Hendrik Hähl

Self-assembly of cellulose nanofibrils by genetically engineered fusion proteins

One central problem for the function and manufacture of materials where performance relies on nanoscale structure is to control the compatibility and interactions of the building blocks. In natural materials, such as nacre, there are examples of multifunctional macromolecules that have combined binding affinities for different materials within the same molecule, thereby bridging these materials and acting as a molecular glue. Here, we describe the use of a designed multifunctional protein that is used for self-assembly of nanofibrillar cellulose. Recent advances in the production of cellulose nanofibrils have given inspiration for new uses of cellulosic materials. Cellulose nanofibrils have mechanical and structural features that open new possibilities for performance in composites and other nanoscale materials. Functionalisation was realised through a bi-functional fusion protein having both an ability to bind to cellulose and a second functionality of surface activity. The cellulose-binding function was obtained using cellulose-binding domains from cellulolytic enzymes and the surface activity through the use of a surface active protein called hydrophobin. Using the bi-functional protein, cellulose nanofibrils could be assembled into tightly packed thin films at the air/water interface and at the oil/water interface. It was shown that the combination of protein and cellulose nanofibrils resulted in a synergistic improvement in the formation and stability of oil-in-water emulsions resulting in emulsions that were stable for several months. The bi-functionality of the protein also allowed the binding of hydrophobic solid drug nanoparticles to cellulose nanofibrils and thereby improving their long-term stability under physiological conditions.

Subsurface influence on the structure of protein adsorbates as revealed by in situ X-ray reflectivity.

The adsorption process of proteins to surfaces is governed by the mutual interactions among proteins, the solution, and the substrate. Interactions arising from the substrate are usually attributed to the uppermost atomic layer. This actual surface defines the surface chemistry and hence steric and electrostatic interactions. For a comprehensive understanding, however, the interactions arising from the bulk material also have to be considered. Our protein adsorption experiments with globular proteins (α-amylase, bovine serum albumin, and lysozyme) clearly reveal the influence of the subsurface material via van der Waals forces. Here, a set of functionalized silicon wafers enables a distinction between the effects of surface chemistry and the subsurface composition of the substrate. Whereas the surface chemistry controls whether the individual proteins are denatured, the strength of the van der Waals forces affects the final layer density and hence the adsorbed amount of proteins. The results imply that van der Waals forces mainly influence surface processes, which govern the structure formation of the protein adsorbates, such as surface diffusion and spreading.

Protein adsorption on tailored substrates: long-range forces and conformational changes

Adsorption of proteins onto solid surfaces is an everyday phenomenon that is not yet fully understood. To further the current understanding, we have performed in situ ellipsometry studies to reveal the adsorption kinetics of three different proteins, lysozyme, α-amylase and bovine serum albumin. As substrates we offer Si wafers with a controlled Si oxide layer thickness and a hydrophilic or hydrophobic surface functionalization, allowing the tailoring of the influence of short- and long-range interactions. Our studies show that not only the surface chemistry determines the properties of an adsorbed protein layer but also the van der Waals contributions of a composite substrate. We compare the experimental findings to results of a colloidal Monte Carlo approach that includes conformational changes of the adsorbed proteins induced by density fluctuations.

Adhesion and tribological properties of hydrophobin proteins in aqueous lubrication on stainless steel surfaces

Macroscale tribological properties of hydrophobin layers bound on stainless steel surfaces were investigated in an aqueous environment. Emphasis was on boundary lubrication because water easily fails in hydrodynamic lubrication due to its low viscosity. We studied the affinities of two different proteins, HFBI and FpHYD5, on stainless steel and their ability to bind water at the surface by combining quartz crystal microbalance (QCM-D) and ellipsometry. Both proteins contained an adhesive hydrophobic domain, but FpHYD5 also had a very strongly hydrating carbohydrate structure attached to it. The lubrication properties of the proteins were studied with two different methods, pin-on-disc (POD) (stainless steel vs. stainless steel) and circular translation pin-on-disc (CTPOD) (UHMWPE vs. stainless steel). It was observed that both hydrophobins could adhere to the stainless steel surface and form highly hydrated layers. Both proteins reduced friction and wear of the sliding contact between two stainless steel surfaces. With UHMWPE against stainless steel, the hydrophobins prevented the polyethylene transfer to the counterface. The lowest coefficient of friction (COF) 0.13 was observed when FpHYD5 hydrophobins were employed in pure water. On the other hand, the lowest wear was observed when FpHYD5 proteins were added in a 50 mM sodium acetate buffer. Increasing the water content and loosening the hydrophobin film structure on the stainless steel surface led to a reduction in friction and wear.

