Functional mimicry of sea urchin biomineralization proteins with CaCO3-binding peptides selected by phage display

The intricate process of biomineralization, e.g. in sea urchins, involves the precise interplay of highly regulated mineralization proteins and the spatiotemporal coordination achieved through compartmentalization. However, the investigation of biomineralization effector molecules, e.g. proteins, is challenging, due to their very low abundance. Therefore, we investigate the functional mimicry in the bioinspired precipitation of calcium carbonate (CaCO3) with artificial peptides selected from a peptide library by phage display based on peptide-binding to calcite and aragonite, respectively. The structure-directing effects of the identified peptides were compared to those of natural protein mixes isolated from skeletal (test) structures of two sea urchin species (Arbacia lixula and Paracentrotus lividus). The calcium carbonate samples deposited in the absence or presence of peptides were analyzed with a set of complementary techniques with regard to morphology, polymorph, and nanostructural motifs. Remarkably, some of the CaCO3-binding peptides induced morphological features in calcite that appeared similar to those obtained in the presence of the natural protein mixes. Many of the peptides identified as most effective in exerting a structure-directing effect on calcium carbonate crystallization were rich in basic amino acid residues. Hence, our in vitro mineralization study further highlights the important, but often neglected, role of positively charged soluble organic matrices associated with biological and bioinspired CaCO3 deposition.

On-Surface Synthesis and Characterization of a High-Spin Aza-[5]-Triangulene

Triangulenes are a class of open-shell triangular graphene flakes with total spin increasing with their size. In the last years, on-surface-synthesis strategies have permitted fabricating and engineering triangulenes of various sizes and structures with atomic precision. However, direct proof of the increasing total spin with their size remains elusive. In this work, we report the combined in-solution and on-surface synthesis of a large nitrogen-doped triangulene (aza-[5]-triangulene) on a Au(111) surface, and the detection of its high-spin ground state. Bond-resolved scanning tunneling microscopy images uncovered radical states distributed along the zigzag edges, which were detected as weak zero-bias resonances in scanning tunneling spectra. These spectral features reveal the partial Kondo screening of a high-spin state. Through a combination of several simulation tools, we find that the observed distribution of radical states is explained by a quintet ground state (S=2), instead of the quartet state (S=3/2) expected for the neutral species. This confirms that electron transfer to the metal substrate raises the spin of the ground state. We further provide a qualitative description of the change of (anti)aromaticity introduced by N-substitution, and its role in the charge stabilization on a surface, resulting in an S=2 aza-triangulene on Au(111).

Measurement of the α-Particle Monopole Transition Form Factor Challenges Theory: A Low-Energy Puzzle for Nuclear Forces?

We perform a systematic study of the α-particle excitation from its ground state 0_{1}^{+} to the 0_{2}^{+} resonance. The so-called monopole transition form factor is investigated via an electron scattering experiment in a broad Q^{2} range (from 0.5 to 5.0  fm^{-2}). The precision of the new data dramatically supersedes that of older sets of data, each covering only a portion of the Q^{2} range. The new data allow the determination of two coefficients in a low-momentum expansion, leading to a new puzzle. By confronting experiment to state-of-the-art theoretical calculations, we observe that modern nuclear forces, including those derived within chiral effective field theory that are well tested on a variety of observables, fail to reproduce the excitation of the α particle.

The impact of accuracy and precision of analytical test methods on the determination of withdrawal periods

Treatment of food-producing animals with veterinary medicinal products (VMPs) can result in residues in foodstuffs (e.g. eggs, meat, milk, or honey) representing a potential consumer health risk. To ensure consumer safety, worldwide regulatory concepts for setting safe limits for residues of VMPs e.g. as tolerances (US) or maximum residue limits (MRLs, EU) are used. Based on these limits so-called withdrawal periods (WP) are determined. A WP represents the minimum period of time required between the last administration of the VMP and the marketing of foodstuff. Usually, WPs are estimated using regression analysis based on residue studies. With high statistical confidence (usually 95% in the EU and 99% in the US) the residues in almost all treated animals (usually 95%) have to be below MRL when edible produce is harvested. Here, uncertainties from both sampling and biological variability are taken into account but uncertainties of measurement associated with the analytical test methods are not systematically considered. This paper describes a simulation experiment to investigate the extent to which relevant sources of measurement uncertainty (accuracy and precision) can impact the length of WPs. A set of real residue depletion data was artificially 'contaminated' with measurement uncertainty related to permitted ranges for accuracy and precision. The results show that both accuracy and precision had a noticeable effect on the overall WP. Due consideration of sources of measurement uncertainty may improve the robustness, quality and reliability of calculations upon which regulatory decisions on consumer safety of residues are based.

