Antje Völkel

Stable Prenucleation Calcium Carbonate Clusters

Calcium carbonate forms scales, geological deposits, biominerals, and ocean sediments. Huge amounts of carbon dioxide are retained as carbonate ions, and calcium ions represent a major contribution to water hardness. Despite its relevance, little is known about the precipitation mechanism of calcium carbonate, and specified complex crystal structures challenge the classical view on nucleation considering the formation of metastable ion clusters. We demonstrate that dissolved calcium carbonate in fact contains stable prenucleation ion clusters forming even in undersaturated solution. The cluster formation can be characterized by means of equilibrium thermodynamics, applying a multiple-binding model, which allows for structural preformation. Stable clusters are the relevant species in calcium carbonate nucleation. Such mechanisms may also be important for the crystallization of other minerals.

A β/γ Motif to Mimic α-Helical Turns in Proteins

The attempt to construct nature’s architecture from nonnatural building blocks has challenged scientists for many decades. One goal of this field of study is to overcome the intrinsic protease susceptibility of natural peptides as it limits their clinical use. Peptides composed of homologous amino acids, those that have additional backbone methylene units compared to the natural a-amino acids, are at present among the most widely studied biomimetic oligomers that adopt well-defined conformations (foldamers). The wide variety of specific secondary structures that can be adopted by band g-peptides becomes especially valuable for the design of higher levels of organization such as tertiary or quaternary structures. Previous efforts towards this goal employing b-amino acid building blocks led to the discovery of both homomeric and heteromeric helix bundles and helical inhibitors of protein–protein interactions; however, there are significant differences between the packing observed in these artificial quaternary assemblies and that in the corresponding natural assemblies. This phenomenon has thus far impeded the combination of both classes into compact protein-like chimeric structures. The aim of the current study was to identify extended sequences of band g-amino acids that can be incorporated into an a-helical coiled coil to produce artificial chimeric folding motifs. Such artificial motifs with their orthogonal structural elements are great candidates for incorporation into natural helical proteins. Because protein–protein interactions involving helical domains determine specificity for important biological processes such as transcriptional control, cellular differentiation, and replication, selective disruption should be an excellent strategy for drug discovery. We were inspired by previous reports in which the principle of “equal backbone atoms” was suggested. Those designs were based on either unsubstituted or conformationally constrained amino acids. In particular b/g-hybrid peptides appear to be well-suited to mimic an a-helical conformation, thus we focused on preserving the natural side chains for the purpose of accurately imitating the natural packing in order to lend stability to the assembly. The a-helical coiled coil is a well-conserved and versatile folding motif that can serve as a model for tertiary and quaternary protein structures. This motif features a canonical heptad repeat, (abcdefg)n, in which hydrophobic residues occupy the a and d positions; these side chains make up the hydrophobic core of the interhelical interface. Charged residues at e and g generally form the second molecular recognition motif by interhelical ionic interactions. One such characteristic heptad, comprising three 13-atom hydrogen-bonded turns of the helix, can be substituted by a pentad repeat of alternating band g-amino acids with retention of the helix dipole and the formation of two 13-membered helix turns. The peptide model system described here comprises a basic a-peptide “Base-pp” which has a high propensity for heterooligomerization to an a-helical coiled coil in the presence of the acidic peptide “Acid-pp” (Figure 1 A). Heterooligomerization is driven by the burial of hydrophobic surface area, primarily contributed by Leu, and is directed by electrostatic interactions between Lys and Glu residues that flank the hydrophobic core. To evaluate b/g-hybrid peptides as a-helix mimics, the two central turns of Base-pp (positions 15–21) were replaced by a pentad of alternating band g-amino acid residues in the chimera B3b2g (Figure 1 B and C). CD spectroscopy (Figure 2 A) indicates random coil and mostly unfolded conformations for B3b2g and Acid-pp, respectively, as was expected based on the design of positions e and g. In contrast, an equimolar mixture of B3b2g and Acid-pp shows significant a-helical structure formation with two well defined minima at 208 and 222 nm. Analysis of the ellipticity at 222 nm as a function of the mole fraction of B3b2g reveals a global minimum at 0.5 (inset in Figure 2 A), which corresponds to the presence of a heteromeric assembly between Acid-pp and B3b2g with 1:1 stoichiometry. Size exclusion chromatography (SEC) was performed to characterize the oligomerization states of the peptides described above. Comparison of retention times with the peptides GCN4-p1, GCN4-pII, and GCN4-pLI as investigated by Harbury et al. suggests the presence of monomeric species (64 min) for the individual peptides Acid-pp, Base-pp and B3b2g, but the formation of four-helix-bundles (57 min) in the equimolar mixtures Acid-pp/Base-pp and Acid-pp/B3b2g (Figure 2 B). Also, [a] R. Rezaei Araghi, M. Salwiczek, S. C. Wagner, S. Wieczorek, Prof. Dr. B. Koksch Institute of Chemistry and Biochemistry, Freie Universit t Berlin Takustraße 3, 14195 Berlin (Germany) Fax: (+ 49) 30-83855644 E-mail : koksch@chemie.fu-berlin.de [b] Dr. C. J ckel Laboratory of Organic Chemistry, Eidgençssische Technische Hochschule Wolfgang-Paulistrasse 10, 8093 Z rich (Switzerland) [c] Dr. H. Cçlfen, A. Vçlkel Department of Colloid Chemistry, Max-Planck-Institute of Colloids and Interfaces 14424 Potsdam (Germany) [d] Dr. C. Baldauf BioQuant, Ruprecht-Karls-Universit t Heidelberg Im Neuenheimer Feld 267, 69120 Heidelberg (Germany) [e] Dr. C. Baldauf MPG-CAS Partner Institute for Computational Biology 320 Yue Yang Road, 200031 Shanghai (P. R. China) Fax: (+ 86) 21-54920451 E-mail : carsten@picb.ac.cn Supporting information for this article is available on the WWW under http ://dx.doi.org/10.1002/cbic.200900700.

