Wolfgang Meyer-Zaika

Switchable supramolecular polymers from the self-assembly of a small monomer with two orthogonal binding interactions.

The low molecular weight heteroditopic monomer 1 forms supramolecular polymers in polar solution as shown, for example, by infrared laser-based dynamic light scattering (DLS), small-angle neutron scattering (SANS), electron microscopy (TEM, cryo-TEM), and viscosity measurements. Self-assembly of 1 is based on two orthogonal binding interactions, the formation of a Fe(II)-terpyridine 1:2 metal-ligand complex and the dimerization of a self-complementary guanidiniocarbonyl pyrrole carboxylate zwitterion. Both binding interactions have a sufficient stability in polar (DMSO) and even aqueous solutions to ensure formation of linear polymers of considerable length (up to 100 nm). The supramolecular polymerization follows a ring-chain mechanism causing a significant increase in the viscosity of the solutions at millimolar concentrations and above. The linear polymers then further aggregate in solution into larger globular aggregates with a densely packed core and a loose shell. Both binding interactions can be furthermore switched on and off either by adding a competing ligand to remove the metal ion and subsequent readdition of Fe(II) or by reversible protonation and deprotonation of the zwitterion upon addition of acid or base. The self-assembly of 1 can therefore be switched back and forth between four different states, the monomer, a metal-complexed dimer or an ion paired dimer, and finally the polymer.

Transfection of cells with custom-made calcium phosphate nanoparticles coated with DNA

Functionalised nanoparticles of calcium phosphate were prepared by controlled precipitation from aqueous solution, followed by coating with DNA. A successful transfection of transformed human endothelial cells was accomplished by adding the dispersion of nanoparticles to the cell culture. The functionalised nanoparticles formed a stable colloid and did not lose their ability for cell transfection during 2–3 weeks of storage. The nanoparticles were approximately spherical with diameters of 10 to 20 nm and covered and stabilised by a hull of DNA.

Functionalisation of calcium phosphate nanoparticles by oligonucleotides and their application for gene silencing

In molecular biology, the production of proteins can be effectively inhibited by introducing specific oligonucleotides into a living cell (gene silencing or antisense strategy; important for gene therapy). Calcium phosphate nanoparticles can serve as carriers for biomolecules in such therapeutic applications due to their high biocompatibility and biodegradability. Stable colloids were prepared by coating the inorganic nanoparticles with single- and double-stranded oligonucleotides. The dispersions were analysed by dynamic light scattering, zeta potential measurements, transmission electron microscopy, and scanning electron microscopy. Particles with a diameter of about 100 nm were obtained under optimized conditions. The efficiency of such nanoparticles to specifically inhibit protein synthesis was tested on HeLa-EGFP cells whose green fluorescence was turned off by the coated nanoparticles (gene silencing with siRNA). If siRNA was incorporated into the calcium phosphate particle and thereby protected from intracellular degradation, the transfection efficiency was significantly increased. The dispersions were stable and could be stored at 4 °C without loss of activity for several weeks, making them available as biochemical reagents.

Strategies for the Anchoring of Metal Complexes, Clusters, and Colloids Inside Nanoporous Alumina Membranes