Femtosecond Thermal and Nonthermal Hot Electron Tunneling Inside a Photoexcited Tunnel Junction

Efficient operation of electronic nanodevices at ultrafast speeds requires understanding and control of the currents generated by femtosecond bursts of light. Ultrafast laser-induced currents in metallic nanojunctions can originate from photoassisted hot electron tunneling or lightwave-induced tunneling. Both processes can drive localized photocurrents inside a scanning tunneling microscope (STM) on femto- to attosecond time scales, enabling ultrafast STM with atomic spatial resolution. Femtosecond laser excitation of a metallic nanojunction, however, also leads to the formation of a transient thermalized electron distribution, but the tunneling of thermalized hot electrons on time scales faster than electron-lattice equilibration is not well understood. Here, we investigate ultrafast electronic heating and transient thermionic tunneling inside a metallic photoexcited tunnel junction and its role in the generation of ultrafast photocurrents in STM. Phase-resolved sampling of broadband terahertz (THz) pulses via the THz-field-induced modulation of ultrafast photocurrents allows us to probe the electronic temperature evolution inside the STM tip and to observe the competition between instantaneous and delayed tunneling due to nonthermal and thermal hot electron distributions in real time. Our results reveal the pronounced nonthermal character of photoinduced hot electron tunneling and provide a detailed microscopic understanding of hot electron dynamics inside a laser-excited tunnel junction.

Toggle-like current-induced Bloch point dynamics of 3D skyrmion strings in a room temperature nanowire

Research into practical applications of magnetic skyrmions, nanoscale solitons with interesting topological and transport properties, has traditionally focused on two dimensional (2D) thin-film systems. However, the recent observation of novel three dimensional (3D) skyrmion-like structures, such as hopfions, skyrmion strings (SkS), skyrmion bundles, and skyrmion braids, motivates the investigation of new designs, aiming to exploit the third spatial dimension for more compact and higher performance spintronic devices in 3D or curvilinear geometries. A crucial requirement of such device schemes is the control of the 3D magnetic structures via charge or spin currents, which has yet to be experimentally observed. In this work, we utilise real-space imaging to investigate the dynamics of a 3D SkS within a nanowire of Co₈Zn₉Mn₃ at room temperature. Utilising single current pulses, we demonstrate current-induced nucleation of a single SkS, and a toggle-like positional switching of an individual Bloch point at the end of a SkS. The observations highlight the possibility to locally manipulate 3D topological spin textures, opening up a range of design concepts for future 3D spintronic devices.

In three dimensional systems with broken bulk inversion symmetry, skyrmions can form extended string-like structures. Here, Birch et al use scanning transmission x-ray microscopy to demonstrate the current induced generation and motion of these three dimensional skyrmion strings.

Toggle-like current-induced Bloch point dynamics of 3D skyrmion strings in a room temperature nanowire

Research into practical applications of magnetic skyrmions, nanoscale solitons with interesting topological and transport properties, has traditionally focused on two dimensional (2D) thin-film systems. However, the recent observation of novel three dimensional (3D) skyrmion-like structures, such as hopfions, skyrmion strings (SkS), skyrmion bundles, and skyrmion braids, motivates the investigation of new designs, aiming to exploit the third spatial dimension for more compact and higher performance spintronic devices in 3D or curvilinear geometries. A crucial requirement of such device schemes is the control of the 3D magnetic structures via charge or spin currents, which has yet to be experimentally observed. In this work, we utilise real-space imaging to investigate the dynamics of a 3D SkS within a nanowire of Co₈Zn₉Mn₃ at room temperature. Utilising single current pulses, we demonstrate current-induced nucleation of a single SkS, and a toggle-like positional switching of an individual Bloch point at the end of a SkS. The observations highlight the possibility to locally manipulate 3D topological spin textures, opening up a range of design concepts for future 3D spintronic devices.

