CONTROL OF CRYSTAL PHASE SWITCHING AND ORIENTATION BY SOLUBLE MOLLUSK-SHELL PROTEINS

IN the initial stages of the biomineralization of abalone shells, a pr imer layer of oriented calcite crystals grows on a nucleating protein sheet(1,2), The deposition of this primer is followed by an abrupt tra nsition to c-axis-oriented crystals of aragonite, another crystalline form of calcium carbonate, The formation of each of the two crystal ty pes is accompanied by the synthesis of specific polyanionic proteins(1 -3), suggesting that cooperative interactions between these proteins a nd the inorganic ions during crystal nucleation and growth control the phase of the deposited mineral and that differential expression of th e proteins allo,rs the organism to induce phase changes, It is known t hat soluble shell proteins can control crystal morphology(4-10), but i t has been suspected that the switch in phase-from calcite to aragonit e-might require the deposition of a new nucleating protein sheet. Here we describe in vitro studies of the crystallization of calcium carbon ate in the presence of soluble polyanionic proteins extracted from aba lone shell, We find that these proteins alone are sufficient to contro l the crystal phase, allowing us to switch abruptly and sequentially b etween aragonite and calcite without the need for deposition of an int ervening protein sheet. These results show that soluble organic compon ents can exert greater control over hierarchical biomineral growth tha n hitherto suspected, offering the prospect of similar phase control i n materials chemistry.