Polysaccharides and proteoglycans in calcium carbonate-based biomineralization.

Biomineralization is a widespread phenomenon in nature leading to the formation of a variety of solid inorganic structures by living organisms, such as intracellular crystals in prokaryotes, exoskeletons in protozoa, algae, and invertebrates, spicules and lenses, bone, teeth, statoliths, and otoliths, eggshells, plant mineral structures, and also pathological biominerals such as gall stones, kidney stones, and oyster pearls. These biologically produced biominerals are inorganic-organic hybrid composites formed by self-assembled bottom up processes under mild conditions, showing interesting properties, controlled hierarchical structures, and remodeling or repair mechanisms which still remain to be developed into a practical engineering process. Therefore, the formation of biominerals provides a unique guide for the design of materials, especially those that need to be fabricated at ambient temperatures. In biominerals, the small amount of organic component not only reinforces the mechanical properties of the resulting composite but also exerts a crucial control on the mineralization process, contributing to the determination of the size, crystal morphology, specific crystallographic orientation, and superb properties of the particles formed. Therefore, biological routes of structuring biominerals are becoming valuable approaches for novel materials synthesis. Although several principles are applicable to the majority of the biominerals, herein we will focus on the role of polysaccharide polymers in calcium carbonate-based biominerals. As a general principle, the assembly of these biominerals consists of a four-stage process. It starts with the fabrication of a hydrophobic solid organic substrate or scaffolding onto which nucleation of the crystalline phase takes place closely associated with some polyanionic macromolecules. Crystal growth is then controlled by the addition of gel-structuring polyanionic macromolecules, and finally mineralization arrest is accompanied by the secretion of a new inert scaffolding of the same type or the deposition of other hydrophobic inhibitory macromolecules. Currently, a large number of proteins have been described which are involved in the control of biomineralization. These proteins are usually highly negatively charged and contain carboxylate, sulfate, or phosphate as functional groups, which may bind Ca ions and could control crystal nucleation and growth by lowering the interfacial energy between the crystal and the macromolecular substrate. However, the precise mechanism involved in controlling crystal nucleation, growth, and morphology is far from being understood. Combinatorial biology techniques have been recently developed for testing the ability of randomly generated peptides to bind different substrates or ions, thus allowing a correlation between peptide structure and ion binding affinity. However, the main focus is on the role of the backbone structure of the polymer due to the primary structure of the protein, because the synthetic technology does not allow the formation of post-translational modifications, such as sulfation and phosphorylation, which do occur in the eukaryotic cell. Even so, the occurrence of negatively charged groups in macromolecules involved in biomineralization, mainly derived from acidic amino acids, has inspired many researchers to produce synthetic polymers having such groups in order to control the size, orientation, phase, and morphology of inorganic crystals. However, since Abolins-Krogis’ work, a slow but increasing interest has been developed to explore the role of polysaccharides in biomineralization, despite the fact that their involvement in biomineralization seems to appear very early in evolution. There is no single type of polysaccharide associated with biominerals, but such polysaccharides are mainly hydroxylated, carboxylated, or sulfated or contain a mixture of these functional moieties.