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Abstract: Rational protein design requires understanding the contribution of each amino acid to a targeted protein fold. For a subset of protein structures, namely, α-helical coiled coils (CCs), knowledge is sufficiently advanced to allow the rational de novo design of many structures, including entirely new protein folds. Current CC design rules center on using aliphatic hydrophobic residues predominantly to drive the folding and assembly of amphipathic α helices. The consequences of using aromatic residues—which would be useful for introducing structural probes, and binding and catalytic functionalities—into these interfaces are not understood. There are specific examples of designed CCs containing such aromatic residues, e.g., phenylalanine-rich sequences, and the use of polar aromatic residues to make buried hydrogen-bond networks. However, it is not known generally if sequences rich in tyrosine can form CCs, or what CC assemblies these would lead to. Here, we explore tyrosine-rich sequences in a general CC-forming background and resolve new CC structures. In one of these, an antiparallel tetramer, the tyrosine residues are solvent accessible and pack at the interface between the core and the surface. In another more complex structure, the residues are buried and form an extended hydrogen-bond network.

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Abstract: Tuning the enzymatic degradation and disassembly rates of polymeric amphiphiles and their assemblies is crucial for designing enzyme-responsive nanocarriers for controlled drug delivery applications. The common methods to control the enzymatic degradation of amphiphilic polymers are to tune the molecular weights and ratios of the hydrophilic and hydrophobic blocks. In addition, to these approaches, the architecture of the hydrophilic block can also serve as a tool to tune enzymatic degradation and disassembly. To gain deeper understanding of the effect of the molecular architecture of the hydrophilic block we prepared two types of well-defined PEG-dendron amphiphiles bearing linear or V-shaped PEG chains as the hydrophilic blocks. The high molecular precision of these amphiphiles, which emerges from the utilization of dendrons as the hydrophobic blocks allowed us to study the self-assembly and enzymatic degradation and disassembly of the two types of amphiphiles with high resolution. Interestingly, the micelles of the V-shaped amphiphiles were significantly smaller and disassembled faster than those of the amphiphiles based on linear PEG. However, the complete enzymatic cleavage of the hydrophobic end-groups was significantly slower for the V-shaped amphiphiles. Our results show that the V-shape architecture can stabilize the unimer state and hence plays a double role in the enzymatic degradation and the induced disassembly and how it can be utilized to control the release of encapsulated or bound molecular cargo.

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Abstract: Hydrogels’ hydrated fibrillar nature makes them the material of choice for the design and engineering of 3D-scaffolds for cell culture, tissue engineering and drug delivery applications. One particular class of hydrogels that has been the focus of significant research is self-assembling peptide hydrogels. In the present work we were interested in exploring how fibre-fibre edge interactions affect the self-assembly and gelation properties of amphipathic peptides. For this purpose we investigated two β-sheet forming peptides, FEFKFEFK (F8) and KFEFKFEFKK (KF8K), the latter one having the fibre edges covered by lysine residues. Our results showed that the addition of the two lysine residues did not affect the ability of the peptides to form β-sheet rich fibres provided that the overall charge carried by the two peptides was kept constant. It did though significantly reduce edge driven hydrophobic fibre-fibre associative interactions resulting in a reduced tendency for KF8K fibres to associate / aggregate laterally and form large fibre bundles and consequently network crosslinks. This effect resulted in the formation of hydrogels with lower moduli but faster dynamics. As a result KF8K fibres could be aligned only under high shear and at high concentration while F8 hydrogel fibres were found to align readily at low shear and low concentration. In addition F8 hydrogels were found to fragment at high concentration due to the high aggregation state stabilising the fibre bundles resulting in fibre breakage rather than disentanglement and alignment.

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Abstract: The biopolymer cellulose is investigated in terms of the crystallographic order within thin films. The films were prepared by spin coating of a trimethylsilyl cellulose precursor followed by an exposure to HCl vapors; two different source materials were used. Careful pre-characterization of the films was performed by infrared spectroscopy and atomic force microscopy. Subsequently, the films were investigated by grazing incidence X-ray diffraction using synchrotron radiation. The results showed broad diffraction peaks, indicating a rather short correlation length of the molecular packing in the range of a few nm. The analysis of the diffraction pattern was based on the known structures of crystalline cellulose, since the observed peak pattern was comparable to cellulose phase II and phase III. The dominant fraction of the film is formed by two different types of layers which are oriented parallel to the substrate surface. The stacking of the layers results in a one-dimensional crystallographic order with a defined interlayer distance of either 7.3 Å or 4.2 Å. As a consequence, two different preferred orientations of the polymer chains are observed. In both cases, polymer chain axes are aligned parallel to the substrate surface, and the orientation of the cellulose molecules are concluded to be either edge-on or flat-on. A minor fraction of the cellulose molecules form nanocrystals that are randomly distributed within the films. In this case, the molecular packing density was found to be smaller in comparison to the known crystalline phases of cellulose.

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Abstract: The activity of antimicrobial peptides stems from their interaction with bacterial membranes, which are disrupted according to a number of proposed mechanisms. Here, we investigate the interaction of a model antimicrobial peptide that contains a single arginine residue with vesicles containing model lipid membranes. The surfactant-like peptide Ala6-Arg (A6R) is studied in the form where both termini are capped (CONH-A6R-NH2, capA6R) or uncapped (NH2-A6R-OH, A6R). Lipid membranes are selected to correspond to model anionic membranes (POPE/POPG) resembling those in bacteria or model zwitterionic membranes (POPC/DOPC) similar to those found in mammalian cells. Viable antimicrobial agents should show activity against anionic membranes but not zwitterionic membranes. We find, using small-angle X-ray scattering (SAXS) and cryogenic-TEM (transmission electron microscopy) that, uniquely, capA6R causes structuring of anionic membranes due to the incorporation of the peptide in the lipid bilayer with peptide β-sheet conformation revealed by circular dichroism spectroscopy (CD). There is a preferential interaction of the peptide with POPG (which is the only anionic lipid in the systems studied) due to electrostatic interactions and bidentate hydrogen bonding between arginine guanidinium and lipid phosphate groups. At a certain composition, this peptide leads to the remarkable tubulation of zwitterionic phosphatidylcholine (PC) vesicles, which is ascribed to the interaction of the peptide with the outer lipid membrane, which occurs without penetration into the membrane. In contrast, peptide A6R has a minimal influence on the anionic lipid membranes (and no β-sheet peptide structure is observed) but causes thinning (lamellar decorrelation) of zwitterionic membranes. We also investigated the cytotoxicity (to fibroblasts) and antimicrobial activity of these two peptides against model Gram positive and Gram negative bacteria. A strong selective antimicrobial activity against Gram positive Listeria monocytogenes, which is an important food-borne pathogen, is observed for capA6R. Peptide A6R is active against all three studied bacteria. The activity of the peptides against bacteria and mammalian cells is related to the specific interactions uncovered through our SAXS, cryo-TEM, and CD measurements. Our results highlight the exquisite sensitivity to the charge distribution in these designed peptides and its effect on the interaction with lipid membranes bearing different charges, and ultimately on antimicrobial activity.