Visualization and identification of single meteoritic organic molecules by atomic force microscopy

Using high-resolution atomic force microscopy (AFM) with CO-functionalized tips, we atomically resolved individual molecules from Murchison meteorite samples. We analyzed powdered Murchison meteorite material directly, as well as processed extracts that we prepared to facilitate characterization by AFM. From the untreated Murchison sample, we resolved very few molecules, as the sample contained mostly small molecules that could not be identified by AFM. By contrast, using a procedure based on several trituration and extraction steps with organic solvents, we isolated a fraction enriched in larger organic compounds. The treatment increased the fraction of molecules that could be resolved by AFM, allowing us to identify organic constituents and molecular moieties, such as polycyclic aromatic hydrocarbons and aliphatic chains. The AFM measurements are complemented by high-resolution mass spectrometry analysis of Murchison fractions. We provide a proof of principle that AFM can be used to image and identify individual organic molecules from meteorites and propose a method for extracting and preparing meteorite samples for their investigation by AFM. We discuss the challenges and prospects of this approach to study extraterrestrial samples based on single-molecule identification.

Order from a Mess: The Growth of 5-Armchair Graphene Nanoribbons

The advent of on-surface chemistry under vacuum has vastly increased our capabilities to synthesize carbon nanomaterials with atomic precision. Among the types of target structures that have been synthesized by these means, graphene nanoribbons (GNRs) have probably attracted the most attention. In this context, the vast majority of GNRs have been synthesized from the same chemical reaction: Ullmann coupling followed by cyclodehydrogenation. Here, we provide a detailed study of the growth process of five-atom-wide armchair GNRs starting from dibromoperylene. Combining scanning probe microscopy with temperature-dependent XPS measurements and theoretical calculations, we show that the GNR growth departs from the conventional reaction scenario. Instead, precursor molecules couple by means of a concerted mechanism whereby two covalent bonds are formed simultaneously, along with a concomitant dehydrogenation. Indeed, this alternative reaction path is responsible for the straight GNR growth in spite of the initial mixture of reactant isomers with irregular metal-organic intermediates that we find. The provided insight will not only help understanding the reaction mechanisms of other reactants but also serve as a guide for the design of other precursor molecules.

π-Diradical Aromatic Soot Precursors in Flames

Soot emitted from incomplete combustion of hydrocarbon fuels contributes to global warming and causes human disease. The mechanism by which soot nanoparticles form within hydrocarbon flames is still an unsolved problem in combustion science. Mechanisms proposed to date involving purely chemical growth are limited by slow reaction rates, whereas mechanisms relying on solely physical interactions between molecules are limited by weak intermolecular interactions that are unstable at flame temperatures. Here, we show evidence for a reactive π-diradical aromatic soot precursor imaged using non-contact atomic force microscopy. Localization of π-electrons on non-hexagonal rings was found to allow for Kekulé aromatic soot precursors to possess a triplet diradical ground state. Barrierless chain reactions are shown between these reactive sites, which provide thermally stable aromatic rim-linked hydrocarbons under flame conditions. Quantum molecular dynamics simulations demonstrate physical condensation of aromatics that survive for tens of picoseconds. Bound internal rotors then enable the reactive sites to find each other and become chemically cross-linked before dissociation. These species provide a rapid, thermally stable chain reaction toward soot nanoparticle formation and could provide molecular targets for limiting the emission of these toxic combustion products.

Intramolecular Coupling of Terminal Alkynes by Atom Manipulation

Glaser-like coupling of terminal alkynes by thermal activation is extensively used in on-surface chemistry. Here we demonstrate an intramolecular version of this reaction performed by atom manipulation. We used voltage pulses from the tip to trigger a Glaser-like coupling between terminal alkyne carbons within a custom-synthesized precursor molecule adsorbed on bilayer NaCl on Cu(111). Different conformations of the precursor molecule and the product were characterized by molecular structure elucidation with atomic force microscopy and orbital density mapping with scanning tunneling microscopy, accompanied by density functional theory calculations. We revealed partially dehydrogenated intermediates, providing insight into the reaction pathway.

