Morphological and Kinetic Transformation of Calcite Crystal Growth by Prismatic-Associated Asprich Sequences

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Abstract

Many of the interesting mechanical and materials properties of the mollusk shell are thought to stem from the prismatic calcite crystal assemblies within this composite structure. It is now evident that proteins play a major role in the formation of these assemblies. Recently, a superfamily of seven conserved prismatic layer-specific mollusk shell proteins, Asprich, were sequenced, and the conserved 42 AA C-terminal sequence region of this protein superfamily was found to introduce surface voids or porosities on calcite crystals in vitro. Using AFM imaging techniques, we further investigate the effect that this 42 AA domain (Fragment-2) and its constituent subdomains, DEAD-17 and Acidic-2, have on the morphology and growth kinetics of calcite dislocation hillocks. We find that Fragment-2 adsorbs on terrace surfaces and pins acute steps, accelerates and then decelerates the growth of obtuse steps, forms clusters and voids on terrace surfaces, and transforms calcite hillock morphology from a rhombohedral form to a rounded one. These results exhibit interesting similarities and differences with some of the earlier findings obtained for nacreous polypeptides and proteins. The subdomains Acidic-2 and DEAD-17 were found to accelerate then decelerate obtuse steps and induce oval rather than rounded hillock morphologies. Unlike DEAD-17, Acidic-2 does form clusters on terrace surfaces and exhibits stronger obtuse velocity inhibition effects than either DEAD-17 or Fragment-2. Hence, the C-terminal Asprich sequences alter the shape of and influence the crystal growth kinetics of calcite. Interestingly, a 1:1 mixture of both subdomains induces an irregular polygonal morphology to hillocks and exhibits the highest degree of acute step pinning and obtuse step velocity inhibition. This suggests that there is some interplay between subdomains within an intra (Fragment-2) or intermolecular (1:1 mixture) context, and sequence interplay phenomena may be employed by biomineralization proteins to exert net effects on crystal growth and morphology.

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