Bacterial crystalline surface layers (S-layers) are the outermost envelope of prokaryotic organisms representing the simplest biological membranes developed during evolution. In this context, the bacterial protein SbpA has already shown its intrinsic ability to reassemble on different substrates forming protein crystals of square lattice symmetry. In this work, we present the interaction between the bacterial protein SbpA and five self-assembled monolayers carrying methyl (CH₃), hydroxyl (OH), carboxylic acid (COOH) and mannose (C₆H₁₂O₆) as functional groups. Protein adsorption and S-layer formation have been characterized by atomic force microscopy (AFM) while protein adsorption kinetics, mass uptake and the protein layer viscoelastic properties were investigated with quartz crystal microbalance with dissipation monitoring (QCM-D). The results indicate that the protein adsorption rate and crystalline domain area depend on surface chemistry and protein concentration. Furthermore, electrostatic interactions tune different protein rate adsorption and S-layer recrystallization pathways. Electrostatic interactions induce faster adsorption rate than hydrophobic or hydrophilic interactions. Finally, the shear modulus and the viscosity of the recrystallized S-layer on CH₃C₆S, CH₃C₁₁S and COOHC₁₁S substrates were calculated from QCM-D measurements. Protein–protein interactions seem to play a main role in the mechanical stability of the formed protein (crystal) bilayer.

We investigate the electrospinning of small molecules, specifically designed peptide derivatives of the pyrazole-isothiazole scaffold. Such non-natural peptides enhance the spectrum of fundamental materials used for electrospinning. Unlike standard electrospun materials, our peptides are not polymeric, but able to aggregate in solution and especially during processing. They contain donor/acceptor groups that can form hydrogen bonds, and groups that are able to generate π-stacking interactions, which are known as important requirements for assembly processes. The pyrazole-isothiazole derivatives were synthesized by means of a 1,3-dipolar cycloaddition reaction, which is completely regioselective, affording only one isomer. We demonstrate that our compounds can be electrospun from fluoroalcohol solution into solid, quasi-endless micro- and nanofibers. The electrospinnability varies substantially, depending on the amino acids linked to the scaffold. Some compounds provide only short fibers, while Fmoc-glycyl-(N-benzyl)-pyrazole-isothiazole-tert-butyl carboxylate-1,1-dioxide forms continuous, homogenous, and bead-free fibers (droplet-like beads are a common problem in electrospinning). We analyzed the compounds and the fibers with various spectroscopic techniques (MS, IR and Raman). Electrospinning does not change chemical composition and configuration, suggesting the monomeric form of the compounds even in the fibers. Interestingly, we found that the stereochemistry of the scaffold can affect the ability of the peptide to be electrospun.

High-resolution microscopy techniques have been extensively used to investigate the structure of soft, biological matter at the nanoscale, from very thin membranes to small objects, like viruses. Electron microscopy techniques allow for obtaining extraordinary resolution by averaging signals from multiple identical structures. In contrast, atomic force microscopy (AFM) collects data from single entities. Here, it is possible to finely modulate the interaction with the samples, in order to be sensitive to their top surface, avoiding mechanical deformations. However, most biological surfaces are highly curved, such as fibers or tubes, and ultimate details of their surface are in the vicinity of steep height variations. This limits lateral resolution, even when sharp probes are used. We overcome this problem by using multifrequency force microscopy on a textbook example, the Tobacco Mosaic Virus (TMV). We achieved unprecedented resolution in local maps of amplitude and phase shift of the second excited mode, recorded together with sample topography. Our data, which combine multifrequency imaging and Fourier analysis, confirm the structure deduced from averaging techniques (XRD, cryoEM) for surface features of single virus particles, down to the helical pitch of the coat protein subunits, 2.3 nm. Remarkably, multifrequency AFM images do not require any image postprocessing.

Biomolecules, cells and the cellular environment have characteristic mechanical properties that determine a range of biological responses. The affected responses include the differentiation and phenotypic expression of cells, an area that has gained prominence due to the current interest in the control of stem cell development. Recent research on biomaterials includes many measurements that have been made on a micro or nano-scale and which are not well described by continuum models. The focus of this review is on the integration and comparison of information obtained from different experimental techniques: mechanical properties are discussed in terms of the wide range of molecular motions and relaxation times that are characteristic of biological materials. Starting at the smaller end of the scale, one component which will be almost universally present in biomolecular samples is water; although bulk water has a relaxation time that would make it fluid in typical experiments, interfacial water and water in confined films will exhibit much slower motions and may therefore show an elastic response, depending on the experimental technique used for the measurements. Water at the surface of hydrophilic solids may thus appear elastic when characterised using high-frequency acoustic devices such as the quartz crystal microbalance (QCM), although the layer will still be fluid in AFM measurements at typical load rates. Likewise, lipid bilayers are viscous at low shear but would be elastic at a sufficiently high frequency. Supported lipid bilayers (SLB) are effectively elastic in acoustic experiments; this could be due to the relaxation time with respect to shear displacements in the bilayer. At the larger end of the size scale, whole cells can also show a frequency-dependent transition to elastic behaviour, at frequencies as low as 0.1 Hz. Other examples mentioned here include proteins and the protein networks of cells.