Synthesis of Cyclo[18]carbon via Debromination of C18Br6

Cyclo[18]carbon (C₁₈, a molecular carbon allotrope) can be synthesized by dehalogenation of a bromocyclocarbon precursor, C₁₈Br₆, in 64% yield, by atomic manipulation on a sodium chloride bilayer on Cu(111) at 5 K, and imaged by high-resolution atomic force microscopy. This method of generating C₁₈ gives a higher yield than that reported previously from the cyclocarbon oxide C₂₄O₆. The experimental images of C₁₈ were compared with simulated images for four theoretical model geometries, including possible bond-angle alternation: D_18h cumulene, D_9h polyyne, D_9h cumulene, and C_9h polyyne. Cumulenic structures, with (D_9h) and without (D_18h) bond-angle alternation, can be excluded. Polyynic structures, with (C_9h) and without (D_9h) bond-angle alternation, both show a good agreement with the experiment and are challenging to differentiate.

Automated structure discovery in atomic force microscopy

Atomic force microscopy (AFM) with molecule-functionalized tips has emerged as the primary experimental technique for probing the atomic structure of organic molecules on surfaces. Most experiments have been limited to nearly planar aromatic molecules due to difficulties with interpretation of highly distorted AFM images originating from nonplanar molecules. Here, we develop a deep learning infrastructure that matches a set of AFM images with a unique descriptor characterizing the molecular configuration, allowing us to predict the molecular structure directly. We apply this methodology to resolve several distinct adsorption configurations of 1S-camphor on Cu(111) based on low-temperature AFM measurements. This approach will open the door to applying high-resolution AFM to a large variety of systems, for which routine atomic and chemical structural resolution on the level of individual objects/molecules would be a major breakthrough.

New Insight in the Q^{2} Dependence of Proton Generalized Polarizabilities

Virtual Compton scattering on the proton has been investigated at three yet unexplored values of the four-momentum transfer Q^{2}: 0.10, 0.20, and 0.45  GeV^{2}, at the Mainz Microtron. Fits performed using either the low-energy theorem or dispersion relations allowed the extraction of the structure functions P_{LL}-P_{TT}/ε and P_{LT}, as well as the electric and magnetic generalized polarizabilities α_{E1}(Q^{2}) and β_{M1}(Q^{2}). These new results show a smooth and rapid falloff of α_{E1}(Q^{2}), in contrast to previous measurements at Q^{2}=0.33  GeV^{2}, and provide for the first time a precise mapping of β_{M1}(Q^{2}) in the low-Q^{2} region.

An sp-hybridized molecular carbon allotrope, cyclo[18]carbon

Carbon allotropes built from rings of two-coordinate atoms, known as cyclo[n]carbons, have fascinated chemists for many years, but until now they could not be isolated or structurally characterized because of their high reactivity. We generated cyclo[18]carbon (C18) using atom manipulation on bilayer NaCl on Cu(111) at 5 kelvin by eliminating carbon monoxide from a cyclocarbon oxide molecule, C24O6. Characterization of cyclo[18]carbon by high-resolution atomic force microscopy revealed a polyynic structure with defined positions of alternating triple and single bonds. The high reactivity of cyclocarbon and cyclocarbon oxides allows covalent coupling between molecules to be induced by atom manipulation, opening an avenue for the synthesis of other carbon allotropes and carbon-rich materials from the coalescence of cyclocarbon molecules.

Elucidating the Geometric Substitution of Petroporphyrins by Spectroscopic Analysis and Atomic Force Microscopy Molecular Imaging

Determination of the molecular structures of petroporphyrins has been crucial to understand the diagenetic pathways and maturation of petroleum. However, these studies have been hampered by their structural complexity and the challenges associated with their isolation. In comparison to the skeletal macrocyclic structures, much less is known about the substitutions, which are more sensitive to the maturation and diagenesis pathways. While these isolated vanadyl petroporphyrins largely consist of etioporphyrin and deoxophylloerythroetioporphyrin as expected, surprisingly, we find evidence that one or a few β hydrogens are present in petroporphyrins of low carbon numbers using a combination of ultraviolet-visible spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry, and non-contact atomic force microscopy. Petroporphyrins with β hydrogens were not anticipated on the basis of their biological precursors. The data support dealkylation under catagenesis but not transalkylation or random alkylation of the β and meso positions, despite the fact that more complex porphyrin structures are formed.

