A structural and physical study of sol–gel methacrylate–silica hybrids: intermolecular spacing dictates the mechanical properties

Sol–gel hybrids are inorganic/organic co-networks with nanoscale interactions between the components leading to unique synergistic mechanical properties, which can be tailored, via a selection of the organic moiety. Methacrylate based polymers present several benefits for class II hybrids (which exhibit formal covalent bonding between the networks) as they introduce great versatility and can be designed with a variety of chemical side-groups, structures and morphologies. In this study, the effect of high cross-linking density polymers on the structure–property relationships of hybrids generated using poly(3-trimethoxysilylpropyl methacrylate) (pTMSPMA) and tetraethyl orthosilicate (TEOS) was investigated. The complexity and fine scale of the co-network interactions requires the development of new analytical methods to understand how network evolution dictates the wide-ranging mechanical properties. Within this work we developed data manipulation techniques of acoustic-AFM and solid state NMR output that provide new approaches to understand the influence of the network structure on the macroscopic elasticity. The concentration of pTMSPMA in the silica sol affected the gelation time, ranging from 2 h for a hybrid made with 75 wt% inorganic with pTMSPMA at 2.5 kDa, to 1 minute for pTMSPMA with molecular weight of 30 kDa without any TEOS. A new mechanism of gelation was proposed based on the different morphologies derived by AC-AFM observations. We established that the volumetric density of bridging oxygen bonds is an important parameter in structure/property relationships in SiO2 hybrids and developed a method for determining it from solid state NMR data. The variation in the elasticity of pTMSPMA/SiO2 hybrids originated from pTMSPMA acting as a molecular spacer, thus decreasing the volumetric density of bridging oxygen bonds as the inorganic to organic ratio decreased

Facet-dependent interactions of islet amyloid polypeptide with gold nanoparticles: Implications for fibril formation and peptide-induced lipid membrane disruption

A comprehensive understanding of the mechanisms of interaction between proteins or peptides and nanomaterials is crucial for the development of nanomaterial-based diagnostics and therapeutics. In this work, we systematically explored the interactions between citrate-capped gold nanoparticles (AuNPs) and islet amyloid polypeptide (IAPP), a 37-amino acid peptide hormone co-secreted with insulin from the pancreatic islet. We utilized diffusion-ordered spectroscopy, isothermal titration calorimetry, localized surface plasmon resonance spectroscopy, gel electrophoresis, atomic force microscopy, transmission electron microscopy (TEM), and molecular dynamics (MD) simulations to systematically elucidate the underlying mechanism of the IAPP–AuNP interactions. Because of the presence of a metal-binding sequence motif in the hydrophilic peptide domain, IAPP strongly interacts with the Au surface in both the monomeric and fibrillar states. Circular dichroism showed that AuNPs triggered the IAPP conformational transition from random coil to ordered structures (α-helix and β-sheet), and TEM imaging suggested the acceleration of IAPP fibrillation in the presence of AuNPs. MD simulations revealed that the IAPP–AuNP interactions were initiated by the N-terminal domain (IAPP residues 1–19), which subsequently induced a facet-dependent conformational change in IAPP. On a Au(111) surface, IAPP was unfolded and adsorbed directly onto the Au surface, while for the Au(100) surface, it interacted predominantly with the citrate adlayer and retained some helical conformation. The observed affinity of AuNPs for IAPP was further applied to reduce the level of peptide-induced lipid membrane disruption

Facet-dependent interactions of islet amyloid polypeptide with gold nanoparticles: implications for fibril formation and peptide-induced lipid membrane disruption