Hybrid colloid analysis combining analytical ultracentrifugation and flow-field flow fractionation

Ferritin, the iron storage protein, is an organic-inorganic hybrid colloid consisting of a hollow protein capsule, which is filled with ferrihydride with up to 4500 iron atoms. Owing to the varying iron content and the resulting density differences, as well as the protein oligomerization, a particle size distribution is superimposed with a density distribution, making a precise analysis of ferritin by analytical ultracentrifugation difficult. This study describes how the information of the sedimentation coefficient distribution can be combined with the diffusion coefficient distribution obtained from flow-field flow fractionation to yield the buoyant molar mass of the oligomers in the mixture, extending the information content of each individual analytical method. In addition, the sedimentation and diffusion coefficients are compatible with a simple hard-sphere aggregation model, suggesting that the ferritin oligomers up to the pentamer have a globular solution structure.

Analytical Ultracentrifugation of Model Nanoparticles: Comparison of Different Analysis Methods

Sedimentation analysis of nanoparticle (ZrO(2) and SiO(2)) suspensions of different particle sizes in various solvents as nanoparticle model systems was carried out using interference optics in the analytical ultracentrifuge. The particles differed in their morphology: SiO(2) particles were spherical, whereas ZrO(2) particles were in one case spherical and in the second case non-spherical and spherical. Different analysis programs, based on different principles of data analysis, were used for the evaluation of the size distributions of these particles, viz. SEDFIT [ls-g*(s), c(s)], UltraScan (vHW, 2DSA-MC), SedAnal (dcdt) and VelXLAI (GFL) method and SedAnal, in order to ascertain the benefits and limitations of these analysis methods in characterising the examined nanoparticles.

