Spatial and temporal resolution of interfaces, phase transitions and isolation of three families of proteins in calcium carbonate based biocomposite materials

The abalone shell is a microlaminate organic-inorganic composite of mineral and biopolymers exhibiting exceptional nanoscale regularity and a strength {dollar}sim{dollar}3,000 times greater than that of the inorganic crystals themselves. Although the integral proteins typically comprise less than 2% (by mass) of the shell, they determine the structural organization and properties of the mineralized composite. Three families of proteins from the red abalone shell have been isolated, purified and characterized and their role in the formation of this composite structure have been studied. I have found that Nature uses two different mechanisms for directing structure during biofabrication of the shell and flat pearl. One controls structure and orientation of the mineral at the atomic and nanoscale levels; and the other controls ordering over macroscale dimensions. The proteins of these three families exert this control: (1) a calcite crystal-nucleating protein (nuclein) (2) polyanionic proteins that determine the phase, orientation, and morphology of calcite and aragonite; and (3) matrix envelope proteins that determine the lamellar spacing and crystal size of aragonite in nacre. Preliminary sequence analysis of these proteins and their cloned genes reveals several unusual structural features. Use of the purified nucleating and polyanionic proteins allow us, in vitro, to abruptly and sequentially switch crystallographic phase from calcite to aragonite and vice-versa, producing multiphase composites with micron-scale domains. Development of in vivo (flat pearl) and in vitro systems allowed for the elucidation of organic-inorganic interfaces and phase transitions of the growing shell to better understand the biomineralization process and development of new multiphase composites.