Diffusion-Controlled Growth of Molecular Heterostructures: Fabrication of Two‑, One‑, and Zero-Dimensional C₆₀ Nanostructures on Pentacene Substrates

A variety of low dimensional C₆₀ structures has been grown on supporting pentacene multilayers. By choice of substrate temperature during growth the effective diffusion length of evaporated fullerenes and their nucleation at terraces or step edges can be precisely controlled. AFM and SEM measurements show that this enables the fabrication of either 2D adlayers or solely 1D chains decorating substrate steps, while at elevated growth temperature continuous wetting of step edges is prohibited and instead the formation of separated C₆₀ clusters pinned at the pentacene step edges occurs. Remarkably, all structures remain thermally stable at room temperature once they are formed. In addition the various fullerene structures have been overgrown by an additional pentacene capping layer. Utilizing the different probe depth of XRD and NEXAFS, we found that no contiguous pentacene film is formed on the 2D C₆₀ structure, whereas an encapsulation of the 1D and 0D structures with uniformly upright oriented pentacene is achieved, hence allowing the fabrication of low dimensional buried organic heterostructures

Patterned Growth of Organic Semiconductors: Selective Nucleation of Perylene on Self-Assembled Monolayers

Organic semiconductors (OSC) have received a large amount of attention because they afford the fabrication of flexible electronic devices. However, the limited resistance to radiation and etching of such materials does not permit their patterning by photolithography, which has been a driving force for the development of integrated circuits and therefore requires alternative structuring techniques. One approach is based on precoating the substrate with self-assembled monolayers (SAMs) to control the nucleation of subsequently deposited OSC layers, but the underlying mechanism is barely understood. Here, we used alkanethiols with different chemical terminations to prepare SAMs on gold substrates serving as model systems to identify the mechanism of selective nucleation for the case of the OSC perylene. Using atomic force microscopy and fluorescence microscopy, we demonstrate that the chemical functionalization of the SAMs determines the adhesion forces for the OSC that are smallest for CF₃-terminated and largest for OH-terminated SAMs, hence yielding distinctly different sticking probabilities upon perylene deposition at room temperature. Microcontact printing and immersion were employed to prepare SAM patterns that enable the selective growth of polycrystalline perylene films. A quite different situation is found upon printing long-chain thiols with low vapor pressure, which leads to the transfer of multilayers and favors the growth of perylene single crystallites. In a more abstract scenario, patterns of silicone oil droplets were printed on a gold substrate, which was previously covered with a repelling fluorinated SAM. Such droplets provide nucleation centers for liquid-mediated growth, often yielding platelet-shaped perylene single crystallites without unwanted perylene nucleation on the remaining surface

Fluorescence titrations of SSB and SSB with poly(dT) at 22°C and different salt conditions

<p><b>Copyright information:</b></p><p>Taken from "Single-stranded DNA-binding protein of : a biophysical characterization"</p><p>Nucleic Acids Research 2005;33(5):1662-1670.</p><p>Published online 21 Mar 2005</p><p>PMCID:PMC1069009.</p><p>© The Author 2005. Published by Oxford University Press. All rights reserved</p> Solid lines are theoretical binding isotherms for the binding of a multidentate ligand to a linear polymer () with binding site size and cooperative affinity ω· as indicated below. () Circles: 0.375 μM SSB in 0.3 M NaCl, 20 mM KP, 100 p.p.m. Tween-20; = 53.9, ω· = 1.2 × 10 M, = 86.2%. Triangles: 0.375 μM SSB in 1 mM NaCl, 1 mM KP, 100 p.p.m. Tween-20; = 47.5, ω· = 5 × 10 M, = 74.9%. () Circles: 0.29 μM SSB in 0.3 M NaCl, 20 mM KP, 0.1 mM EDTA, 100 p.p.m. Tween-20; = 63.7, ω· = 1.5 × 10 M, = 92%. Triangles: 0.44 μM SSB in 1 mM NaCl, 1 mM KP, 0.1 mM EDTA, 100 p.p.m. Tween-20; = 39.5, ω· = 9.3 × 10 M, = 70%

Interaction of SSB and DNA polymerase III χ subunit detected by analytical ultracentrifugation