Influence of the Subsurface Composition of a Material on the Adhesion of Staphylococci

Controlling the interface between bacteria and solid materials has become an important task in biomedical science. For a fundamental and comprehensive understanding of adhesion it is necessary to seek quantitative information about the involved interactions. Most studies concentrate on the modification of the surface (chemical composition, hydrophobicity, or topography) neglecting, however, the influence of the bulk material, which always contributes to the overall interaction via van der Waals forces. In this study, we applied AFM force spectroscopy and flow chamber experiments to probe the adhesion of Staphylococcus carnosus to a set of tailored Si wafers, allowing for a separation of short- and long-range forces. We provide experimental evidence that the subsurface composition of a substrate influences bacterial adhesion. A coarse estimation of the strength of the van der Waals forces via the involved Hamaker constants substantiates the experimental results. The results demonstrate that the uppermost layer is not solely responsible for the strength of adhesion. Rather, for all kinds of adhesion studies, it is equally important to consider the contribution of the subsurface.

Is adhesion superficial? Silicon wafers as a model system to study van der Waals interactions

Adhesion is a key issue for researchers of various fields, it is therefore of uppermost importance to understand the parameters that are involved. Commonly, only surface parameters are employed to determine the adhesive forces between materials. Yet, van der Waals forces act not only between atoms in the vicinity of the surface, but also between atoms in the bulk material. In this review, we describe the principles of van der Waals interactions and outline experimental and theoretical studies investigating the influence of the subsurface material on adhesion. In addition, we present a collection of data indicating that silicon wafers with native oxide layers are a good model substrate to study van der Waals interactions with coated materials.

Pure Protein Bilayers and Vesicles from Native Fungal Hydrophobins.

Pure protein bilayers and vesicles are formed using the native, fungal hydrophobin HFBI. Bilayers with hydrophobic (red) and hydrophilic (blue) core are produced and, depending on the type of core, vesicles in water, oily media, and even in air can be created using microfluidic jetting. Vesicles in water are even able to incorporate functional gramicidin A pores.

Influence of bidisperse self-assembled monolayer structure on the slip boundary condition of thin polymer films

Alkylsilane self-assembled monolayers (SAMs) are often used as model substrates for their ease of preparation and hydrophobic properties. We have observed that these atomically smooth monolayers also provide a slip boundary condition for dewetting films composed of unentangled polymers. This slip length, an indirect measure of the friction between a given liquid and different solids, is switchable and can be increased [R. Fetzer et al., Phys. Rev. Lett. 95, 127801 (2005); O. Bäumchen et al., J. Phys.: Condens. Matter 24, 325102 (2012)] if the alkyl chain length is changed from 18 to 12 backbone carbons, for example. Typically, this change in boundary condition is affected in a quantized way, using one or the other alkyl chain length, thus obtaining one or the other slip length. Here, we present results in which this SAM structure is changed in a continuous way. We prepare bidisperse mixed SAMs of alkyl silanes, with the composition as a control parameter. We find that all the mixed SAMs investigated show an enhanced slip boundary condition as compared to the single-component SAMs. The slip boundary condition is accessed using optical and atomic force microscopy, and we describe these observations in the context of X-ray reflectivity measurements. The slip length, varying over nearly two orders of magnitude, of identical polymer melts on chemically similar SAMs is found to correlate with the density of exposed alkyl chains. Our results demonstrate the importance of a well characterized solid/liquid pair, down to the angstrom level, when discussing the friction between a liquid and a solid.

Single-Molecule Force Spectroscopy Study on Modular Resilin Fusion Protein

The adhesive and mechanical properties of a modular fusion protein consisting of two different types of binding units linked together via a flexible resilin-like-polypeptide domain are quantified. The adhesive domains have been constructed from fungal cellulose-binding modules (CBMs) and an amphiphilic hydrophobin HFBI. This study is carried out by single-molecule force spectroscopy, which enables stretching of single molecules. The fusion proteins are designed to self-assemble on the cellulose surface, leading into the submonolayer of proteins having the HFBI pointing away from the surface. A hydrophobic atomic force microscopy (AFM) tip can be employed for contacting and lifting the single fusion protein from the HFBI-functionalized terminus by the hydrophobic interaction between the tip surface and the hydrophobic patch of the HFBI. The work of rupture, contour length at rupture and the adhesion forces of the amphiphilic end domains are evaluated under aqueous environment at different pHs.