INTERACTION OF MINERAL SURFACES WITH POLYPEPTIDE NETWORKS: DYNAMIC SIMULATIONS FOR CALCIUM CARBONATE CRYSTAL SEED FORMATION

Calcite is a rhombohedral polymorph of calcium carbonate (CaCO₃). Calcite deposition on the exoskeleton of organisms can be related to their DNA pattern, and hence, to the protein sequence. Our first objective is to find suitable orientations of amino-acid residues in these peptide chains, where these peptide chains align themselves parallel to the calcite surface or surface step. Various sequences of small-chain peptide residues on surface steps of calcite (104) faces have been studied. The residue with the highest adsorption energy is phe-leu-lys^4- with a total adsorption energy of -1.071eV (on a non-polar calcite surface step) or –0.3571 eV/residue. The adsorption energies for 12-amino acid long peptide chain's interaction with (a) polar calcite step in alkaline condition and (b) non-polar step in acidic condition are respectively –0.09824 eV/amino acid residue and -0.1978 eV/amino acid residue. The 12-amino acid long peptide chain with alternating glycine and alanine shows better parallel alignment.

To understand the nucleating mechanism during biomineralization, experiments have focused on synthetic bioorganic templates, such as ß-pleated poly amino acids and Langmuir films. In the case of Langmuir monolayers, the amphiphilic molecules can act as artificial two-dimensional nuclei for the promotion of crystal nucleation and have been used as templates to direct the crystal nucleation and growth of calcium carbonate. This is the starting point of our molecular dynamics simulations. After deriving a pure-core potential set for fast molecular dynamics simulations, we have created different two-dimensional networks of amide-containing phospholipids that serve as templates for Ca-carbonate seed formation. We applied pressure on both side of the two-dimensional amphiphilic molecule on the film, and increased the pressure until the disruption of the film. The pressure-area curve matches the experimental data. We studied the Langmuir film interaction with calcite surfaces along the (104), (100), and (001) faces and observe the effect of phosphate group interaction with calcium and carbonate up to first 3-4 layers in (001) face.. The ultimate goal of this project is to provide systematic insight into template and, thus, seed formation control from a theoretical point of view.

Reference 1) Corstjens, L.A.M et al. 1998, J. Phycol., 34, 622. 2) Buijnsters, P.J.J.A. et al., Langmuir 2001, 17, 3623.