Two complementary strategies are presented for the anchoring of molecular palladium complexes, of cobalt or platinum clusters or of gold colloids inside the nanopores of alumina membranes. The first consists in the one step condensation of an alkoxysilyl functional group carried by the metal complex with the hydroxy groups covering the surface of the membrane pores. Thus, using the short-bite alkoxysilyl-functionalized diphosphane ligands (Ph2P)2N(CH2)3Si(OMe)3 (1) and (Ph2P)2N(CH2)4SiMe2(OMe)] (2) derived from (Ph2P)2NH (dppa) (dppa bis(diphenylphosphanyl)amine), the palladium complexes [Pd(dmba)(kappa2-P,P-(Ph2P)2N(CH2)3Si(OMe)3)] Cl (3) and [Pd(dmba)[kappa2-P,P-(Ph2P)2N(CH2)4SiMe2(OMe)]]Cl (4) (dmba-H = dimethylbenzylamine). respectively, were tethered to the pore walls. After controlled thermal treatment. confined and highly dispersed palladium nanoparticles were formed and characterized by transmission electron microscopy (TEM). This method could not be applied to the cobalt cluster [Co4(CO)8(mu-dppa)[mu-P,P-(Ph2P)2N(CH2)4SiMe2(OMe)]] (7) owing to its too limited solubility. However, its anchoring was achieved by using the second method which consisted of first derivatizing the pore walls with 1 or 2. The covalent attachment of the diphosphane ligands provides a molecular anchor that allows subsequent reaction with the cluster [Co4(CO)10(mu-dppa)] 6 to generate anchored 7 and this step was monitored by UV/Vis spectroscopy. In addition, the presence of carbonyl ligands in the cluster provides for the first time a very sensitive spectroscopic probe in the IR region which confirms both cluster incorporation and the retaining of its molecular nature inside the membrane. The presence of the bridging dppa ligand in 6 provides additional stabilization and accounts for the selectivity of the procedure. Using this method, platinum clusters (diameter ca. 2 nm) and gold colloids (diameter ca. 13 nm) were immobilized after passing their solution through the functionalized membrane pores. The resulting membranes were characterized by TEM which demonstrated the efficiency of the complexation and showed the high dispersion of the metal loading. The successful application of these methods has demonstrated that nanoporous alumina membranes are not only unique supports to incorporate metal complexes, clusters, or colloids but can also be regarded as functional matrices or microreactors, thus opening new fields for applications.

Naked Au55 Clusters: Dramatic Effect of a Thiol‐Terminated Dendrimer

Reaction of the thiol-terminated fourth-generation dendrimer 2-G4 (96 SH groups) with the gold cluster compound Au55(PPh3)12Cl6 in a 3:1 molar ratio in dichloromethane results in the formation of bare Au55 clusters. The cuboctahedrally shaped Au55 particles coalesce to well-formed microcrystals (Au55) infinity. The role of the dendrimer is not only to remove the phosphine and chlorine ligands but also to act as an ideal matrix for perfect crystal growth. Transmission electron microscopy (TEM), small- and wide-angle X-ray diffraction (SAXRD and WAXRD) measurements indicate a structure where rows of edge-linked Au55 building blocks form a distorted cubic lattice. The X-ray data fit best if a 5% reduction of the Au-Au bond length in the Au55 clusters is assumed, in agreement with previous extended X-ray absorption fine structure (EXAFS) measurements. Energy-dispersive X-ray spectroscopy (EDX) analyses and IR investigations show the absence of PPh3 and Cl in the microcrystals.

Rational design of β-sheet ligands against Aβ42-induced toxicity.

A β-sheet-binding scaffold was equipped with long-range chemical groups for tertiary contacts toward specific regions of the Alzheimers Aβ fibril. The new constructs contain a trimeric aminopyrazole carboxylic acid, elongated with a C-terminal binding site, whose influence on the aggregation behavior of the Aβ(42) peptide was studied. MD simulations after trimer docking to the anchor point (F19/F20) suggest distinct groups of complex structures each of which featured additional specific interactions with characteristic Aβ regions. Members of each group also displayed a characteristic pattern in their antiaggregational behavior toward Aβ. Specifically, remote lipophilic moieties such as a dodecyl, cyclohexyl, or LPFFD fragment can form dispersive interactions with the nonpolar cluster of amino acids between I31 and V36. They were shown to strongly reduce Thioflavine T (ThT) fluorescence and protect cells from Aβ lesions (MTT viability assays). Surprisingly, very thick fibrils and a high β-sheet content were detected in transmission electron microscopy (TEM) and CD spectroscopic experiments. On the other hand, distant single or multiple lysines which interact with the ladder of stacked E22 residues found in Aβ fibrils completely dissolve existing β-sheets (ThT, CD) and lead to unstructured, nontoxic material (TEM, MTT). Finally, the triethyleneglycol spacer between heterocyclic β-sheet ligand and appendix was found to play an active role in destabilizing the turn of the U-shaped protofilament. Fluorescence correlation spectroscopy (FCS) and sedimentation velocity analysis (SVA) provided experimental evidence for some smaller benign aggregates of very thin, delicate structure (TEM, MTT). A detailed investigation by dynamic light scattering (DLS) and other methods proved that none of the new ligands acts as a colloid. The evolving picture for the disaggregation mechanism by these new hybrid ligands implies transformation of well-ordered fibrils into less structured aggregates with a high molecular weight. In the few cases where fibrillar components remain, these display a significantly altered morphology and have lost their acute cellular toxicity.

