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Reduced Acid Dissociation of Amino-Acids at the Surface of Water
Get PDFWe use surface-specific intensity vibrational sum-frequency generation (VSFG) and attenuated total reflection spectroscopy (ATR) to probe the ionization state of the amino-acids L-alanine and L-proline at the air/water surface and in the bulk. The ionization state is determined by probing the vibrational signatures of the carboxylic acid group, representing the non-dissociated acid form, and the carboxylate anion group, representing the dissociated form, over a wide range of pH values. We find that the carboxylic acid group deprotonates at a significantly higher pH at the surface than in the bulk
Evidence for Intramolecular Antiparallel Beta-Sheet Structure in Alpha-Synuclein Fibrils from a Combination of Two-Dimensional Infrared Spectroscopy and Atomic Force Microscopy
Get PDFThe aggregation of the intrinsically disordered protein alpha-synuclein (αS) into amyloid fibrils
is thought to play a central role in the pathology of Parkinson’s disease. Using a combination of
techniques (AFM, UV-CD, XRD, and amide-I 1D- and 2D-IR spectroscopy) we show that the structure
of αS fibrils varies as a function of ionic strength: fibrils aggregated in low ionic-strength buffers
([NaCl] ≤ 25 mM) have a significantly different structure than fibrils grown in higher ionic-strength
buffers. The observations for fibrils aggregated in low-salt buffers are consistent with an extended
conformation of αS molecules, forming hydrogen-bonded intermolecular β-sheets that are loosely
packed in a parallel fashion. For fibrils aggregated in high-salt buffers (including those prepared in
buffers with a physiological salt concentration) the measurements are consistent with αS molecules in
a more tightly-packed, antiparallel intramolecular conformation, and suggest a structure characterized
by two twisting stacks of approximately five hydrogen-bonded intermolecular β-sheets each. We find
evidence that the high-frequency peak in the amide-I spectrum of αS fibrils involves a normal mode
that differs fundamentally from the canonical high-frequency antiparallel β-sheet mode. The high
sensitivity of the fibril structure to the ionic strength might form the basis of differences in αS-related
pathologies
Membrane Binding of Parkinson's Protein α-Synuclein : Effect of Phosphorylation at Positions 87 and 129 by the S to D Mutation Approach
Get PDFHuman α-synuclein, a protein relevant in the brain with so-far unknown function, plays an important role in Parkinson's disease. The phosphorylation state of αS was related to the disease, prompting interest in this process. The presumed physiological function and the disease action of αS involves membrane interaction. Here, we study the effect of phosphorylation at positions 87 and 129, mimicked by the mutations S87A, S129A (nonphosphorylated) and S87D, S129D (phosphorylated) on membrane binding. Local binding is detected by spin-label continuous-wave electron paramagnetic resonance. For S87A/D, six positions (27, 56, 63, 69, 76, and 90) are probed; and for S129A/D, three (27, 56, and 69). Binding to large unilamellar vesicles of 100 nm diameter of 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1′-rac-glycerol) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine in a 1 : 1 composition is not affected by the phosphorylation state of S129. For phosphorylation at S87, local unbinding of αS from the membrane is observed. We speculate that modulating the local membrane affinity by phosphorylation could tune the way αS interacts with different membranes; for example, tuning its membrane fusion activity
Systematic study of the hybrid plasmonic-photonic band structure underlying lasing action of diffractive plasmon particle lattices
Get PDFWe study lasing in distributed feedback lasers made from square lattices of silver particles in a dye-doped waveguide. We present a systematic analysis and experimental study of the band structure underlying the lasing process as a function of the detuning between the particle plasmon resonance and the lattice Bragg diffraction condition. To this end, as gain medium we use either a polymer doped with Rh6G only, or polymer doped with a pair of dyes (Rh6G and Rh700) that act as a Förster energy transfer (FRET) pair. This allows for gain, respectively, at 590 or 700 nm when pumped at 532 nm, compatible with the achievable size tunability of silver particles embedded in the polymer. By polarization-resolved spectroscopic Fourier microscopy, we are able to observe the plasmonic/photonic band structure of the array, unraveling both the stop gap width, as well as the loss properties of the four involved bands at fixed lattice Bragg diffraction condition and as a function of detuning of the plasmon resonance. To explain the measurements we derive an analytical model that sheds insights on the lasing process in plasmonic lattices, highlighting the interaction between two competing resonant processes, one localized at the particle level around the plasmon resonance, and one distributed across the lattice. Both are shown to contribute to the lasing threshold and the overall emission properties of the arra
Integrating Sphere Microscopy for Direct Absorption Measurements of Single Nanostructures
Get PDFNanoscale materials are promising for optoelectronic devices because their physical dimensions are on the order of the wavelength of light. This leads to a variety of complex optical phenomena that, for instance, enhance absorption and emission. However, quantifying the performance of these nanoscale devices frequently requires measuring absolute absorption at the nanoscale, and remarkably, there is no general method capable of doing so directly. Here, we present such a method based on an integrating sphere but modified to achieve submicron spatial resolution. We explore the limits of this technique by using it to measure spatial and spectral absorptance profiles on a wide variety of nanoscale systems, including different combinations of weakly and strongly absorbing and scattering nanomaterials (Si and GaAs nanowires, Au nanoparticles). This measurement technique provides quantitative information about local optical properties that are crucial for improving any optoelectronic device with nanoscale dimensions or nanoscale surface texturin
Optimization of Sample Preparation and Instrumental Parameters for the Rapid Analysis of Drugs of Abuse in Hair samples by MALDI-MS/MS Imaging