A comprehensive understanding of the mechanisms of interaction between proteins or peptides and nanomaterials is crucial for the development of nanomaterial-based diagnos-tics and therapeutics. In this work, we systematically explored the interactions between citrate-capped gold nanoparticles (AuNPs) and islet amyloid polypeptide (IAPP), a 37-amino acid peptide hormone co-secreted with insulin from the pancreatic islet. We uti-lized diffusion-ordered spectroscopy, isothermal titration calorimetry, localized surface plasmon resonance spectroscopy, gel electrophoresis, atomic force microscopy, transmis-sion electron microscopy (TEM), and molecular dynamics (MD) simulations to systemati-cally elucidate the underlying mechanism of the IAPP−AuNP interactions. Because of the presence of a metal-binding sequence motif in the hydrophilic peptide domain, IAPP strongly interacts with the Au surface in both the monomeric and fibrillar states. Circular dichroism showed that AuNPs triggered the IAPP conformational transition from random coil to ordered structures (α-helix and β-sheet), and TEM imaging suggested the accelera-tion of IAPP fibrillation in the presence of AuNPs. MD simulations revealed that the IAPP−AuNP interactions were initiated by the N-terminal domain (IAPP residues 1−19), which subsequently induced a facet-dependent conformational change in IAPP. On a Au(111) surface, IAPP was unfolded and adsorbed directly onto the Au surface, while for the Au(100) surface, it interacted predominantly with the citrate adlayer and retained some helical conformation. The observed affinity of AuNPs for IAPP was further applied to reduce the level of peptide-induced lipid membrane disruption

Probing amylin fibrillation at an early stage via a tetracysteine-recognising fluorophore

Amyloid fibrillation is a nucleation-dependent process known be involved in the development of more than 20 progressive and chronic diseases. The detection of amyloid formation at the nucleation stage can greatly advance early diagnoses and treatment of diseases. In this work, we developed a new assay for the early detection of amylin fibrillation using the biarsenical dye 4,5-bis(1,3,2-dithiarsolan-2-yl)fluorescein (FlAsH), which could recognise tetracysteine motifs and transform from non-fluorescent form into strongly fluorescent complexes. Due to the close proximity of two cysteine residues within the hydrophilic domain of amylin, a non-contiguous tetracysteine motif can form upon amylin dimerisation or oligomerisation, which can be recognised by FlAsH and emit strong fluorescence. This enables us to report the nucleation-growth process of amylin without modification of the protein sequence. We showed that the use of this assay not only allowed the tracking of initial nucleation events, but also enabled imaging of amyloid fibrils and investigation of the effects of amyloid inhibitor/modulator toward amylin fibrillation

Temperature and force dependence of nanoscale electron transport via the Cu protein Azurin

The mechanisms of solid-state electron transport (ETp) via a monolayer of immobilized Azurin (Az) was examined by conducting probe atomic force microscopy (CP-AFM), both as function of temperature (248 - 373K) and of applied tip force (6-12 nN). By varying both temperature and force in CP-AFM, we find that the ETp mechanism can alter with a change in the force applied via the tip to the proteins. As the applied force increases, ETp via Az changes from temperature-independent to thermally activated at high temperatures. This is in contrast to the Cu-depleted form of Az (apo-Az), where increasing the applied force causes only small quantitative effects, that fit with a decrease in electrode spacing. At low force ETp via holo-Az is temperature-independent and thermally activated via apo-Az. This observation agrees with macroscopic-scale measurements, thus confirming that the difference in ETp dependence on temperature between holo- and apo-Az is an inherent one that may reflect a difference in rigidity between the two forms. An important implication of these results, which depend on CP-AFM measurements over a significant temperature range, is that for ETp measurements on floppy systems, such as proteins, the stress applied to the sample should be kept constant or, at least controlled during measurement.Comment: 24 pages, 6 figures, plus Supporting Information with 4 pages and 2 figure

A dual photobase system for directing the pathway of pH-sensitive chemical reactions with light

Light-gated chemical reactions allow spatial and temporal control of chemical processes. Here, we suggest a new system for controlling pH-sensitive processes with light using two photobases of Arrhenius and Brønsted types. Only after light excitation do Arrhenius photobases undergo hydroxide ion dissociation, while Brønsted photobases capture a proton. However, none can be used alone to reversibly control pH due to the limitations arising from excessively fast or overly slow photoreaction timescales. We show here that combining the two types of photobases allows light-triggered and reversible pH control. We show an application of this method in directing the pH-dependent reaction pathways of the organic dye Alizarin Red S simply by switching between different wavelengths of light, i.e., irradiating each photobase separately. The concept of a light-controlled system shown here of a sophisticated interplay between two photobases can be integrated into various smart functional and dynamic systems

