Chitin and collagen as universal and alternative templates in biomineralization

Biomineralized structures and tissues are composites, containing a biologically produced organic matrix and nano- or microscale amorphous or crystalline minerals. Two main examples of organic matrices - the amino-polysaccharide chitin and the asymmetric protein collagen - are presented and discussed as the basic structural modules and organo-templates for calcium and silica biomineralization in nature. Both serve as templates, providing preferential sites for nucleation and controlling the location and orientation of mineral phases. Here, for the first time, chitin and collagen are analysed from evolutionary, structural, and functional points of view with respect to their templating properties in calcification and silicification phenomena, using both in vivo and in vitro data. It is proposed that these biopolymers be characterized as fundamental templates in biomineralization, inasmuch as they are very ancient from an evolutionary point of view, common to many species and biological systems with a global distribution. The two polymers also exhibit very similar hierarchical structural organizations, in spite of the possible alternatives they provide in chemical nature and origin. In addition, the phenomenon of multi-phase mineralization - where two minerals, amorphous and crystalline CaCO3, form from one biomolecule, chitin - is also described, analysed, and discussed for the first time.