Get PDFMatrix-assisted laser desorption/ionization-mass spectrometry imaging (MALDI-MSI) has been employed to rapidly screen longitudinally sectioned drug user hair samples for cocaine and its metabolites using continuous raster imaging. Optimization of the spatial resolution and raster speed were performed on intact cocaine contaminated hair samples. The optimized settings (100 × 150 μm at 0.24 mm/s) were subsequently used to examine longitudinally sectioned drug user hair samples. The MALDI-MS/MS images showed the distribution of the most abundant cocaine product ion at m/z 182. Using the optimized settings, multiple hair samples obtained from two users were analyzed in approximately 3 h: six times faster than the standard spot-to-spot acquisition method. Quantitation was achieved using longitudinally sectioned control hair samples sprayed with a cocaine dilution series. A multiple reaction monitoring (MRM) experiment was also performed using the ‘dynamic pixel’ imaging method to screen for cocaine and a range of its metabolites, in order to differentiate between contaminated hairs and drug users. Cocaine, benzoylecgonine, and cocaethylene were detectable, in agreement with analyses carried out using the standard LC-MS/MS method
Combining Time-of-Flight Secondary Ion Mass Spectrometry Imaging Mass Spectrometry and CARS Microspectroscopy Reveals Lipid Patterns Reminiscent of Gene Expression Patterns in the Wing Imaginal Disc of Drosophila melanogaster
Get PDFUsing label-free ToF-SIMS imaging mass spectrometry, we generated a map of small molecules differentially expressed in the Drosophila wing imaginal disc. The distributions of these moieties were in line with gene expression patterns observed during wing imaginal disc development. Combining ToF-SIMS imaging and coherent anti-Stokes Raman spectroscopy (CARS) microspectroscopy allowed us to locally identify acylglycerols as the main constituents of the pattern differentiating the future body wall tissue from the wing blade tissue. The findings presented herein clearly demonstrate that lipid localization patterns are strongly correlated with a developmental gene expression. From this correlation, we hypothesize that lipids play a so far unrecognized role in organ development
Interfacial engineering of metal-insulator-semiconductor junctions for efficient and stable photoelectrochemical water oxidation
Get PDFSolar-assisted water splitting can potentially provide an efficient route for large-scale
renewable energy conversion and storage. It is essential for such a system to provide a
sufficiently high photocurrent and photovoltage to drive the water oxidation reaction. Here we
demonstrate a photoanode that is capable of achieving a high photovoltage by engineering
the interfacial energetics of metal–insulator–semiconductor junctions. We evaluate the
importance of using two metals to decouple the functionalities for a Schottky contact and a
highly efficient catalyst. We also illustrate the improvement of the photovoltage upon
incidental oxidation of the metallic surface layer in KOH solution. Additionally, we analyse the
role of the thin insulating layer to the pinning and depinning of Fermi level that is responsible
to the resulting photovoltage. Finally, we report the advantage of using dual metal overlayers
as a simple protection route for highly efficient metal–insulator–semiconductor photoanodes
by showing over 200 h of operational stability
Experimental and theoretical evidence for bilayer-by-bilayer surface melting of crystalline ice
Get PDFOn the surface of water ice, a quasi-liquid layer (QLL) has been extensively reported at temperatures below its bulk melting point at 273 K. Approaching the bulk melting temperature from below, the thickness of the QLL is known to increase. To elucidate the precise temperature variation of the QLL, and its nature, we investigate the surface melting of hexagonal ice by combining noncontact, surfacespecific vibrational sum frequency generation (SFG) spectroscopy and spectra calculated from molecular dynamics simulations. Using SFG, we probe the outermost water layers of distinct single crystalline ice faces at different temperatures. For the basal face, a stepwise, sudden weakening of the hydrogen-bonded structure of the outermost water layers occurs at 257 K. The spectral calculations from the molecular dynamics simulations reproduce the experimental findings; this allows us to interpret our experimental findings in terms of a stepwise change from one to two molten bilayers at the transition temperature
Quantitative modelling of legume root nodule primordium induction by a diffusive signal of epidermal origin that inhibits auxin efflux
Get PDFBackground: Rhizobium nitrogen fixation in legumes takes place in specialized organs called root nodules. The initiation of these symbiotic organs has two important components. First, symbiotic rhizobium bacteria are recognized at the epidermis through specific bacterially secreted lipo-chitooligosaccharides (LCOs). Second, signaling processes culminate in the formation of a local auxin maximum marking the site of cell divisions. Both processes are spatially separated. This separation is most pronounced in legumes forming indeterminate nodules, such as model organism Medicago truncatula, in which the nodule primordium is formed from pericycle to most inner cortical cell layers.
Results: We used computer simulations of a simplified root of a legume that can form indeterminate nodules. A diffusive signal that inhibits auxin transport is produced in the epidermis, the site of rhizobium contact. In our model, all cells have the same response characteristics to the diffusive signal. Nevertheless, we observed the fastest and strongest auxin accumulation in the pericycle and inner cortex. The location of these auxin maxima correlates with the first dividing cells of future nodule primordia in M. truncatula. The model also predicts a transient reduction of the vascular auxin concentration rootward of the induction site as is experimentally observed. We use our model to investigate how competition for the vascular auxin source could contribute to the regulation of nodule number and spacing.
Conclusion: Our simulations show that the diffusive signal may invoke the strongest auxin accumulation response in the inner root layers, although the signal itself is strongest close to its production site