Nanoparticle-host interactions in natural systems

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Two-dimensional interactions: Calcite seed formation by interaction with Langmuir films

So far, we have discussed interaction of peptides with calcite surfaces in one dimension. To better understand and mimic the interaction taking place in the exoskeleton of biominerals, we need to study the organic matrix-inorganic mineral surface interaction in two-dimensional aspects. The relationship between the templating biomolecular substrate and the inorganic phase lies in the epitaxial matching of lattice spacings of specific crystal planes with some ordered arrangement of molecular units in the template (Mann, 1988; Buijnsters et al., 2001). Different approaches to understand these processes include focusing on synthetic templates such as polymers (Berman et al., 1995), macromolecular complexes (Donners et al., 2000), phospholipid vesicles (Mann et al., 1986) and Langmuir films (Landau et al., 1985, 1989). In the case of Langmuir monolayers the amphiphilic molecules can be designed in such a way that they act as artificial two-dimensional nuclei for the promotion of crystal nucleation. Such films have been used as templates to direct the crystal nucleation and growth of amino acids, proteins, and calcium carbonate. Thus, Biswas & Becker (submitted) studied the growth and crystallization of calcium carbonate seed crystals under the influence of Langmuir films. Monolayer crystallizations are carried on highly compressed surfactants which acted as 2D crystals (Landau et al., 1989). It is found that modification of the polar group and the head group of the surfactant has a marked effect on the crystallization process (Heywood & Mann, 1994). Mobility of the molecules in the monolayer can also have a significant effect on the crystallization. In the experimental study, the Langmuir monolayer was composed of benzyl (-)-(2S)-3-phenoxy-2-octadecylaminopropan-1-ylmethylphosphate (“compound 1”, see also Fig. 25):

The effect of piston pressure on the surface area per surfactant molecule (compound 1) was studied by Buijnsters et al., 2001. These results can be compared with the computational simulation of the Langmuir film composed of compound 1 by decreasing the area available for each surfactant molecule and calculating the resulting strain, which can be converted to pressure. Subsequently, the growth of calcite seed nucleation under these Langmuir films at different pressure/(surface area) conditions was evaluated. Compound 1 is an amide-containing phospholipid and is an ideal choice for studying the organic matrix-mineral interaction. The presence of a hydrophilic head group and a hydrophobic hydrocarbon chain closely depicts the physico-chemical properties of exopolymers of living organisms.

Simulation procedure

Using the Cerius2 software package, compound 1 is aligned in a two-dimensional infinitely periodic setup (the periodicity is infinite parallel to the film) to create the Langmuir film (Fig. ). The elongated hydrocarbon chain is pointing upwards and the amide-containing phosphate head groups point downwards in contact with the inorganic mineral surface. First the influence of the external force on the unit area of the Langmuir film needs to be evaluated. Starting with a unit area of 46 Å2, the external force applied is 0 N/m. By decreasing the surface area and applying molecular dynamics simulations to allow the film to relax due to this external pressure, the surface pressure and the resulting change in surface area are monitored. Up to a piston pressure of 25.4 N/m, the decrease in surface area is continuous and the final surface area per surfactant molecule decreases to 29.011 Å2 (Fig. ). When the force is increased to 25.5 N/m on the pistons on both sides of the Langmuir film, the whole film ruptures.

In order to study the interaction of different calcite faces with such a film, the different calcite faces are placed under the Langmuir film at a particular force and surface area condition. Molecular dynamics and energy minimization are performed for each face. Two-dimensional periodic boundary conditions are applied in each case, indicating that the surface is extended infinitely in both directions parallel to the interface. Three different calcite faces, (001), (100), and (10-14) were studied in such a way (Fig. ). It was observed that the interaction with the organic compounds do not affect the molecules in the calcite seed beyond the fourth layer. In addition, molecular dynamics simulations were performed with the Langmuir film in contact with the most preferred face of calcite, which promotes the nucleus of seed formation for calcite. The adsorption energy per unit area provides an estimate on the most energetically favourable interface for the onset of calcite seed formation under the Langmuir film. Both the surface and interfacial energy in each case of organic-calcite interaction are calculated, and these energies are used to decide on which would be the most likely interface.

Molecular simulations of calcite seed formation

Of the three calcite faces under the Langmuir film, the (100) face yielded the lowest interfacial energy (-0.05 eV/Å2) and the (10-14) face (this rhombohedral face is the most stable one if calcite is grown inorganically) the most unfavourable one. This suggests that a Langmuir film consisting of amides containing phospholipids is not suitable to promote calcite seed formation with the (10-14) face forming the interface.

Therefore, the calcite (100) face was chosen for molecular dynamics simulations to model the dynamic development of calcite seeds in contact with Langmuir films containing amide-phospholipids (Fig. ). The hydrophilic head group in the Langmuir film consists of a phosphate group, a benzene ring, and an amide bond. The phosphate group is deprotonated during the interaction with the calcite surface and it is attracted to the Ca2+ ions at the surface. The H atoms of the phospholipids and the carbonate groups have a weak van-der-Waals attraction, which causes minimal distortion of the calcite face and helps promote the nucleation of the calcite seed. The hydrophobic benzene ring is aligned away from the calcite surface, and more tilted towards the long hydrophobic hydrocarbon chain. A close look at the orientation of organic molecules in the Langmuir film justifies the favourable interaction between the organic lattice and the calcite. The hydrophobic part of each organic compound in the film, which is the long hydrocarbon chain, and the benzene ring, are aligned at an angle of 45o with respect to the interfacial surface of organic and inorganic lattice. The hydrophilic part, the amide bond and the phosphate group are aligned almost perpendicular to the above mentioned interfacial surface. This allows maximum interaction between the hydrophilic parts of the organic molecule and the calcium atoms and carbonate group on the calcite surface. The spacing, stoichiometry, and the favourable chemical interaction between the hydrophilic part and calcite (100) face promote the nucleation of a calcite seed, as shown in Fig. . Water molecules around the calcite atoms form the hydration sphere and help in the interaction between organic and inorganic phase, thus mimicking the natural process more closely.
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