Interactions of Mineral Surfaces and Adsorbates: A Computational Modeling Approach.

Abstract

Calcite and apatite are the two major minerals on the Earth’s surface that interact with organic molecules to create biominerals. We have studied the interaction of small, 3-amino acid peptides and long, 12-amino acid peptides with the energetically stable calcite (104) surface (and steps thereon). These organic adsorbate molecules were chosen to test the hypothesis that building blocks of complex biomolecules can modify the morphology of biominerals. Our studies indicate that the arrangement of polar functional groups in peptides and their direct interaction with mineral surface ions is more important than steric hindrance of different side-chains of amino acids to control the growth of biominerals. Langmuir films have been used in previous experimental studies to mimic matrix-controlled biomineralization. In our simulations on the growth of calcite nuclei under Langmuir films consisting of amide-containing phospholipids, the polar calcite (100) surface resulted in the most negative interfacial energy. These molecular-dynamics simulations give insight in the mechanism of matrix-controlled calcite nuclei formation in aqueous environments. Hydroxyapatite and carbonated apatite are the major inorganic minerals found in vertebrates, and their interactions with cell-adhesion peptides govern mineral growth and bone-regeneration processes. Molecular modeling studies on the interaction with RGD and YIGSR peptides with the hydroxyapatite (001) surface and surface steps show that final orientation of the peptides parallel to the [010] step direction is most favorable. A quantum-mechanical approach was chosen to study electron transfer between redox couples through a mineral surface to investigate the long-range interaction of As (OH)3 through a galena (100) surface with different oxidants. As(III) is a toxic element that can be immobilized by adsorption/oxidation on a mineral surface to limit its concentration in groundwater. In all cases studied, the galena surface was found to be an effective medium to shuttle electrons from As(III) over some distance to the respective oxidants (O, O2, and Fe(III)). Molecular modeling studies applied in this thesis to adsorbate-mineral surface interactions can elucidate reaction mechanisms that are usually out-of reach in bench-top experiments, while providing insight in predicting and mimicking natural biomineralization and redox phenomena.