<p><b>Copyright information:</b></p><p>Taken from "Single-stranded DNA-binding protein of : a biophysical characterization"</p><p>Nucleic Acids Research 2005;33(5):1662-1670.</p><p>Published online 21 Mar 2005</p><p>PMCID:PMC1069009.</p><p>© The Author 2005. Published by Oxford University Press. All rights reserved</p> Analytical sedimentation velocity centrifugation of 2.65 μM SSB (dotted line) and a mixture of 2.65 μM SSB and 31.8 μM χ (solid line) in 0.3 M NaCl, 20 mM KP at 20°C and 50 000 r.p.m. Differential sedimentation coefficient distributions were obtained using the program SEDFIT (). In the presence of χ, a complex is formed which sediments faster than any of the free proteins. Inset: for different mixtures concentrations of free and bound χ were determined from the areas under the separated peaks of the differential sedimentation coefficient distributions and were used to construct a binding isotherm. The solid line represents the best non-linear least squares fit for independent binding of 2.2 χ molecules to a SSB dimer with = 7.4 × 10 M

Epitaxial Tetrathiafulvalene–Tetracyanoquinodimethane Thin Films on KCl(100): New Preparation Methods and Observation of Interface-Mediated Thin Film Polymorph

Combining organic compounds of complementary ionization potential and electron affinity allows fabrication of charge-transfer complexes that exhibit remarkable properties, resulting, for example, in very high conductivity. Though the bulk properties of the prototypical organic conductor tetrathiafulvalene–tetracyanoquinodimethane (TTF-TCNQ) have been studied in detail, the influence of defects and crystallite size on the resulting electronic properties, as well as an integration of these materials in organic thin film devices, is barely explored. One important requirement for such a comprehension is the precise control over crystallite size and quality. In this study, we report on different strategies to prepare crystalline TTF-TCNQ thin films and compare their structural quality. While conventional organic molecular beam deposition of TTF-TCNQ onto KCl(100) substrates enables the growth of epitaxial thin films with grain dimensions of up to 2 μm, further enhancement of the crystallite dimensions by raising the growth temperature is thermally limited by vanishing sticking and onset of vaporization. Using more sophisticated methods like hot wall evaporation, however, allows one to overcome these limitations and yields crystalline islands with extensions enhanced by 2 orders of magnitude. Furthermore, we identify and provide a full structure solution of a yet unknown interface-mediated thin film polymorph of TTF-TCNQ, which is adopted in films of thicknesses below 1 μm

SSB and SSB show different influence on the melting behavior of poly(dA–dT)

<p><b>Copyright information:</b></p><p>Taken from "Single-stranded DNA-binding protein of : a biophysical characterization"</p><p>Nucleic Acids Research 2005;33(5):1662-1670.</p><p>Published online 21 Mar 2005</p><p>PMCID:PMC1069009.</p><p>© The Author 2005. Published by Oxford University Press. All rights reserved</p> Samples containing 38 μM nucleotides of poly(dA–dT) in a buffer containing 75 mM NaCl and 20 mM KP were melted in the presence of 3.25 μM SSB (a), 3.25 μM SSB (b) and in the absence of protein (c)

Molecular Packing Determines Singlet Exciton Fission in Organic Semiconductors

Carrier multiplication by singlet exciton fission enhances photovoltaic conversion efficiencies in organic solids. This decay of one singlet exciton into two triplet states allows the extraction of up to two electrons per harvested photon and, hence, promises to overcome the Shockley–Queisser limit. However, the microscopic mechanism of singlet exciton fission, especially the relation between molecular packing and electronic response, remains unclear, which therefore hampers the systematic improvement of organic photovoltaic devices. For the model system perfluoropentacene, we experimentally show that singlet exciton fission is greatly enhanced for a slip-stacked molecular arrangement by addressing different crystal axes featuring different packing schemes. This reveals that the fission process strongly depends on the intermolecular coupling: slip-stacking favors delocalization of excitations and allows for efficient exciton fission, while face-to-edge molecular orientations commonly found in the prevailing herringbone molecular stacking patterns even suppress it. Furthermore, we clarify the controversially debated role of excimer states as intermediary rather than competitive or precursory. Our detailed findings serve as a guideline for the design of next-generation molecular materials for application in future organic light-harvesting devices exploiting singlet exciton fission

DdBrk1 forms stable trimers in solution.

