Control over aragonite crystal nucleation and growth: An in vitro study of biomineralization

Calcite and aragonite are the two most stable polymorphs of calcium carbonate. Even though thy have very similar structures, many organisms are able to selectively deposit one polymorph and not the other. Recent in vitro studies have shown that one or more mollusk shell matrix macromolecules are capable of specific polymorph nucleation, provided they are in an appropriate microenvironment.([1]) In this study we examine aspects of the structure and function of some of the components of this β-chitin-silk fibroin in vitro system. We also show, by scanning electron microscopy, that the chitin framework is very porous, thus facilitating the diffusion of ions and macromolecules into the structure. Fluorescent light microscopy and scanning electron microscopy (SEM) demonstrate that the silk fibroin is intimately associated with the chitin framework. One particular fraction purified from the assemblage of mollusk-shell macromolecules extracted from an aragonitic shell layer is able to specifically induce aragonite crystal formation in vitro. These crystals are needle-shaped, whereas the aragonite crystals induced by the total assemblage of macromolecules are shortened into ellipsoids. This implies that other components in the assemblage modulate crystal growth. Finally, testing of a series of Asp and Leu or Glu and Leu containing synthetic peptides in the in vitro assay system shows that only (Asp-Leu)(n) is capable of specifically inducing aragonite formation. All these observations demonstrate that polymorph specificity is dependent upon the amino acid sequence, the conformation of specific protein(s) in the mollusk shell and the microenvironment in which crystal nucleation and growth takes place.