Nadav Amdursky

Blue Luminescence Based on Quantum Confinement at Peptide Nanotubes

We report on observation of photoluminescence (PL) in blue and UV regions of exciton origin in bioinspired material-peptide nanotubes (PNTs). Steplike optical absorption and PL measurements have allowed finding quantum confined (QC) phenomenon in PNTs. The estimations show that QC in these nanotubes occurs due to a crystalline structure of subnanometer scale dimension formed under the self-assembly process. Our new findings pave the way for the integration of PNT in a new generation of optical devices. A blue PL array of a PNT-patterned device is demonstrated.

Quantum Confinement in Self-Assembled Bioinspired Peptide Hydrogels

Adv. Mater. 2010, 22, 2311–2315 2010 WILEY-VCH Verlag G T IO N Hydrogels can be composed of natural or synthetic polymers. They form a three-dimensional (3D) scaffold that can absorb a large quantity of water (>99% by volume). They can mimic the extracellular matrix, having good biocompatible and biodegradable qualities, which enables them to support the growth of cultured cells. Among the various polymers for forming hydrogels, short peptides are an important group. It has been shown that several peptides can undergo gelation and form the 3D structure of a hydrogel, such as peptides that influence the selective differentiation of neural cells, short alternating charged amino acid peptides, and peptide clusters from the nuclear pore complex. Moreover, the addition of the protective group N-fluorenylmethoxycarbonyl (Fmoc) to the short peptides may improve the hydrogel formation process. Naturally self-assembled nanostructures of protein fibrils are associated with neurodegenerative conditions such as Alzheimer’s disease, where the fibril structure is made of amyloid-b (Ab) peptides. It has been found that the minimal core recognition motif of Ab peptides is a diphenylalanine element. The chemically synthesized dipeptide NH2Phe-Phe-COOH (FF) can self-assemble into well-ordered peptide nanotubes (PNTs). By using the FFmotif connected to an Fmoc moiety (Fmoc-FF), we have shown the formation of a peptidebased hydrogel made from a PNT network. In the work presented here, we have examined the optical properties, optical absorption and photoluminescence (PL), of hydrogels that were self-assembled from Fmoc-FF building blocks. From the optical properties we were able to follow the formation of a quantum confined structure within the hydrogel nanotubes. The most common example of quantum confinement (QC) is a 2D-QC—also called a quantum well (QW)—system of GaAs. This system contains various structures, the most noticeable of which is GaAs/AlGaAs, along with GaAs/InGaAs and InGaN/GaN. These structures have a double heterostructure consisting of a thin layer of GaAs ca. 10 nm thick, whose bandgap is smaller than that of the surrounding AlGaAs bulk. Another QC system, which was first revealed by Canham in 1990, is found in mesoporous silicon layers. Canham showed that by increasing the pore size, hence decreasing the size of the bulk silicon skeleton between the pores, a QC effect can occur. In order to observe the QC effects, the bulk silicon dimensions need to be less than the dimensions of the free exciton Bohr radius of bulk silicon of ca. 50 Å. Recently QC has been demonstrated in inorganic nanotubes, such as nanowires of ZnO or InGaN/ GaN multiple QWs. QW structures can produce remarkable changes in the optical and electrical properties of semiconductor structures; hence QW structures are most commonly characterized by spectroscopy measurements. We have very recently demonstrated the QC phenomenon in several peptide nanostructures. We showed a QW structure within PNTs, which were formed by vapor deposition of the FF building blocks. Furthermore, we showed a quantum dot (QD) structure within self-assembled peptide spheres, made from an analogue of the FF building block. By understanding the phenomenon, we were able to demonstrate a unique blue luminescence from the bioinspired PNTs. The hydrogel’s self-assembled PNT network is composed of Fmoc-FF building blocks (Fig. 1A) with a final diameter of 7–15 nm (Figs. 1B,C). When the environment of the solution is not suitable for their self-assembly, the Fmoc-FF molecules form aggregates in the solution. The optical absorption of the hydrogel and the aggregates is presented in Figure 2. In Figure 2A in the curves of the high-concentration samples, characterized by PNT network formation, pronounced step-like behavior of the absorption spectra is observed. The spectrum indicates a major step in the UV region with a peak at its ‘‘red’’ edge with wavelength