Poly(ethylene oxide)-b-poly(3-sulfopropyl methacrylate) Block Copolymers for Calcium Phosphate Mineralization and Biofilm Inhibition

Poly(ethylene oxide) (PEO) has long been used as an additive in toothpaste, partly because it reduces biofilm formation on teeth. It does not, however, reduce the formation of dental calculus or support the remineralization of dental enamel or dentine. The present article describes the synthesis of new block copolymers on the basis of PEO and poly(3-sulfopropyl methacrylate) blocks using atom transfer radical polymerization. The polymers have very large molecular weights (over 10(6) g/mol) and are highly water-soluble. They delay the precipitation of calcium phosphate from aqueous solution but, upon precipitation, lead to relatively monodisperse hydroxyapatite (HAP) spheres. Moreover, the polymers inhibit the bacterial colonization of human enamel by Streptococcus gordonii, a pioneer bacterium in oral biofilm formation, in vitro. The formation of well-defined HAP spheres suggests that a polymer-induced liquid precursor phase could be involved in the precipitation process. Moreover, the inhibition of bacterial adhesion suggests that the polymers could be utilized in caries prevention.

Application of the density variation method on calcium carbonate nanoparticles

The simultaneous determination of particle size and density distributions by the so called density variation method via measurement of the same sample in H 2 O and D 2 O proved to be of great value for latex systems. However, many colloids of practical interest are inorganic or inorganic-organic hybrid colloids. Their density is usually much higher than that of the solvents so that the density variation method appears of limited applicability. In addition, these systems are usually charged - a complication, which was so far not yet considered in the theory for the density variation method. In this work, we apply this method to the determination of the particle size and density of CaCO 3 precursor particles, which form superstructures in order to elucidate their crystal modification. Interestingly, the particle densities can be determined rather reasonably, whereas the particle size is much more influenced by the nonideality.

Poly(ethylene oxide)-based block copolymers with very high molecular weights for biomimetic calcium phosphate mineralization

The present article is among the first reports on the effects of poly(ampholyte)s and poly(betaine)s on the biomimetic formation of calcium phosphate. We have synthesized a series of di- and triblock copolymers based on a non-ionic poly(ethylene oxide) block and several charged methacrylate monomers, 2-(trimethylammonium)ethyl methacrylate chloride, 2-((3-cyanopropyl)-dimethylammonium)ethyl methacrylate chloride, 3-sulfopropyl methacrylate potassium salt, and [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide. The resulting copolymers are either positively charged, ampholytic, or betaine block copolymers. All the polymers have very high molecular weights of over 106 g mol−1. All polymers are water-soluble and show a strong effect on the precipitation and dissolution of calcium phosphate. The strongest effects are observed with triblock copolymers based on a large poly(ethylene oxide) middle block (nominal Mn = 100 000 g mol−1). Surprisingly, the data show that there is a need for positive charges in the polymers to exert tight control over mineralization and dissolution, but that the exact position of the charge in the polymer is of minor importance for both calcium phosphate precipitation and dissolution.

Modification of salt-templated carbon surface chemistry for efficient oxidation of glucose with supported gold catalysts

Gold nanoparticles dispersed on high‐surface‐area carbon materials were investigated as heterogeneous catalysts for the selective oxidation of d‐glucose to d‐gluconic acid in aqueous solution with molecular oxygen. Salt‐templated porous carbon supports were obtained from different precursors with and without nitrogen and treated under air or hydrogen atmosphere to functionalize the surface with nitrogen, oxygen, or hydrogen. The influence of the surface atomic structure of the carbonaceous supports with similar pore structure on the size and catalytic properties of the metallic nanoparticles was studied at gold nanoparticle loadings of 0.4–0.7 wt %. The functionalisation significantly influences the surface polarity of the support materials and the strength of the interaction with the gold nanoparticles. The surface polarity influences the structure and properties of the catalysts because both the gold deposition and the glucose oxidation reaction take place in the aqueous phase. Rather hydrophilic supports are obtained by doping with oxygen and nitrogen and lead to large gold nanoparticles with low catalytic activity. In contrast, the rather hydrophobic as‐made and hydrogen‐treated supports provide higher catalytic activity (metal time yield up to 1.5 molGlucose molAu−1 s−1) resulting from their smaller gold particles of 3–5 nm in diameter.