pH-controlled aggregation polymorphism of amyloidogenic Aβ(16-22): insights for obtaining peptide tapes and peptide nanotubes, as function of the N-terminal capping moiety

Modulating Kinetics of the Amyloid-Like Aggregation of S. aureus Phenol-Soluble Modulins by Changes in pH

The pathogen Staphylococcus aureus is recognized as one of the most frequent causes of biofilm-associated infections. The recently identified phenol-soluble modulin (PSM) peptides act as the key molecular effectors of staphylococcal biofilm maturation and promote the formation of an aggregated fibril structure. The aim of this study was to evaluate the effect of various pH values on the formation of functional amyloids of individual PSM peptides. Here, we combined a range of biophysical, chemical kinetics and microscopic techniques to address the structure and aggregation mechanism of individual PSMs under different conditions. We established that there is a pH-induced switch in PSM aggregation kinetics. Different lag times and growth of fibrils were observed, which indicates that there was no clear correlation between the rates of fibril elongation among different PSMs. This finding confirms that pH can modulate the aggregation properties of these peptides and suggest a deeper understanding of the formation of aggregates, which represents an important basis for strategies to interfere and might help in reducing the risk of biofilm-related infections.

pH-Dependent fibril maturation of a Pmel17 repeat domain isoform revealed by tryptophan fluorescence

The pre-melanosomal protein (Pmel17) aggregates within melanosomes to form functional amyloid fibrils that facilitate melanin polymerization. The repeat domain (RPT) of Pmel17 fibrillates under strict acidic melanosomal pH. Alternative splicing results in a shortened repeat domain (sRPT), which also forms amyloid fibrils. Here, we explored the effects of pH and protein concentration on sRPT aggregation by monitoring the intrinsic fluorescence of the sole tryptophan at position 381 (³⁸¹W). ³⁸¹W emission properties revealed changes of local environment polarity for sRPT fibrils formed at different pH. At pH 4, fibrils formed rapidly with no lag phase. A high ³⁸¹W intensity was observed with a slight blue shift (10 nm). These fibrils underwent further structural rearrangements at intermediate pH (5-6), mirroring that of melanosome maturation, which initiates at pH 4 and increases to near neutral pH. In contrast, typical sigmoidal kinetics were observed at pH 6 with slower rates and ³⁸¹W exhibited quenched emission. Interestingly, biphasic kinetics were observed at pH 5 in a protein concentration-dependent manner. A large ³⁸¹W blue shift (23 nm) was measured, indicating a more hydrophobic environment for fibrils made at pH 5. Consistent with ³⁸¹W fluorescence, Raman spectroscopy revealed molecular level perturbations in sRPT fibrils that were not evident from circular dichroism, transmission electron microscopy, or limited proteolysis analysis. Finally, sRPT fibrils did not form at pH ≥7 and preformed fibrils rapidly disaggregated under these solution conditions. Collectively, this work yields mechanistic insights into pH-dependent sRPT aggregation in the context of melanosome maturation.

Responsive Hybrid (Poly)peptide-Polymer Conjugates

(Poly)peptide-polymer conjugates continue to garner significant interest in the production of functional materials given their composition of natural and synthetic building blocks that confer select and synergistic properties. Owing to opportunities to design predefined architectures and structures with different morphologies, these hybrid conjugates enable new approaches for producing micro- or nanomaterials. Their modular design enables the incorporation of multiple responsive properties into a single conjugate. This review presents recent advances in (poly)peptide-polymer conjugates for drug-delivery applications, with a specific focus on the utility of the (poly)peptide component in the assembly of particles and nanogels, as well as the role of the peptide in triggered drug release.

Self-assembling peptide and protein amyloids: from structure to tailored function in nanotechnology

Self-assembled peptide and protein amyloid nanostructures have traditionally been considered only as pathological aggregates implicated in human neurodegenerative diseases. In more recent times, these nanostructures have found interesting applications as advanced materials in biomedicine, tissue engineering, renewable energy, environmental science, nanotechnology and material science, to name only a few fields. In all these applications, the final function depends on: (i) the specific mechanisms of protein aggregation, (ii) the hierarchical structure of the protein and peptide amyloids from the atomistic to mesoscopic length scales and (iii) the physical properties of the amyloids in the context of their surrounding environment (biological or artificial). In this review, we will discuss recent progress made in the field of functional and artificial amyloids and highlight connections between protein/peptide folding, unfolding and aggregation mechanisms, with the resulting amyloid structure and functionality. We also highlight current advances in the design and synthesis of amyloid-based biological and functional materials and identify new potential fields in which amyloid-based structures promise new breakthroughs.

Self-Assembly, Hydrogelation, and Nanotube Formation by Cation-Modified Phenylalanine Derivatives

Fluorenylmethoxycarbonyl-protected phenylalanine (Fmoc-Phe) derivatives are a privileged class of molecule that spontaneously self-assemble into hydrogel fibril networks. Fmoc-Phe-derived hydrogels are typically formed by dilution of the hydrogelator from an organic cosolvent into water, by dissolution of the hydrogelator under basic aqueous conditions followed by adjustment of the pH with acid, or by other external triggering forces, including sonication and heating. These conditions complicate biological applications of these hydrogels. Herein, we report C-terminal cation-modified Fmoc-Phe derivatives that are positively charged across a broad range of pH values and that can self-assemble and form hydrogel networks spontaneously without the need to adjust pH or to use an organic cosolvent. In addition, these cationic Fmoc-Phe derivatives are found to self-assemble into novel sheet-based nanotube structures at higher concentrations. These nanotube structures are unique to C-terminal cationic Fmoc-Phe derivatives; the parent Fmoc-Phe carboxylic acids form only fibril or worm-like micelle structures. Nanotube formation by the cationic Fmoc-Phe molecules is dependent on positive charge at the C-terminus, since at basic pH where the positive charge is reduced only fibrils/worm-like micelles are formed and nanotube formation is suppressed. These studies provide an important example of Fmoc-Phe derivatives that can elicit hydrogelation without organic cosolvent or pH modification and also provide insight into how subtle modification of structure can perturb the self-assembly pathways of Fmoc-Phe derivatives.