Scientific publications

The unicellular, simply shaped desmid Netrium digitus inhabiting acid bog ponds grows in two phases. Prior to division, the cell elongates at its central zone, whereas in a second phase, polar tip growth occurs. Electron microscopy demonstrates that Netrium is surrounded by a morphologically homogeneous cell wall, which lacks pores. Immunocytochemical and biochemical analyses give insight into physical wall properties and, thus, into adaptation to the extreme environment. The monoclonal antibodies JIM5 and JIM7 directed against pectic epitopes with different degrees of esterification label preferentially growing wall zones in Netrium. In contrast, 2F4 marks the cell wall only after experimental de-esterification. Electron energy loss spectroscopy reveals Ca-binding capacities of pectins and gives indirect evidence for the degree of their esterification. An antibody raised against Netrium mucilage is not only specific to mucilage but also recognizes wall components in transmission electron microscopy and dot blots. These results indicate a smooth transition between mucilage and the cell wall in Netrium.

The formation of defined shapes by cells is one of the challenging questions in biology. Due to the anisotropy of cell walls and of certain biominerals, the LC-PolScope represents a promising tool for tracking dynamic structural changes in vivo non-invasively and, to some extent, quantitatively. A complex three-dimensional biogenic system, the in vitro precipitation of calcium oxalate induced by cellulose stalks produced by Dictyostelium discoideum, was analyzed. Although the retardance values and orientation of the crystals with respect to the stalk were quickly and easily detected, this study raised a number of issues that were addressed in this work. The effect of the refractive index of the embedding medium was examined by taking advantage of the homogeneous size and shape distribution of kiwifruit raphides, a biologically controlled calcium oxalate biomineral and of cotton (Gossypium) seed fibers. The retardance remained consistent when embedding these samples in media with increasing refractive indices from 1.33 to 1.42 or 1.47 for sucrose or glycerol gradients, respectively. The general applicability of LC-PolScope image processing for biominerals and cell wall formation during development in vivo was demonstrated in a particular group of green algae, the Desmidiaceae. Various organization levels of the cell wall were identified, thus confirming earlier findings based on electron microscopy and immunostaining investigations. It can be concluded that LC-PolScope microscopy is an attractive tool for studying dynamic ordering of biomolecules, such as plant cell walls, when additional parameters regarding the structure, composition, and refractive indices of the specimen are available.

We use the existing data of force-extension experiments on F-actin molecules tied into knots to compute a value of 0.15 for the static friction coefficient for contact between different parts of the same molecule with itself. This estimate for protein-protein friction is relevant for the stabilization of the 273 known proteins with knots, one percent of the structures deposited in the Protein Data Bank.

The keratin structure in the cortex of peacocks' feathers is studied by X-ray diffraction along the feather, from the calamus to the tip. It changes considerably over the first 5 cm close to the calamus and remains constant for about 1 m along the length of the feather. Close to the tip, the structure loses its high degree of order. We attribute the X-ray patterns to a shrinkage of a cylindrical arrangement of beta-sheets, which is not fully formed initially. In the final structure, the crystalline beta-cores are fixed by the rest of the keratin molecule. The hydrophobic residues of the beta-core are locked into a zip-like arrangement. Structurally there is no difference between the blue and the white bird.

The aragonite-specific proteins, Pif97 and Pif80, provide new views on remote control of nacre formation. A chitinous membrane is pinned to matured nacre in certain spots until the onset of nucleation of the next layer. In this model, diffusible low-molecular-weight acidic proteins would fulfill an important role as Ca2+/H+ shuttles to avoid local acidification and balance mineralization gradients.

In quill embroidery, leather goods are beautified with patterns stitched with stripes of the cortex of peacock feathers. Our new structural, geometric and mechanical data for the tail cover feathers of the peacock allow an understanding of the mechanisms of embroidery. This study serves as an example of how biological materials can be analyzed like engineering materials, giving insight into the optimization processes during evolution.

A second-order nonlinear elastic model is developed for biogels, whereby the elastic constants are estimated from the free energy densities for polymer networks and polymer-water mixtures. For typical values of the constants, important physical insights are revealed: a biogel stiffens under tension, contracts longitudinally under torsion (inverse Poynting effect), and experiences a negative normal stress under simple shear. This approach of extracting nonlinear elastic constants works for any constitutive law, and has the advantage of exploring physical phenomena without recourse to computationally intensive simulations.

To mimic the adhesive effects of gecko toes, artificial surfaces have been manufactured recently using polydimethylsiloxanes (PDMS). However, the effects of repeated contacts on the adhesive properties remain largely unexplored. In this paper we report on the effect of repeated pull-off force measurements on the adhesion behavior of PDMS (polymer kit Sylgard 184, Dow Corning) tested with a borosilicate glass probe. A decrease in pull-off force with increase in number of test cycles is found until a plateau is reached. The initial value and the rate of change in pull-off force strongly depend on the sample preparation procedure, including curing time and cross-linking. It is proposed that the behavior is due to steady coverage of the probe with free oligomers. The results are crucial for developing reusable, durable, and residue-free bioinspired adhesives.

Porous metal thin films have high potential for use in applications such as catalysis, electrical contacts, plasmonics, as well as energy storage and conversion. Structuring metal thin films on the nanoscale to generate high surface areas poses an interesting challenge as metals have high surface energy. In this communication, we demonstrate direct access to nanostructured metal nanoparticle hybrid thin films with high nanoparticle loadings through spin coating of a mixture of block copolymer and ligand stabilized platinum and palladium nanoparticles. Plasma cleaning to remove the organics results in a conductive metal thin film. We expect that the methods described here can be generalized to other metals, mixtures of metal nanoparticles, and intermetallics. We report on direct access to nanoporous metal thin films using block copolymer self-assembly. Nanostructured metal nanoparticle hybrid thin films with high nanoparticle loadings were generated through spin coating of a mixture of block copolymer and ligand stabilized platinum and palladium nanoparticles. Plasma cleaning to remove the organics results in a conductive metal thin film. We expect that the methods described here can be generalized to other metals, mixtures of metal nanoparticles and intermetallics.