Gold nanoparticles: dispersibility in biological media and cell-biological effect

Spherical gold nanoparticles with a hydrodynamic diameter between 25 and 37 nm were prepared and stabilised with poly(N-vinylpyrrolidone) (PVP) or tris(sodium-m-sulfonatophenyl)phosphine (TPPTS). They were subjected to different cell culture media, e.g. pure RPMI, RPMI containing up to 10% of fetal calf serum (FCS), and RPMI containing up to 10% of bovine serum albumin (BSA), and the rate of agglomeration was studied by dynamic light scattering. In pure RPMI, a strong agglomeration was observed whereas in the RPMI–FCS and RPMI–BSA mixtures the particles remained well dispersed above 1 wt% protein concentration. The effect of PVP-stabilised gold nanoparticles on human mesenchymal stem cells (hMSC) was studied as well. No significant influence on the viability and chemotaxis was observed after incubation of hMSC with gold nanoparticles. However, gold nanoparticles induced the activation of hMSC as indicated by the release of IL-6 and IL-8.

Cooperative self-assembly of discoid dimers: hierarchical formation of nanostructures with a pH switch.

Derivatives of the self-complementary 2-guanidiniocarbonyl pyrrole 5-carboxylate zwitterion (1) (previously reported by us to dimerize to 1•1 with an aggregation constant of ca. >10(10) M(-l) in DMSO) aggregate in a diverse manner depending on, e.g., variation of concentration or its protonation state. The mode of aggregation was analyzed by spectroscopic (NMR, UV) and microscopic (AFM, SEM, HIM, and TEM) methods. Aggregation of dimers of these zwitterions to higher supramolecular structures was achieved by introduction of sec-amide substituents at the 3-position, i.e., at the rearward periphery of the parent binding motif. A butyl amide substituent as in 2b enables the discoid dimers to further aggregate into one-dimensional (rod-like) stacks. Quantitative UV dilution studies showed that this aggregation is strongly cooperative following a nucleation elongation mechanism. The amide hydrogen seems to be essential for this rod-like aggregation, as neither 1 nor a corresponding tert-amide congener 2a form comparable structures. Therefore, a hydrogen bond-assisted π-π-interaction of the dimeric zwitterions is suggested to promote this aggregation mode, which is further affected by the nature of the amide substituent (e.g., steric demand), enabling the formation of bundles of strands or even two-dimensional sheets. By exploiting the zwitterionic nature of the aggregating discoid dimers, a reversible pH switch was realized: dimerization of all compounds is suppressed by protonation of the carboxylate moiety, converting the zwitterions into typical cationic amphiphiles. Accordingly, typical nanostructures like vesicles, tubes, and flat sheets are formed reversibly under acidic conditions, which reassemble into the original rod-like aggregates upon readjustment to neutral pH.

Continuous preparation of functionalised calcium phosphate nanoparticles with adjustable crystallinity

Calcium phosphate nanoparticles can be prepared in almost uniform size and shape by a continuous precipitation process that also allows their functionalisation by organic molecules (DNA, surfactants).

Calcium phosphate nanoparticles as templates for nanocapsules prepared by the layer-by-layer technique

Uniform nanocapsules with typical diameters of 150–240 nm were prepared with calcium phosphate nanoparticles as a fully biocompatible core material by alternate coating with cationic (PAH) and anionic (PSS) polymers (layer-by-layer technique: LbL), followed by dissolution of the calcium phosphate core with hydrochloric acid by dialysis in the presence of ion exchangers. The nanocapsules were characterized by dynamic light scattering (DLS), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (TEM), and atomic force microscopy (AFM).