A Single-Molecule Chemical Reaction Studied by High-Resolution Atomic Force Microscopy and Scanning Tunneling Microscopy Induced Light Emission

Atomic force microscopy (AFM) as well as scanning tunneling microscopy induced light emission (STM-LE) are, each on their own, powerful tools used to investigate a large variety of properties of single molecules adsorbed on a surface. However, accessing both structural information by AFM as well as optical information by STM-LE on the same molecule so far remains elusive. We present a combined high-resolution AFM and STM-LE study on single metal-oxide phthalocyanines. Using atomic manipulation, the molecules can be deliberately reduced. We demonstrate structure elucidation and adsorption geometry determination of single molecules with atomic resolution combined with optical characterization by STM-LE and the possibility of investigating the change in a molecule's exciton emission intensity by a chemical reaction.

Exploring a Route to Cyclic Acenes by On-Surface Synthesis

A route to generate cyclacenes by on-surface synthesis is explored. We started by synthesizing two tetraepoxycyclacenes by sequences of Diels-Alder cycloadditions. Subsequently, these molecules were deposited onto Cu(111) and scanning-tunneling-microscopy(STM)-based atom manipulation was employed to dissociate the oxygen atoms. Atomic force microscopy (AFM) with CO-functionalized tips enabled the detailed characterization of the reaction products and revealed that, at most, two oxygens per molecule could be removed. Importantly, our experimental results suggest that the generation of cyclacenes by the described route might be possible for larger epoxycyclacenes.

Flexible enzymatic activation of artificial polyketide extender units by Streptomyces cinnamonensis into the monensin biosynthetic pathway

Streptomyces cinnamonensis A495 is a variant of the monensin producer which instead of the native polyether antibiotic gives rise to antibiotic and anti-tumour shunt-product premonensin. Through the supplementation of the fermentation medium with suitable precursors, premonensin can be derivatized via the incorporation of new-to-nature extender units into the biosynthetic machinery. Polyketide extender units require activation, typically in form of coenzyme A-thioesters. These are membrane impermeable and thus in the past an artificial mimic was employed. Here, we show the use and preliminary characterization of a highly substrate promiscuous new enzyme for the endogenous thioester formation in a Streptomyces strain. These intracellularly activated alternative extender units are significantly better incorporated into premonensin than the synthetically activated counterparts.

SIGNIFICANCE AND IMPACT OF THE STUDY

Polyketide natural products are of enormous relevance in medicine. The hit-rate in finding active compounds for the potential treatment of various diseases among this substance family of microbial origin is high. However, most polyketides require derivatization to render them suitable for the application. Of relevance in this field is the incorporation of artificial substances into the biogenesis of polyketides, hampered by both the microbial metabolism and the complexity of the enzymes involved. This manuscript describes the straightforward and selective biosynthetic incorporation of synthetic substances into a reduced polyketide and showcases a promising new enzyme to aid this purpose.

First Measurement of the Q^{2} Dependence of the Beam-Normal Single Spin Asymmetry for Elastic Scattering off Carbon

We report on the first Q^{2}-dependent measurement of the beam-normal single spin asymmetry A_{n} in the elastic scattering of 570 MeV vertically polarized electrons off ^{12}C. We cover the Q^{2} range between 0.02 and 0.05  GeV^{2}/c^{2} and determine A_{n} at four different Q^{2} values. The experimental results are compared to a theoretical calculation that relates A_{n} to the imaginary part of the two-photon exchange amplitude. The result emphasizes that the Q^{2} behavior of A_{n} given by the ratio of the Compton to charge form factors cannot be treated independently of the target nucleus.

