Molecular Dynamics of Cation Hydration in the Presence of Carboxylated Molecules: Implications for Calcification
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Molecular Dynamics of Cation Hydration in the Presence of Carboxylated Molecules: Implications for Calcification Laura M. Hamm1, Adam F. Wallace2 and Patricia M. Dove1 Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 2 Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720
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ABSTRACT Biomolecules rich in aspartic acid (Asp) are known to play a role in biomineral morphology and polymorph selection, and have been shown to greatly enhance the growth kinetics of calcite. The mechanism by which these compounds favor calcification may be related to their effects upon cation solvation. Using molecular dynamics, we investigated the influence of small carboxylated molecules on the hydration states and water exchange rates of divalent cations. We show that the carboxylate moieties of Asp promote dehydration of Ca2+ and Sr2+ and that contact ion pair (CIP) formation is not required to disrupt the hydration of these cations. Ca2+- Asp and Sr2+ - Asp CIP formation decreases the total inner sphere coordination from an average of 8.0 and 8.4 in bulk water to 7.5 and 8.0, respectively. Water residence times estimated for Mg2+, Ca2+and Sr2+ follow the expected trend of decreasing residence time with increasing ionic radius. In the presence of Asp, both solvent-separated ion pair (SSIP) and CIP formation decrease the residence times of Ca2+and Sr2+ inner sphere water molecules. Comparable impacts on Mg2+ hydration are not observed. Mg2+ - Asp CIP formation is energetically unfavorable and Asp does not affect Mg2+ inner sphere water residence times. INTRODUCTION Calcified tissues are comprised of biogenic minerals that nucleate and grow in close association with an organic matrix of proteins and other macromolecules. These molecules are widely believed to have a myriad of regulatory functions in different stages of biomineral formation, including crystal nucleation [1], crystal growth [2], and the stabilization of amorphous intermediate phases [3]. Matrix biomolecules are also thought to influence the mineral phase [4] and morphology [5] that forms. Studies of calcification across many taxa report that the macromolecules are characterized by an abundance of carboxylated amino acids and carbohydrate species [6]. This unique association of biopolymer and mineral has motivated an extensive effort to determine the physical basis by which the compositions and conformations of acidic molecules modulate the onset of different calcified phases and morphologies. Although the findings of many in vitro and in vivo investigations provide important clues, the mechanistic basis for how organic components modulate mineralization is not well understood. Anecdotal evidence from a number of studies suggests that a critical role of macromolecules in biomineralization may reside in their ability to modify solvent properties in the local environment where mineral nucleation and growth occurs. A recent experimental study of calcification processes proposed the solvent-modifyi
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