Thermoresponsive BSA hydrogels with phase tunability

Amyloids are fibrillar structures formed due to protein aggregation or misfolding when the molecules undergo a conformational change from α-helix to β-sheet. Although this self-assembly is associated with many neurodegenerative diseases in vivo, the highly ordered amyloidic structures formed in vitro are ideal scaffolds for many bionanotechnological applications. Amyloid fibrillar networks under specific stimuli can also form stable hydrogels. We have used bovine serum albumin (BSA) as a model amyloidogenic protein to obtain thermally-induced hydrogels that display tunable sol-gel-sol transitions spanning over minutes to days. High concentrations of BSA (14-22% w/v) were heated at 65 °C for less than 3 min without any cross-linking agent to yield soft, injectable gels that were non-toxic to mammalian cells. A detailed investigation of temperature, concentration, incubation time and ionic strength on the formation and reversal of these gels was carried out using visual inspection, rheology, electron microscopy, fluorescence spectroscopy, UV-visible spectroscopy and circular dichroism spectroscopy. The optimum gelation temperature (T_g) for phase reversal of BSA gels was found to lie between 60 and 70 °C. An increase in protein concentration led to a reduction in the gelation time and increase in the gel-to-rev sol transition time. Gels heated for longer duration than their minimum gelation time yielded irreversible gels suggesting that low incubation periods were favourable for partial protein denaturation and hydrogel formation. This was supported by time-resolved secondary and tertiary structural ensemble studies. Further, the hydrogel networks demonstrated a zero-order drug release kinetics and the rev sol was found to be cytocompatible with HaCaT skin cell lines. Overall, our approach demonstrates rapid, crosslinker-free thermoresponsive BSA gelation with wide tunability and control on the time and material property, ideal for topical drug delivery applications.

A Colloidal Description of Intermolecular Interactions Driving Fibril-Fibril Aggregation of a Model Amphiphilic Peptide

We apply a kinetic analysis platform to study the intermolecular interactions underlying the colloidal stability of dispersions of charged amyloid fibrils consisting of a model amphiphilic peptide (RADA 16-I). In contrast to the aggregation mechanisms observed in the large majority of proteins and peptides, where several elementary reactions involving both monomers and fibrils are present simultaneously, the system selected in this work allows the specific investigation of the fibril-fibril aggregation process. We examine the intermolecular interactions driving the aggregation reaction at pH 2.0 by changing the buffer composition in terms of salt concentration, type of ion as well as type and concentration of organic solvent. The aggregation kinetics are followed by dynamic light scattering, and the experimental data are simulated by Smoluchowski population balance equations, which allow to estimate the energy barrier between two colliding fibrils in terms of the Fuchs stability ratio (W). When normalized on a dimensionless time weighted on the Fuchs stability ratio, the aggregation profiles under a broad range of conditions collapse on a single master curve, indicating that the buffer composition modifies the aggregation kinetics without affecting the aggregation mechanism. Our results show that the aggregation process does not occur under diffusion-limited conditions. Rather, the reaction rate is limited by the presence of an activation energy barrier that is largely dominated by electrostatic repulsive interactions. Such interactions could be reduced by increasing the concentration of salt, which induces charge screening, or the concentration of organic solvent, which affects the dielectric constant. It is remarkable that the dependence of the activation energy on the ionic strength can be described quantitatively in terms of charge screening effects in the frame of the DLVO theory, although specific anion and cation effects are also observed. While anion effects are mainly related to the binding to the positive groups of the fibril surface and to the resulting decrease of the surface charge, cation effects are more complex and involve additional solvation forces.

Stability and cytotoxicity of crystallin amyloid nanofibrils

Previous work has identified crystallin proteins extracted from fish eye lenses as a cheap and readily available source for the self-assembly of amyloid nanofibrils. However, before exploring potential applications, the biophysical aspects and safety of this bionanomaterial need to be assessed so as to ensure that it can be effectively and safely used. In this study, crude crystallin amyloid fibrils are shown to be stable across a wide pH range, in a number of industrially relevant solvents, at both low and high temperatures, and in the presence of proteases. Crystallin nanofibrils were compared to well characterised insulin and whey protein fibrils using Thioflavin T assays and TEM imaging. Cell cytotoxicity assays suggest no adverse impact of both mature and fragmented crystallin fibrils on cell viability of Hec-1a endometrial cells. An IR microspectroscopy study supports long-term structural integrity of crystallin nanofibrils.

The interplay between carbon nanomaterials and amyloid fibrils in bio-nanotechnology

Recent advances in bio-nanotechnology have not only rapidly broadened the applications and scope of hybrid nanomaterials in biological fields, but also greatly enriched the examples of ordered materials based on supramolecular self-assembly. Among eminent examples of functional nanostructured materials of undisputed impact in nanotechnology and biological environments, carbon nanomaterials (such as fullerenes, carbon nanotubes and graphene) and amyloid fibrils have attracted great attention because of their unique architectures and exceptional physical properties. Nonetheless, combination of these two classes of nanomaterials into functional hybrids is far from trivial. For example, the presence of carbon nanomaterials can offer either an inhibitory effect or promotion of amyloid fibrillation, depending on the structural architectures of carbon nanomaterials and the starting amyloid proteins/peptides considered. To date, numerous studies have been devoted to evaluating both the biological toxicity of carbon nanomaterials and their use in developing therapies for amyloidosis. At the same time, hybridization of these two classes of nanomaterials offers new possibilities for combining some of their desirable properties into nanocomposites of possible use in electronics, actuators, sensing, biomedicine and structural materials. This review describes recent developments in the hybridization of carbon nanomaterials and amyloid fibrils and discusses the current state of the art on the application of carbon nanomaterial-amyloid fibril hybrids in bio-nanotechnology.

Proteins and peptides as biological nanowires: towards biosensing devices

The current landscape of nanotechnology is such that attention is being given to those materials that self-assemble, as a mode of "bottom-up" fabrication of nanomaterials. The field of nanotubes and nanowires has long been dominated by carbon nanotubes and inorganic materials. However in more recent years, the search for materials with desirable properties, such as self-assembly, has unsurprisingly led to the biological world, where functional nanoscale biomolecular assemblies are in abundance.Potential has been seen for a number of these assemblies to be translated into functional nanomaterials. The early days of bionanotechnology saw a lot of attention given to DNA molecules as nanowires, and proteins and peptides have now also been seen to have promise in this area. With most of the biological structures investigated having low conductivity in the native state, the use of biomolecules as templates for the formation of metallic and semiconductor nanowires has been the direction taken.This chapter will discuss the use of various biomolecules and biomolecular assemblies as nanowires, with a particular emphasis on proteins, beginning with an introduction into the field of nanotubes and nanowires. Many applications are now recognized for nanowires, but for brevity, this chapter will focus solely on their use as biosensors, using glucose sensors as a case study.