GTS-21 attenuates loss of body mass, muscle mass, and function in rats having systemic inflammation with and without disuse atrophy

Muscle changes of critical illness are attributed to systemic inflammatory responses and disuse atrophy. GTS-21 (3-(2,4-dimethoxy-benzylidene)anabaseine), also known as DMBX-A) is a synthetic derivative of the natural product anabaseine that acts as an agonist at α7-acetylcholine receptors (α7nAChRs). Hypothesis tested was that modulation of inflammation by agonist GTS-21 (10 mg/kg b.i.d. intraperitoneally) will attenuate body weight (BW) and muscle changes. Systemic sham inflammation was produced in 125 rats by Cornyebacterium parvum (C.p.) or saline injection on days 0/4/8. Seventy-four rats had one immobilized-limb producing disuse atrophy. GTS-21 effects on BW, tibialis muscle mass (TMM), and function were assessed on day 12. Systemically, methemoglobin levels increased 26-fold with C.p. (p < 0.001) and decreased significantly (p < 0.033) with GTS-21. Control BW increased (+ 30 ± 9 g, mean ± SD) at day 12, but decreased with C.p. and superimposed disuse (p = 0.005). GTS-21 attenuated BW loss in C.p. (p = 0.005). Compared to controls, TMM decreased with C.p. (0.43 ± 0.06 g to 0.26 ± 0.03 g) and with superimposed disuse (0.18 ± 0.04 g); GTS-21 ameliorated TMM loss to 0.32 ± 0.04 (no disuse, p = 0.028) and to 0.22 ± 0.03 (with disuse, p = 0.004). Tetanic tensions decreased with C.p. or disuse and GTS-21 attenuated tension decrease in animals with disuse (p = 0.006) and in animals with C.p. and disuse (p = 0.029). C.p.-induced 11-fold increased muscle α7nAChR expression was decreased by > 60% with GTS-21 treatment. In conclusion, GTS-21 modulates systemic inflammation, evidenced by both decreased methemoglobin levels and decrease of α7nAChR expression, and mitigates inflammation-mediated loss of BW, TMM, fiber size, and function.

Elemental Identification by Combining Atomic Force Microscopy and Kelvin Probe Force Microscopy

There are currently no experimental techniques that combine atomic-resolution imaging with elemental sensitivity and chemical fingerprinting on single molecules. The advent of using molecular-modified tips in noncontact atomic force microscopy (nc-AFM) has made it possible to image (planar) molecules with atomic resolution. However, the mechanisms responsible for elemental contrast with passivated tips are not fully understood. Here, we investigate elemental contrast by carrying out both nc-AFM and Kelvin probe force microscopy (KPFM) experiments on epitaxial monolayer hexagonal boron nitride (hBN) on Ir(111). The hBN overlayer is inert, and the in-plane bonds connecting nearest-neighbor boron and nitrogen atoms possess strong covalent character and a bond length of only ∼1.45 Å. Nevertheless, constant-height maps of both the frequency shift Δ f and the local contact potential difference exhibit striking sublattice asymmetry. We match the different atomic sites with the observed contrast by comparison with nc-AFM image simulations based on the density functional theory optimized hBN/Ir(111) geometry, which yields detailed information on the origin of the atomic-scale contrast.

Beam-Recoil Polarization Measurement of π^{0} Electroproduction on the Proton in the Region of the Roper Resonance

The helicity-dependent recoil proton polarizations P_{x}^{'} and P_{z}^{'} as well as the helicity-independent component P_{y} have been measured in the p(e[over →],e^{'}p[over →])π^{0} reaction at four-momentum transfer Q^{2}≃0.1  GeV^{2}, center-of-mass proton emission angle θ_{p}^{*}≃90°, and invariant mass W≃1440  MeV. This first precise measurement of double-polarization observables in the energy domain of the Roper resonance P_{11}(1440) by exploiting recoil polarimetry has allowed for the extraction of its scalar electroexcitation amplitude at an unprecedentedly low value of Q^{2}, establishing a powerful instrument for probing the interplay of quark and meson degrees of freedom in the nucleon.