<p>Analytical ultracentrifugation experiments of 43 µM DdBrk1 in PBS at a detection wavelength of 280 nm. (A) Sedimentation equilibrium gradients were measured at rotor speeds of 18,000 rpm and 26,000 rpm at 10°C. Global fitting of the data with a model of a single species using the program BPCfit <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0021327#pone.0021327-Witte1" target="_blank">[46]</a> yielded a molar mass of 24.7 (±2) kg/mol (solid lines) indicating that the protein forms trimers in solution. (B) Sedimentation coefficient distribution as obtained from a sedimentation velocity experiment at 20°C using the program SEDFIT <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0021327#pone.0021327-Schuck1" target="_blank">[48]</a>. DdBrk1 sediments as a single species with a sedimentation coefficient s_20,W = 2.1 S. Experiments performed in a concentration rage of 15 µM to 340 µM DdBrk1 gave no indication of aggregation or dissociation of the DdBrk1 trimers, as indicated by a slight decrease of the sedimentation coefficient with increasing protein concentration (data not shown). The inset shows a Coomassie stained 16%-Tris/Tricin SDS-PAGE of the DdBrk1 sample used in these experiments.</p

Comparison of DdBrk1 with human WAVE-complex.

<p>(A) Superimposition of homotrimeric DdBrk1 with the human hetero-pentameric Scar/WAVE-complex <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0021327#pone.0021327-Chen1" target="_blank">[23]</a> (pdb code: 3P8C). The subunits HSPC300/HsBrk1 (yellow), WAVE1 (purple) and Abi2 (orange) are shown as cartoon representations using the same color code as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0021327#pone.0021327-Chen1" target="_blank">[23]</a>, Nap1 and Sra1 (also known as Pir1) are shown as surfaces in grey and beige. The DdBrk1-triple helix (blue cartoon) was superimposed onto the HsBrk1/WAVE1/Abi2-subcomplex to illustrate the structural similarity between homotrimeric DdBrk1 and the heterotrimeric subcomplex within the mature Scar/WAVE-complex (RMSD = 1.055Å). (B) Contact surface of the human triple-coil assembly to the Nap1/Sra1-platform as shown in (A). The triple-coil assembly is shown from the binding interface side, obtained by rotation of 180° around the horizontal axis. Residues within 5Å of the platform are shown with their surfaces (for reason of clarity the surface of HsBrk1-contact residues is shown in transparent grey to enhance contrast). HsBrk1 is responsible for the majority of contacts between the triple-coil arrangement and the Nap1/Sra1-platform. (C) Isolated superimposition of the triple-coil domains from the heterotrimer HsBrk1/WAVE1/Abi2/ (purple, orange, yellow) and the DdBrk1 homotrimer (blue) in two orthogonal projections. (D) A detailed superposition with exemplary labeled side chains of HsBrk1 (grey) and DdBrk1 (orange) underlining the high similarity both in sequence and structure of the two adapter-proteins.</p

X-ray Structure of DdBrk1.

<p>(A) Portions of experimental (left) and final refined 2fo-fc electron density shown with residues (right) of the region F30-L31 contoured at 1σ. (B) Cartoon model of the completely α-helical DdBrk1-molecule in the asymmetric unit. The approximately 20° kink at K-33 creates the N-terminally funnel shape in the trimer. (C) Two orthogonal views of the trimeric DdBrk1, built up from symmetry mates from the 3-fold symmetry axis. Whereas the C-termini are tightly packed, the N-termini seem to open up and create a funnel-like shape. (D) Solvent accessible electrostatic surface potential of one DdBrk1-chain (ranging from blue = 3kT/e to red = -3kT/e) calculated with APBS/PyMol <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0021327#pone.0021327-Baker1" target="_blank">[57]</a>. The other two chains of the trimer are shown as cartoon representations in green and orange. The helix-helix-interaction surface is neutral whereas the outside of the supercoil is clearly hydrophilic and shows negatively charged patches at the N- and C-termini and a positive patch in the central part. (E) Most of the highly conserved residues are located in the hydrophobic interface (e.g. cross-section with F30, L31, F34), but charged residues on the surface-patches are also highly conserved. Identical residues depicted in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0021327#pone-0021327-g001" target="_blank">Figure 1A</a> are shown in yellow and highly conserved residues are shown in green. (F) Section of the triple-coil showing a typical leucine heptad-repeat (L52, L59, shaded in grey) and the ring-like stabilization between Q55 and D57/E60 (shown as sticks) of the neighboring chain. (G) The crystal lattice of DdBrk1-crystals is stabilized by two Ca²⁺-ions in a head-to-tail orientation of the Brk1-chains. The two ions are coordinated by one N-terminal charged patch (W11, E15, E18)_{chain 1} and two C-terminal patches from two chains (D53, D57, E60)_{chain 2} and (Q55)_{chain 3}. The picture depicts an exemplary part of the crystal lattice (right part, some molecules were omitted for clarity) and a cross-section (left part) to visualize the coordinated Ca²⁺-ions (yellow spheres). DdBrk1 chains are shown as cartoon representations and colored as biological units.</p