Search

Stable, single-nanometer thin, and free-standing two-dimensional layers with controlled molecular architectures are desired for several applications ranging from (opto-)electronic devices to nanoparticle and single-biomolecule characterization. It is, however, challenging to construct these stable single molecular layers via self-assembly, as the cohesion of those systems is ensured only by in-plane bonds. We herein demonstrate that relatively weak noncovalent bonds of limited directionality such as dipole-dipole (-CN⋅⋅⋅NC-) interactions act in a synergistic fashion to stabilize crystalline monomolecular layers of tetrafunctional calixarenes. The monolayers produced, demonstrated to be free-standing, display a well-defined atomic structure on the single-nanometer scale and are robust under a wide range of conditions including photon and electron radiation. This work opens up new avenues for the fabrication of robust, single-component, and free-standing layers via bottom-up self-assembly.

A Boersch phase plate can shift the phase of electrons proportionally to the applied electrical potential, thereby allowing for in situ control of the electron phase shift. A device comprising multiple Boersch phase shifter elements will be able to modulate the wavefront of a coherent electron beam and control electron interference. Recently, fabrication of single and 2 × 2 element Boersch phase shifter devices by focused ion beam milling has been reported. Realization of a large-scale Boersch phase shifter array would demand further developments in the device design and the fabrication strategy, e.g., using lithographic processes. In the present work, we develop a fabrication method utilizing the state-of-the-art electron beam lithography and reactive ion etching processes, a combination that is widely used for high-throughput and large-scale micro- and nanofabrication of electronic and photonic devices. Using the developed method, we fabricated a three-element phase shifter device with a metal–insulator–metal structure with 100-nm-thick ring electrodes and tested its electron transmission characteristics in a transmission electron microscope with a beam energy of 200 keV. We observed voltage-controlled evolution of electron interference, demonstrating the voltage-controlled electron phase shift using the fabricated device with a phase shift of π rad per 1 V. We analyze the experimental results in comparison with a three-dimensional electrostatic simulation. Furthermore, we discuss the possible improvements in terms of beam deflection and crosstalk between phase shifter elements in a five-layer device structure. [ABSTRACT FROM AUTHOR]

Endosporulation is a complex morphophysiological process resulting in a more resistant cellular structure that is produced within the mother cell and is called endospore. Endosporulation evolved in the common ancestor of Firmicutes, but it is lost in descendant lineages classified as asporogenic. While Kurthia spp. is considered to comprise only asporogenic species, we show here that strain 11kri321, which was isolated from an oligotrophic geothermal reservoir, produces phase‐bright spore‐like structures. Phylogenomics of strain 11kri321 and other Kurthia strains reveals little similarity to genetic determinants of sporulation known from endosporulating Bacilli. However, morphological hallmarks of endosporulation were observed in two of the four Kurthia strains tested, resulting in spore‐like structures (cryptospores). In contrast to classic endospores, these cryptospores did not protect against heat or UV damage and successive sub‐culturing led to the loss of the cryptosporulating phenotype. Our findings imply that a cryptosporulation phenotype may have been prevalent and subsequently lost by laboratory culturing in other Firmicutes currently considered as asporogenic. Cryptosporulation might thus represent an ancestral but unstable and adaptive developmental state in Firmicutes that is under selection under harsh environmental conditions. [ABSTRACT FROM AUTHOR]

Antithrombin, a plasma serpin, is relatively inactive as an inhibitor of the coagulation proteases until it binds to the heparan side chains that line the microvasculature. The binding specifically occurs to a core pentasaccharide present both in the heparans and in their therapeutic derivative heparin. The accompanying conformational change of antithrombin is revealed in a 2.9-[Angstrom] structure of a dimer of latent and active antithrombins, each in complex with the high-affinity pentasaccharide. Inhibitory activation results from a shift in the main sheet of the molecule from a partially six-stranded to a five-stranded form, with extrusion of the reactive center loop to give a more exposed orientation. There is a tilting and elongation of helix D with the formation of a 2-turn helix P between the C and D helices. Concomitant conformational changes at the heparin binding site explain both the initial tight binding of antithrombin to the heparans and the subsequent release of the antithrombin-protease complex into the circulation. The pentasaccharide binds by hydrogen bonding of its sulfates and carboxylates to Arg-129 and Lys-125 in the D-helix, to Arg-46 and Arg-47 in the A-helix, to Lys-114 and Glu-113 in the P-helix, and to Lys-11 and Arg-13 in a cleft formed by the amino terminus. This clear definition of the binding site will provide a structural basis for developing heparin analogues that are more specific toward their intended target antithrombin and therefore less likely to exhibit side effects.

In the structure of bovine mitochondrial [F.sub.1]-ATPase that was previously determined with crystals grown in the presence of adenylyl-imidodiphosphate (AMP-PNP) and ADP, the three catalytic [Beta]-subunits have different conformations and nucleotide occupancies. Adenylyl-imidodiphosphate is bound to one [Beta]-subunit ([[Beta].sub.TP]), ADP is bound to the second ([[Beta].sub.DP]), and no nucleotide is bound to the third ([[Beta].sub.E]). Here we show that the uncompetitive inhibitor aurovertin B binds to bovine [F.sub.1] at two equivalent sites in [[Beta].sub.TP] and [[Beta].sub.E], in a cleft between the nucleotide binding and C-terminal domains. In [[Beta].sub.DP], the aurovertin B pocket is incomplete and is inaccessible to the inhibitor. The aurovertin B bound to [[Beta].sub.TP] interacts with [Alpha]-Glu399 in the adjacent [[Alpha].sub.TP] subunit, whereas the aurovertin B bound to [[Beta].sub.E] is too distant from [[Alpha].sub.E] to make an equivalent interaction. Both sites encompass [Beta]Arg-412, which was shown by mutational studies to be involved in binding aurovertin. Except for minor changes around the aurovertin pockets, the structure of bovine [F.sub.1]-ATPase is the same as determined previously. Aurovertin B appears to act by preventing closure of the catalytic interfaces, which is essential for a catalytic mechanism involving cyclic interconversion of catalytic sites.

The three catalytic beta-subunits that constitute bovine mitochondrial F(1)-ATPase reveal differences in conformation and nucleotide binding properties, as revealed by the crystal structure of bovine mitochondrial F(1)-ATPase at a resolution of 2.8 angstroms. This indicates the presence of a catalytic mechanism in intact ATP synthase. The subunits are in three different states during a catalytic cycle. Rotation of the alpha(3)beta(3) subassembly with respect to the gamma-subunit's alpha-helical region allows interconversion between states.