Charge-Transfer-Driven Nonplanar Adsorption of F4TCNQ Molecules on Epitaxial Graphene

π-conjugated organic molecules tend to adsorb in a planar configuration on graphene irrespective of their charge state. In contrast, here we demonstrate charging-induced strong structural relaxation of tetrafluorotetracyanoquinodimethane (F₄TCNQ) on epitaxial graphene on Ir(111) (G/Ir(111)). The work function modulation over the graphene moiré unit cell causes site-selective charging of F₄TCNQ. Upon charging, the molecule anchors to the face-centered cubic sites of the G/Ir(111) moiré through one or two cyano groups. The reaction is reversible and can be triggered on a single molecule by moving it between different adsorption sites. We introduce a model taking into account the trade-off between tilt-induced charging and reduced van der Waals interactions, which provides a general framework for understanding charging-induced structural relaxation on weakly interacting substrates. In addition, we argue that the partial sp³ rehybridization of the underlying graphene and the possible bonding mechanism between the cyano groups and the graphene substrate are also relevant for the complete understanding of the experiments. These results provide insight into molecular charging on graphene, and they are directly relevant for potential device applications where the use of molecules has been suggested for doping and band structure engineering.

Effective PEGylation of gold nanorods

Standard procedures to coat gold nanorods (AuNR) with poly(ethylene glycol) (PEG)-based ligands are not reliable and high PEG-grafting densities are not achieved. In this work, the ligand exchange of AuNR with PEGMUA, a tailored PEG-ligand bearing a C10 alkylene spacer, is studied. PEGMUA provides AuNR with very high stability against oxidative etching with cyanide. This etching reaction is utilized to study the ligand exchange in detail. Ligand exchange is faster, less ligand consuming and more reproducible with assisting chloroform extraction. Compared to PEG ligands commonly used, PEGMUA provides much higher colloidal and chemical stability. Further analyses based on NMR-, IR- and UV/Vis-spectroscopy reveal that significantly higher PEG-grafting densities, up to ∼3 nm(-2), are obtained with PEGMUA. This demonstrates how the molecular structure of the PEG ligand can be used to dramatically improve the ligand exchange and to synthesize PEGylated AuNR with high chemical and colloidal stability and high PEG grafting densities. Such AuNR are especially interesting for applications in nanomedicine.

Biosynthesis-driven structure–activity relationship study of premonensin-derivatives

The controlled derivatization of natural products is of great importance for their use in drug discovery.

Electronic states at the graphene-hexagonal boron nitride zigzag interface

The electronic properties of graphene edges have been predicted to depend on their crystallographic orientation. The so-called zigzag (ZZ) edges haven been extensively explored theoretically and proposed for various electronic applications. However, their experimental study remains challenging due to the difficulty in realizing clean ZZ edges without disorder, reconstructions, or the presence of chemical functional groups. Here, we propose the ZZ-terminated, atomically sharp interfaces between graphene and hexagonal boron nitride (BN) as experimentally realizable, chemically stable model systems for graphene ZZ edges. Combining scanning tunneling microscopy and numerical methods, we explore the structure of graphene-BN interfaces and show them to host localized electronic states similar to those on the pristine graphene ZZ edge.

Search at the Mainz Microtron for light massive gauge bosons relevant for the muon g-2 anomaly

A massive, but light, Abelian U(1) gauge boson is a well-motivated possible signature of physics beyond the standard model of particle physics. In this Letter, the search for the signal of such a U(1) gauge boson in electron-positron pair production at the spectrometer setup of the A1 Collaboration at the Mainz Microtron is described. Exclusion limits in the mass range of 40  MeV/c^{2} to 300  MeV/c^{2}, with a sensitivity in the squared mixing parameter of as little as ε^{2}=8×10^{-7} are presented. A large fraction of the parameter space has been excluded where the discrepancy of the measured anomalous magnetic moment of the muon with theory might be explained by an additional U(1) gauge boson.

Templated self-assembly and local doping of molecules on epitaxial hexagonal boron nitride

Using low-temperature scanning tunneling microscopy, we show that monolayer hexagonal boron nitride (h-BN) on Ir(111) acts as ultrathin insulating layer for organic molecules, while simultaneously templating their self-assembly. Tunneling spectroscopy experiments on cobalt phthalocyanine (CoPC) reveal narrow molecular resonances and indicate that the charge state of CoPC is periodically modulated by the h-BN moiré superstructure. Molecules in the second layer show site-selective adsorption behavior, allowing the synthesis of molecular dimers that are spatially ordered and inaccessible by usual chemical means.