Biological Modification in the Brushite Crystallization
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Biological Modification in the Brushite Crystallization Ruikang Tang,1 Molly R. Darragh,2 Christine A. Orme,2 John R. Hoyer3 and George H. Nancollas1 1 Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, NY 14260, USA 2 Department of Chemistry and Materials Science, Lawrence Livermore National Laboratory, Livermore, CA 94551, USA 3 The Children’s Hospital of Philadelphia, University of Pennsylvania, Philadelphia, PA 19104, USA ABSTRACT The crystallization of brushite has been investigated in the presence of typical ionic biological components, magnesium, fluoride and citrate and a natural protein, osteopontin, using a constant composition method capable of nano-mole sensitivity. While the simple ionic additives, magnesium and fluoride do not have a significant effect, citrate ions can adsorb on lateral faces due to electrostatic interactions involving the functional carboxylate groups, resulting in crystal morphology changes. Osteopontin was the most effective inhibitor influencing the growth of virtually all brushite crystal faces. INTRODUCTION Numerous inorganic and organic compounds are involved in biomineralization processes[1,2]. Some, especially protein matrices, can modify the formation of biominerals by participating in the control of nucleation events, growth kinetics, crystal orientation, crystal morphology and even the nature of the mineral phase formed. This control can range from specific requirements related to the compatibility of crystal faces and the binding functional groups on a protein or macromolecule to non-specific control in which the biological surface presents a solution environment containing ions that increase the thermodynamic driving force for precipitation or crystallization. It is now generally agreed that the proteins most active in mediating biologically directed mineral growth contain acidic amino acid residues, specifically carboxylic acid-rich regions, which interact with mineral surfaces to influence both the rates of formation and crystal morphology[3,4]. Chemical and interfacial regulation of precipitation, matrix-induced or matrix-mediated nucleation and crystal growth, and inhibition by binding of molecules to specific faces of the crystals are suggested to be the most important mechanisms which determine/control the biomineralization. Calcium phosphates, the most important inorganic constituents of biological hard tissues, are present in bones, teeth and tendons and give these organs stability, hardness and function[5,6]. There are several different phases of calcium phosphates, of which hydroxyapatite is the most thermodynamically most stable. However it is difficult for this phase to nucleate directly under a mild biological conditions [6,7] and phase transformations are probably always involved in its formation [5,7]. Brushite (dicalcium phosphate dihydrate, CaHPO4⋅2H2O) is one of the important biominerals[8,9] that has frequently been invoked as a precursor phase as it readily crystallizes from aqueous solutions, and it is
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