Model Studies on the Function of Acidic Proteins in Biomineralization
Final Report Abstract
In the final phase of this SPP project, a series of amphiphilic acidic hairpin peptides was synthesized by solid phase peptide synthesis and characterized with respect to conformation in solution and conformation in a monolayer at the air-water interface. Moreover, four acidic peptides with hairpin structure were analysed with respect to their influence on CaCO3 crystallization beneath these peptide monolayers. This slight structure difference causes a significant variation of the CaCO3 crystals growing underneath them. The crystals were investigated by SEM, TEM, SAED, and in situ optical and BAM investigations. Unlike in solution, these peptides all adopt a β-hairpin conformation at the air/water interface due to their primary sequence with alternating hydrophilic and hydrophobic amino acid residues. The content of ordered hairpin structure could be correlated with the density of packing in the monolayer. Glu-containing peptide 1b forms a highly ordered head-to-head packing arrangement in the monolayer, while peptide 1a forms a less ordered packing arrangement. Major points of the directing effects exerted on CaCO3 crystal growth are summarized as follows: An amorphous precursor was found at the initial crystallization stage. It was shown that these amorphous precursors are composed of nanosized aggregates of crystal nuclei. Crystalline domains were found inside the amorphous layer indicating that the crystallization starts after the aggregation of the amorphous precursor. Nucleation of amorphous calcium carbonate particles mainly occured at the phase boundary of LC domains or in the LE phase of the peptide monolayers; the monolayer domain movement seems to be crucial for diffusion or aggregation of the nuclei, which controls the crystallization kinetically. In summary, non-specific factors such as the surface charge density or the molecular mean dipole moment were found to be a key factor to determine the CaCO3 crystal morphology and polymorph, which has been demonstrated for a vast variety of crystal growth model systems, employing molecular (organic) templates.6 In addition, the monolayer flexibility and the monolayer structure evolution during crystallization have been recently suggested as crucial factors in the mineralization process. Both the specific structure factors, i.e. the monolayer packing arrangement and structure flexibility or monolayer fluidity influence the crystallization in a synergetic way. The kinetic factors including monolayer fluidity influence mainly the initial nucleation process, while the structure factors including monolayer flexibility affect mainly the subsequent crystal growth stage. Therefore, the mineralization process beneath Langmuir monolayers is a dynamic process with synergetic and constant interactions with the mineral phase. Considering the complex interactions between such molecular templates and the amorphous or crystalline CaCO3 phases which nucleate and grow at the common interface, it appears somewhat unlikely that natural biomineralizing organisms might employ such templating strategies to create intricately and uniformly shaped CaCO3 mineral deposits such as nacre or coccolithophorids. In our hands, all such kinds of model systems have failed to provide substantial evidence for a direct and simple correlation between the structural organisation of the molecules forming the organic template and the crystal face(s) being in contact with the monolayer template. The investigations during the last funding period in the other hand have demonstrated the existence and importance of amorphous CaCO3 precursor phases. The mechanistic rules by which ACC phases transform into oriented arrangements of single or polycrystalline CaCO3 yet remain to be elucidated, albeit model systems have demonstrated the principal utility of this approach.
Publications
- Crystallization of Calcium Carbonate Beneath Insoluble Monolayers: Suitable Models of Mineral–Matrix Interactions in Biomineralization? Top. Curr. Chem. 270 (2007) 1–41
Fricke M, Volkmer D
(See online at https://doi.org/10.1007/128_063) - Formation of Single-Crystalline Aragonite Tablets/Films via an Amorphous Precursor. Langmuir 23 (2007) 1988–1994
Amos FF, Sharbaugh DM, Talham DR, Gower LB, Fricke M, Volkmer D
(See online at https://doi.org/10.1021/la061960n) - Synthesis of Poly(methacrylic acid) Brushes via Surface-Initiated Atom Transfer Radical Polymerization of Sodium Methacrylate and Their Use as Substrates for the Mineralization of Calcium Carbonate. Macromolecules 40 (2007) 168–177
Tugulu S, Barbey R, Harms M, Fricke M, Volkmer D, Rossi A, Klok HA
(See online at https://doi.org/10.1021/ma060739e)
