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Crystal Engineering

 

Model Studies on CaCO3 Biomineralization

 

Much attention has been devoted to the characterization of hybrid materials which occur in biominerals and calcified tissues. Among the many open questions, one of the most challenging scientific goals is to gain insights into the molecular interactions that occur at the interface between the inorganic mineral and the macromolecular organic matrix. For the most widespread calcified tissues it is frequently assumed that a structurally rigid composite matrix consisting of fibrous proteins and thereon adsorbed acidic macromolecules acts as a supramolecular “blueprint” that templates nucleation of the inorganic phase.

 

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Scheme 1: Structure model of a highly acidic pre-organized crystal nucleation site in molluscan tissues according to Addadi & Weiner (J. Struct. Biol. 2001, 135, 8-17).

 

Since three-dimensional structures of natural acidic proteins extracted from calcified tissues are currently not available, we develop synthetic molecules which mimic structural and functional aspects of their natural counterparts.

 

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Fig.1: Amphiphilic oligo-carboxylic acids which are used in our workgroup

 

Thus, we employ amphiphilic polyacids (i.e. macrocyclic calixarenes and resorcarenes, see Fig.1, or nylon oligomers) as supramolecular templates to induce a heteroepitaxial growth of CaCO3 crystals.

The amphiphilic molecules are spread at the air-water interface which gives rise to formation of monolayers. Crystallization of CaCO3 crystals underneath these monolayers (Fig.2) often leads to calcite crystals which display a uniform orientation in relation to the monolayer.
However, depending on the average surface charge of the monolayer, and its internal supramolecular organisation, switching between different CaCO3 polymorphs (i.e. calcite, aragonite, or vaterite) might occur and the shape of crystal assemblies growing underneath the monolayer can become quite intricate.
(Fig.3)

 

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Fig.2: Left: optical micrograph (brightfield) of (012) oriented calcite single crystals grown underneath a monolayer of an amphiphilic calixarene after 3h. Right: Same crystal specimen observed in plane polarized light

 

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Fig.3: Scanning electron micrograph of an assembly of vaterite (red) and aragonite crystals (blue) grown underneath the monolayer of a highly charged amphiphilic resorcarene.

 

The big challenge is to gain insights into the structure-function relationships of the templating matrix and the nucleated crystal face, and convincing solutions for this problem have been reported to date only for a few simple cases. Therefore, considerable efforts are necessary to characterize the structure of the amphiphilic matrix for which we employ surface potential measurements, Brewster-angle microscopy (Fig.4), and occasionally, grazing-incidence X-ray diffraction (GIXD).

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Fig.4: Typical Brewster-angle micrographs of monolayers of an amphiphilc calixarene at 24ºC on 10 mM CaCl2. Monolayer domains appear as light regions. (Scanned image area: 450 × 400 µm)

 

Furthermore, the single crystal X-ray structures of the amphiphilic molecules and their Ca complexes (Fig.5) are solved. The growth of CaCO3 single crystals underneath the monolayers is monitored in situ by (polarisation) optical microscopy. The orientation of the calcite crystals with respect to the monolayer is determined by means of X-ray diffraction (Fig.6), scanning electron microscopy and optical microscopy.

 

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Fig.5: Wire model of the Ca coordination polymer of an amphiphilic calixarene showing the packing arrangement of the one-dimensional polymeric strands in the crystal lattice

 

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Fig.6: X-ray powder diffraction pattern of calcite crystals grown underneath a monolayer of an amphiphilic calixarene (a), and at the air-water interface without monolayer (b). The dashed line represents the calculated diffraction pattern of randomly oriented calcite crystals

 

On the basis of these complementary informations we are capable of constructing idealized structure models of the monolayer/crystal interface which are further refined by means of theoretical methods (computer simulations).

 

In order to grow crystals of inorganic materials underneath a monolayer we have replaced the stage of an upright microscope by a miniature Langmuir trough. Crystal growth can be monitored in situ and by video imaging while the compression state of the monolayer is controlled by the trough. Crystallization underneath monolayers has many practical advantages since the water surface provides a clean and smooth interface unlikely to many solid substrates which are often used in studies of self-assembled monolayers.