Photocurrent generation in artificial light-harvesting protein matrices

Global interest in solar energy utilization is driving the search for new materials that allow light harvesting and photocurrent generation under a light stimulus. Light harvesting is also one of the most important natural phenomena, of which photosynthesis is an example, and such natural systems have been as well for photocurrent generation. Inspired by natural light-harvesting complexes, we present here a synthetic and artificial solid-state protein-based biopolymer that facilitates the formation of a photocurrent in a wide range of wavelengths upon the molecular doping of the natural light-harvesting chlorophyll molecules. Interfacing of the doped protein matrix with electrodes yielded a photocurrent in the order of a few microamperes when the matrix was exposed to light. We show a switchable (flipping in the) photocurrent behavior when: 1) the magnitude of the applied bias is changed, 2) the location of the irradiated area is changed with respect to the electrodes, and 3) a gradient doping, enabled by the facile molecular doping approach, is formed. Finally, the synthetic artificial nature of the protein matrix allows the exploration of several light-harvesting cofactors not used in natural systems, where we further show photocurrent generation by doped metal-free porphyrins

Manipulating the electronic properties and structure of MoO3 nanosheets with light via an excited-state proton transfer mechanism

Light is an attractive source of energy to regulate stimuli-responsive chemical systems, enabling control over chemical and physical processes. Here, we use light as a gating source to control the redox state, the formation of a localized surface plasmonic resonance (LSPR), and the structure of molybdenum oxide (MoO3) nanosheets, which are important for a wide array of applications. However, the light excitation is not of the MoO3 nanosheets but rather of a pyranine (HPTS) photoacid, which in turn undergoes an excited state proton transfer (ESPT) process. We show that the ESPT process from HPTS to the nanosheets and the intercalation of protons within the MoO3 nanosheets triggers the reduction of the nanosheets and the formation of an LSPR peak, a process that is reversible, meaning that in the absence of light, the LSPR peak disappears and the nanosheets return to their oxidized form. We further show that this reversible process is accompanied by a change in the nanosheet size and morphology

Auramine‑O as a Fluorescence Marker for the Detection of Amyloid Fibrils

There is an indispensable need for a fluorescence marker for the detection of amyloid fibrils, where, at present, the most used marker is thioflavin-T (ThT). Here, we present the use of auramine-O (AuO) as a possible alternative to ThT. As with ThT, the increase in the emission of AuO upon binding to amyloid fibrils is the result of inhibition of the free rotation of the two dimethylamino arms of the molecule. This inhibition prevents the excited-state electronic wave function from moving from the emissive locally excited state to the dark charge-transfer state. We further show that not only AuO is comparable to ThT as a fluorescent marker for amyloid fibrils but also it has a unique spectroscopic signature. AuO has distinct two modes that are characterized by a large shift in the absorption and emission peak positions between its unbound and bound states (before and after the fibrils formation, respectively). In this context, we show that, whereas the emission band position is red-shifting, the absorption peak shifts to the blue and the spectrum exhibits an isosbestic point. The large shifts in emission and absorption peak positions can be explained by the photoacid activity of AuO exhibiting an excited-state proton-transfer process

Exploring fast proton transfer events associated with lateral proton diffusion on the surface of membranes

Proton diffusion (PD) across biological membranes is a fundamental process in many biological systems, and much experimental and theoretical effort has been employed for deciphering it. Here, we report on a spectroscopic probe, which can be tightly tethered to the membrane, for following fast (nanosecond) proton transfer events on the surface of membranes. Our probe is composed of a photoacid that serves as our light-induced proton source for the initiation of the PD process. We use our probe to follow PD, and its pH dependence, on the surface of lipid vesicles composed of a zwitterionic headgroup, a negative headgroup, a headgroup that is composed only from the negative phosphate group, or a positive headgroup without the phosphate group. We reveal that the PD kinetic parameters are highly sensitive to the nature of the lipid headgroup, ranging from a fast lateral diffusion at some membranes to the escape of protons from surface to bulk (and vice versa) at others. By referring to existing theoretical models for membrane PD, we found that while some of our results confirm the quasi-equilibrium model, other results are in line with the nonequilibrium model.peerReviewe