Crystals are normally symmetrical with well-defined angles between flat crystal faces; however, the macroscopic size and shape of a crystal can be significantly modified by the addition of molecules to the growing crystal surfaces.  In fact, in some instances the crystal growth can be completely halted due to the addition of growth inhibitors.  These additives (from ions to organic molecules to proteins) control the nucleation, shape, size, rigidity, and form of the crystals.

        The mechanism by which this control occurs is not fully understood.  It has been proposed that certain functionalities of the molecules may substitute for similar functionalities on the growing crystal surfaces.  Coulombic and intermolecular bonding between molecules and growing crystal surfaces has also been proposed as a possible mechanism of habit modification and incorporation into biominerals.  To this end, I began studying the effect of molecules on the control of crystal growth in order to elucidate the mechanism of habit modification by naturally occurring proteins within given organisms and pre-designed inhibitors of calcium sulfate.

Abalone Shell Proteins

        In order to adapt to changing environments, many organisms have evolved vital supporting structures.  These structures range from the complex skeletal systems found in mammals and reptiles to the simple shells of mollusks such as abalone.  Typically comprised of calcium-based biominerals, these remarkable frameworks take on numerous functions.  The nucleation, composition, growth rate, shape, and strength of these structures vary with the type of organic macromolecules utilized.  These macromolecules play an important role in the morphogenesis of calcium carbonate crystals in the genus Haliotis that have two layers of exoskeleton, the calcite and aragonite layers. 

        Previous research by Morse and co-workers on the Haliotis rufescens, red abalone, revealed that five proteins in the calcite layer and nine proteins in the aragonite layer play a significant role in the crystal growth of the shell by binding non-uniformly to growing calcium carbonate surfaces.  It has been demonstrated that these proteins control the shape, size, and form of the calcium carbonate crystals present in each layer of the red abalone shell.  However, similar studies have not been previously reported on other California abalone species. 

        We are currently pursuing the isolation and purification of the water-soluble proteins from the calcite and aragonite layers of the red and green abalone in order to determine the importance of each protein in the exoskeleton growth process and to compare proteins isolated from different species of California abalone.  During the past three years, we have isolated and purified a group of water-soluble proteins extracted from the Haliotis fulgens (green abalone) and Haliotis rufescens (red abalone).  As part of their Senior Theses last year, Rob Hickey and Justus Guerrieri focused on developing separation procedures in order to isolated individual proteins or smaller groups of these proteins in order to begin our comparison.  They were able to isolate a couple of proteins that are now ready for characterization.  Ultimately, we are interested in the whether there is a specific domain of the proteins that interact with the crystal surfaces.  Once the importance of each protein has been determined, we will synthesize protein domains.  We will then compare the habit modification that has occurred due to the interaction of the proteins/domains and the crystal surfaces.  At this point, we would begin to model the possible interactions between the domains and the crystal surfaces.

Zebra Mussel Shell Proteins

        As an extension of the above projects, Elyce Link began a project involving the isolation of proteins from Zebra Mussel shells in the Summer of 2006.  Our goal for this project is to isolate the proteins responsible for the nucleation and control of crystal growth responsible for shell formation.  To date we have determined by Powder X-ray diffraction that Zebra Mussel shells are composed of aragonite, and we have isolated four water-soluble proteins from the shell of Zebra Mussel.  Our attempts to separate the proteins have not been fruitful to date.  We hope that this project will provide insight into the growth of zebra mussel shells that can be used to design molecules or genetic procedures that halt the population growth of the organism.  The population growth of the zebra mussel has recently become a huge problem in the midwestern portion of the United States.