Info

Biomimetic Synthesis of Calcite Using Foldamers

Calcium carbonate is one of the most widespread biominerals found in nature. Numerous reports of the control of morphology and polymorph selectivity of calcium carbonate have been published. Calcium carbonate has six polymorphs: calcite, aragonite, vaterite, two hydrated forms and one amorphous form [90]. Of these, calcite and aragonite are the most commonly found biominerals while va-terite is rather rare. Calcite, the thermodynamically most stable form of CaCO3, crystallizes in a trigonal unit cell and has rhombohedral morphology. Aragonite crystallizes in an orthorhombic crystal system and forms needle like crystals. Finally, vaterite crystals are spherical with a hexagonal unit cell.

Ueyama et al. have reported the crystallization of CaCO3 in the presence of aromatic polyamide foldamers with regularly repeating carboxylates (Fig. 7.23) [116]. The structures are held in a fixed conformation by intramolecular H bonds as evidenced by IR and NMR. When CaCO3 crystals were grown in the presence of compound 24 the morphology of the calcite obtained was affected. Inspection of the crystals obtained with 24 revealed that the growth of the {401} face had been inhibited. This plane has a Ca2+ ion spacing of about 4.96 A between adjacent calcium ions. Since 24 linearly projects carboxylates at a repeat distance of @10A, the negative charges are optimally placed to interact with alternate Ca2+ ions. On the other hand, the trans geometry of the double bonds in 25 precludes the linear arrangement of carboxylates and it is unable to induce a morphological change in calcite. The authors also detected 24 occluded within the inorganic crystals by using cross polarization/magic angle spinning NMR techniques. This further corroborated the fact that 24 was involved in the crystallization of calcite.

In a later paper, the same group reported alternately amidated poly(acrylate)s which form 8-membered rings stabilized by hydrogen bonds between the

Fig. 7.23 (a) The polyamide 24 with carboxylates regularly repeating at a distance of 10A optimal for interaction with the calcite lattice; (b) The polyamide 25 with the trans geometry of the fumaryl spacer not suitable for interaction with the calcite lattice.

carboxylate oxygen and the amide NH (Fig. 7.24) [117]. When the poly(acid)s or their sodium salts were used as additives during the crystallization process, the CaCO3 crystals obtained were found to be predominantly the vaterite polymorph. In contrast, the crystals obtained with the calcium salts were mainly calcite. The authors proposed that these polymers control the polymorph selectivity at the nu-cleation stage presumably by stabilizing the nascent nuclei of one polymorph over the others.

Fig. 7.24 Alternately amidated poly(acrylates) showing 8-membered H-bonded rings used by Ueyama and co-workers.

Fig. 7.25 (a) The polyisocyanide-based scaffold, showing the H-bonding arrays between the side chains used by Sommerdijk and co-workers; Calcite crystals obtained in the presence of the polyisocyanide-based scaffold; (b) at low magnification; (c) at high magnification showing the apple core morphology. (Reprinted in part with permission from J. Am. Chem. Soc. 2002, 124, p. 9700. Copyright 2002 American Chemical Society).

Fig. 7.25 (a) The polyisocyanide-based scaffold, showing the H-bonding arrays between the side chains used by Sommerdijk and co-workers; Calcite crystals obtained in the presence of the polyisocyanide-based scaffold; (b) at low magnification; (c) at high magnification showing the apple core morphology. (Reprinted in part with permission from J. Am. Chem. Soc. 2002, 124, p. 9700. Copyright 2002 American Chemical Society).

Sommerdijk and co-workers have used peptide based polyisocyanides as organic templates for the crystallization of calcium carbonate [118]. These molecules adopt a helical conformation due to restricted rotation of the polymer backbone and H-bonding interactions between the dipeptide side chains (Fig. 7.25a) [119]. CD data showed that the helical conformation was further stabilized in the presence of calcium ions. Interestingly, the handedness of the helix could be changed by switching the chirality of the amino acids. When the sodium salt of poly(l-isocyanoalanyl-d-alanine) was added to the crystallization solution, the calcite crystals obtained had an ''apple core-type morphology'' (Fig. 7.25 b, c). The molecular weight and polydispersity of the polymers were shown to have no effect on crystal growth. The faces of the {hk.0} family were expressed in the crystals obtained. They identified the nucleating plane as (01.1) and suggested that the orientation of the carboxylates in the polymer mimics the orientation of the carbonates on this face. When the corresponding l,l isomer was used, the crystals obtained were not as uniform, presumably due to a less well-defined structure for the polymer as seen from its CD spectrum. This underlines the importance of well-folded structures and controlled projection of functionality to effectively

Fig. 7.26 (a) The foldamer used by Hamilton and co-workers; (b) Calcite crystals (90x) obtained in the presence of the foldamer under optical microscope (left), under scanning electron microscope (scale bar: 5 mm) (right) and control calcite crystals (inset).

modify crystal growth. Similar results were obtained with these foldamers in the case of calcium phosphate where rod-like hydroxyapatite crystals were formed [120]. From thermogravimetric analysis, the authors were able to show that the crystals contained 5% by mass of the polymer adsorbed in the crystal.

Hamilton and co-workers reported a synthetic foldamer with a polyamide backbone that is held in a linear conformation due to bifurcated H bonds and projects carboxylates in an ordered manner from one face of the molecule (Fig. 7.26a) [111]. The design was based on a a-helix mimetic that was reported earlier from the Hamilton group [50]. The side chains were designed to include carboxylates that would mimic the aspartates and recognize the arrangement of calcium ions on the surface of a growing calcite crystal. The CaCO3 crystals grown in the presence of these foldamers were shown to be calcite from the characteristic peaks in the IR spectrum at 713 and 876 cm-1. The crystals were elongated and had a "saw-tooth" morphology very different from the usual rhombohedral geometry of control crystals (Fig. 7.26b). Overgrowth experiments were performed in which the elongated calcite crystals were re-introduced into the crystallization solution to study the orientation and identity of the new crystal faces formed. From these results and modeling data, the expressed faces were identified as the {-101} where 0.5 < l < 1. The arrangement of the carbonates on these faces was proposed to be well mimicked by the carboxylates of the foldamer.

Volkmer et al. have used short peptides (4 and 8 residues) containing an alternating Phe-Asp repeat to obtain calcite crystals similar to those obtained from biological samples [121]. They assigned the newly formed faces as {11.0} and {01.2} which are also commonly found in biomineralized calcite. Although the authors have yet to determine the three-dimensional structure adopted by these peptides in solution, they hypothesized that the peptides take up preferred conformations in which the spacing of the carboxylates matches the positions of the calcium ions on the expressed crystal faces.

Fig. 7.26 (a) The foldamer used by Hamilton and co-workers; (b) Calcite crystals (90x) obtained in the presence of the foldamer under optical microscope (left), under scanning electron microscope (scale bar: 5 mm) (right) and control calcite crystals (inset).

Fig. 7.27 The peptidomimetic used by Kelly and co-workers to obtain controlled growth of CdS crystals.

Was this article helpful?

0 0

Post a comment