenomenon of crystallization has been known for a very long time, the roles that factors such as solvent, temperature and impurities play in this process are still not well understood [81]. A precise control over the crystallization process, which is essentially a type of molecular recognition, is still a formidable challenge [82]. The most remarkable example of such control comes from Nature in the form of biomineralization. In Nature, the growth of inorganic minerals (biominerals) is rigorously controlled by organic biomacromolecules like proteins and polysaccharides. The highly regulated conditions give rise to structures with shape, size, morphology and properties very different from those obtained in the laboratory [83-90]. Interestingly, the biomacromolecules are often incorporated into the inorganic crystal lattice during the growth process, giving rise to organic-inorganic hybrid structures. These composite materials combine the rigidity of inorganic substances and the toughness of organic materials to provide organisms with stronger building units [90, 91]. Biominerals are often superior to man-made materials in terms of their mechanical strength, resistance to fracture and other physical properties [91]. An example of this is calcite, a polymorph of calcium carbonate, which grows in the laboratory as a brittle rhombohe-dral crystal (Fig. 7.20a). Under biological control, however, it attains a very different shape (Fig. 7.20b) and enough strength to impart protection and mechanical support to the animal, e.g. mollusk shells. One of the most remarkable features of biomineralization is that it occurs under physiological conditions unlike the manufacture of man-made materials, like cement, that requires extremes of temperature and pressure [92]. Due to the desirable properties of biominerals, there is much interest in mimicking biomineralization to obtain superior materials with tailored properties [93].

In Nature, biomineralization is controlled by biomacromolecules that project functionality in an ordered way to selectively recognize the growing faces of a

Fig. 7.20 (a) Calcite crystals grown in the laboratory; (b) A mature sea urchin spine composed of calcium carbonate and organic macromolecules (Reprinted with permission from Science 2003, 299, p. 1192. Copyright 2003 AAAS).
Fig. 7.21 Schematic diagram showing crystal growth in the presence of inhibitor for face B (Reproduced with permission from the authors).

crystal. An effective strategy to mimic this process is to present arrays of functionality designed to recognize and bind to specific crystal faces [90, 94]. This involves molecular recognition between the organic additive and the inorganic ions at the surface of a growing crystal. When an organic additive selectively recognizes a specific face of a growing inorganic crystal, it is adsorbed and further accumulation of inorganic ions on that face is hindered (Fig. 7.21). These faces gradually increase in area while others that are not recognized by the additive, decrease in area and ultimately disappear as the crystal grows. As a result, only the faces that are recognized by the organic molecule are manifested in the equilibrium morphology of the crystal while the rest of the faces are not observed. This implies that examination of the faces that appear in a crystal can provide information about the specific interactions between the additive and the affected face [94]. Conversely, it is possible to design additives that are complementary to a specific face of a crystal that can be used to alter the morphology of a crystal (Fig. 7.22)

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Fig. 7.22 Schematic diagram showing structural complementarity between the interacting groups.

[89]. Charge, size, distance and stereochemical complementarity are important parameters to be considered when designing effective crystal growth modulators [84, 90]. The role of interfacial interactions on the conformational preference of foldamers is discussed in detail in Chapter 13.

During the last two decades, numerous types of templates have been used to obtain various morphologies of common minerals. These include conformation-ally constrained peptides [95], polymers [96-101], self-assembled monolayers [102-105], other supramolecular assemblies [106] and small molecules [107111] and have been reviewed elsewhere [85, 112-114]. Since many organic mac-romolecules involved in biomineralization are known to have acidic moieties, a common design strategy involves projecting negatively charged groups (e.g. car-boxylates, phosphates) in an ordered manner [115]. The negatively charged groups presumably recognize the arrangement of cations in the initial stages of crystallization. Foldamers designed to project negatively charged functionality in a rigid manner could interact selectively with specific crystal faces and act as effective crystal growth modifiers.

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