Electronic Transport via Proteins

A central vision in molecular electronics is the creation of devices with functional molecular components that may provide unique properties. Proteins are attractive candidates for this purpose, as they have specific physical (optical, electrical) and chemical (selective binding, self-assembly) functions and offer a myriad of possibilities for (bio-)chemical modification. This Progress Report focuses on proteins as potential building components for future bioelectronic devices as they are quite efficient electronic conductors, compared with saturated organic molecules. The report addresses several questions: how general is this behavior; how does protein conduction compare with that of saturated and conjugated molecules; and what mechanisms enable efficient conduction across these large molecules? To answer these questions results of nanometer-scale and macroscopic electronic transport measurements across a range of organic molecules and proteins are compiled and analyzed, from single/few molecules to large molecular ensembles, and the influence of measurement methods on the results is considered. Generalizing, it is found that proteins conduct better than saturated molecules, and somewhat poorer than conjugated molecules. Significantly, the presence of cofactors (redox-active or conjugated) in the protein enhances their conduction, but without an obvious advantage for natural electron transfer proteins. Most likely, the conduction mechanisms are hopping (at higher temperatures) and tunneling (below ca. 150-200 K).

Self-assembled bioinspired quantum dots: Optical properties

Until now, the wide research field of quantum dots (QDs) focused on inorganic structures. In the present study, we report on quantum confinement phenomena found in peptide nanocrystalline regions formed within self-assembly peptide nanospheres. These bioinspired nanostructures exhibit the optical absorption characteristics of QDs with pronounced luminescence of excitons whose origin is at the UV region. Theoretical estimations based on experimental data show that the radius of the self assembled peptide QDs is 1.3 nm.

Modeling the nonradiative decay rate of electronically excited thioflavin T.

A computational model of nonradiative decay is developed and applied to explain the time-dependent emission spectrum of thioflavin T (ThT). The computational model is based on a previous model developed by Glasbeek and co-workers (van der Meer, M. J.; Zhang, H.; Glasbeek, M. J. Chem. Phys. 2000, 112, 2878) for auramine O, a molecule that, like ThT, exhibits a high nonradiative rate. The nonradiative rates of both auramine O and ThT are inversely proportional to the solvent viscosity. The Glasbeek model assumes that the excited state consists of an adiabatic potential surface constructed by adiabatic coupling of emissive and dark states. For ThT, the twist angle between the benzothiazole and the aniline is responsible for the extensive mixing of the two excited states. At a twist angle of 90°, the S(1) state assumes a charge-transfer-state character with very small oscillator strength, which causes the emission intensity to be very small as well. In the ground state, the twist angle of ThT is rather small. The photoexcitation leads first to a strongly emissive state (small twist angle). As time progresses, the twist angle increases and the oscillator strength decreases. The fit of the experimental results by the model calculations is good for times longer than 3 ps. When a two-coordinate model is invoked or a solvation spectral-shift component is added, the fit to the experimental results is good at all times.

Apoptosis induced by islet amyloid polypeptide soluble oligomers is neutralized by diabetes-associated specific antibodies

Soluble oligomeric assemblies of amyloidal proteins appear to act as major pathological agents in several degenerative disorders. Isolation and characterization of these oligomers is a pivotal step towards determination of their pathological relevance. Here we describe the isolation of Type 2 diabetes-associated islet amyloid polypeptide soluble cytotoxic oligomers; these oligomers induced apoptosis in cultured pancreatic cells, permeated model lipid vesicles and interacted with cell membranes following complete internalization. Moreover, antibodies which specifically recognized these assemblies, but not monomers or amyloid fibrils, were exclusively identified in diabetic patients and were shown to neutralize the apoptotic effect induced by these oligomers. Our findings support the notion that human IAPP peptide can form highly toxic oligomers. The presence of antibodies identified in the serum of diabetic patients confirms the pathological relevance of the oligomers. In addition, the newly identified structural epitopes may also provide new mechanistic insights and a molecular target for future therapy.

Temperature and Force Dependence of Nanoscale Electron Transport via the Cu Protein Azurin

Solid-state electron transport (ETp) via a monolayer of immobilized azurin (Az) was examined by conducting probe atomic force microscopy (CP-AFM), as a function of both temperature (248-373K) and applied tip force (6-15 nN). At low forces, ETp via holo-Az (with Cu(2+)) is temperature-independent, but thermally activated via the Cu-depleted form of Az, apo-Az. While this observation agrees with those of macroscopic-scale measurements, we find that for holo-Az the mechanism of ETp at high temperatures changes upon an increase in the force applied by the tip to the proteins; namely, above 310 K and forces >6 nN ETp becomes thermally activated. This is in contrast to apo-Az, where increasing applied force causes only small monotonic increases in currents due to decreased electrode separation. The distinct ETp temperature dependence of holo- and apo-Az is assigned to a difference in structural response to pressure between the two protein forms. An important implication of these CP-AFM results (of measurements over a significant temperature range) is that for reliable ETp measurements on flexible macromolecules, such as proteins, the pressure applied during the measurements should be controlled or at least monitored.

Temperature dependence of the fluorescence properties of thioflavin-T in propanol, a glass-forming liquid.

Steady-state and time-resolved emission techniques were employed to study the nonradiative process of Thioflavin-T (ThT) in 1-propanol as a function of temperature. We found that the nonradiative rate, k(nr), decreased by about 3 orders of magnitude when the temperature was lowered to 88 K. We found remarkably good correspondence between the temperature dependence of k(nr) of ThT and the dielectric relaxation times of the 1-propanol solvent.

Ferroelectric and Related Phenomena in Biological and Bioinspired Nanostructures

We review ferroelectric and related phenomena in biological materials. Also the results of the study of peptide nanotubes (PNT), which are formed by self-assembly of aromatic peptides building blocks, are presented. We describe a new preparation process for making highly ordered alignments of PNT by vapor deposition of bio-molecules. The conducted optical studies show that PNTs possess nanocrystalline ceramics structure, characterized by quantum confinement and pronounced luminescence of exciton origin. PNTs demonstrate strong second harmonic generation based on their hexagonal asymmetric structure. This bio-inspired ceramic nanostructural material shows strong piezoelectric activity, pointing to electric polarization directed along the tube axis.

Pressure effect on the nonradiative process of thioflavin-T.

Time-resolved emission techniques were employed to study the nonradiative process of thioflavin-T (ThT) in 1-propanol, 1-butanol, and 1-pentanol as a function of the hydrostatic pressure. Elevated hydrostatic pressure increases the alcohol viscosity, which in turn strongly influences the nonradiative rate of ThT. A diamond-anvil cell was used to increase the pressure up to 2.4 GPa. We found that the nonradiative rate constant, k(nr), decreases with pressure. We further found a remarkable linear correlation between a decrease in k(nr) (increase in the nonradiative lifetime, τ(nr)) and an increase in the solvent viscosity. The viscosity was varied by a factor of 1000 and k(nr) was measured at high pressures, at which the nonradiative rate constant of the molecules decreased from (7 ps)(-1) to (13 ns)(-1), (13 ps)(-1) to (17 ns)(-1) and (17 ps)(-1) to (15 ns)(-1) for 1-propanol, 1-butanol, and 1-pentanol, respectively. The viscosity-dependence of k(nr) is explained by the excited-state rotation rate of the two-ring systems, with respect to each other.