Enantioselective Au-Catalyzed Synthesis of Thia[5]- and Thia[6]helicenes and Their Transformation into Bowl-shaped Pleiadenes.

A series of helically shaped benzo[b]chryseno[4,3-d]thiophenes, naphtho[1,2-b]phenanthro[4,3-d]thiophenes, and chryseno[3,4-b]naphtho[1,2-d]thiophenes is synthesized via a highly enantioselective Au-catalyzed intramolecular alkyne hydroarylation reaction. The inversion barriers of the structures obtained are determined both theoretically and experimentally, and their chiroptical properties are reported. Preliminary studies on the post-synthetic functionalization of these thiahelicenes and their transformation into azahelicenes are also presented. In addition, a straightforward one-step protocol is developed, which wraps the initially obtained chryseno[3,4-b]naphtho[1,2-d]thiophenes into bowl-shaped pleiadene derivatives without erosion of the enantiopurity. The number of structurally related products that are obtained with high enantioselectivity enables the establishment of comprehensive correlations between the structure and conformational stability or (chir)optical properties.

Formal Synthesis of Indolizidine and Quinolizidine Alkaloids from Vinyl Cyclic Carbonates.

Cyclic carbonates have long been considered relatively inert molecules acting as protecting groups in complex multistep synthetic routes. This study shows that a concise, yet modular synthesis of indolizidine and quinolizidine alkaloids can be developed from vinyl-substituted cyclic carbonate (VCC) intermediates. Through a highly stereoselective palladium-catalyzed allylic alkylation reaction, these alkaloid motifs can be assembled in four synthetic and only two column purification steps. The combined results help to further advance functionalized cyclic carbonates as useful and reactive intermediates in natural product synthesis.

Molecular Motors in Aqueous Environment.

Molecular motors are Nature's solution for (supra)molecular transport and muscle functioning and are involved in most forms of directional motion at the cellular level. Their synthetic counterparts have also found a myriad of applications, ranging from molecular machines and smart materials to catalysis and anion transport. Although light-driven rotary molecular motors are likely to be suitable for use in an artificial cell, as well as in bionanotechnology, thus far they are not readily applied under physiological conditions. This results mainly from their inherently aromatic core structure, which makes them insoluble in aqueous solution. Here, the study of the dynamic behavior of these motors in biologically relevant media is described. Two molecular motors were equipped with solubilizing substituents and studied in aqueous solutions. Additionally, the behavior of a previously reported molecular motor was studied in micelles, as a model system for the biologically relevant confined environment. Design principles were established for molecular motors in these media, and insights are given into pH-dependent behavior. The work presented herein may provide a basis for the application of the remarkable properties of molecular motors in more advanced biohybrid systems.

Does space-time torsion determine the minimum mass of gravitating particles?

We derive upper and lower limits for the mass-radius ratio of spin-fluid spheres in Einstein-Cartan theory, with matter satisfying a linear barotropic equation of state, and in the presence of a cosmological constant. Adopting a spherically symmetric interior geometry, we obtain the generalized continuity and Tolman-Oppenheimer-Volkoff equations for a Weyssenhoff spin fluid in hydrostatic equilibrium, expressed in terms of the effective mass, density and pressure, all of which contain additional contributions from the spin. The generalized Buchdahl inequality, which remains valid at any point in the interior, is obtained, and general theoretical limits for the maximum and minimum mass-radius ratios are derived. As an application of our results we obtain gravitational red shift bounds for compact spin-fluid objects, which may (in principle) be used for observational tests of Einstein-Cartan theory in an astrophysical context. We also briefly consider applications of the torsion-induced minimum mass to the spin-generalized strong gravity model for baryons/mesons, and show that the existence of quantum spin imposes a lower bound for spinning particles, which almost exactly reproduces the electron mass.

Modified teleparallel theories of gravity: Gauss-Bonnet and trace extensions.

We investigate modified theories of gravity in the context of teleparallel geometries with possible Gauss-Bonnet contributions. The possible coupling of gravity with the trace of the energy-momentum tensor is also taken into account. This is motivated by the various different theories formulated in the teleparallel approach and the metric approach without discussing the exact relationship between them. Our formulation clarifies the connections between different well-known theories. For instance, we are able to formulate the correct teleparallel equivalent of Gauss-Bonnet modified general relativity, amongst other results. Finally, we are able to identify modified gravity models which have not been studied in the past. These appear naturally within our setup and would make a interesting starting point for further studies.

Septation of infectious hyphae is critical for appressoria formation and virulence in the smut fungus Ustilago maydis.

Differentiation of hyphae into specialized infection structures, known as appressoria, is a common feature of plant pathogenic fungi that penetrate the plant cuticle. Appressorium formation in U. maydis is triggered by environmental signals but the molecular mechanism of this hyphal differentiation is largely unknown. Infectious hyphae grow on the leaf surface by inserting regularly spaced retraction septa at the distal end of the tip cell leaving empty sections of collapsed hyphae behind. Here we show that formation of retraction septa is critical for appressorium formation and virulence in U. maydis. We demonstrate that the diaphanous-related formin Drf1 is necessary for actomyosin ring formation during septation of infectious hyphae. Drf1 acts as an effector of a Cdc42 GTPase signaling module, which also consists of the Cdc42-specific guanine nucleotide exchange factor Don1 and the Ste20-like kinase Don3. Deletion of drf1, don1 or don3 abolished formation of retraction septa resulting in reduced virulence. Appressorium formation in these mutants was not completely blocked but infection structures were found only at the tip of short filaments indicating that retraction septa are necessary for appressorium formation in extended infectious hyphae. In addition, appressoria of drf1 mutants penetrated the plant tissue less frequently.

The germinal centre kinase Don3 triggers the dynamic rearrangement of higher-order septin structures during cytokinesis in Ustilago maydis.

The dimorphic phytopathogenic fungus Ustilago maydis grows in its haploid phase by budding. Cytokinesis and separation of daughter cells are accomplished by the consecutive formation of two distinct septa. Here, we show that both septation events involve the dynamic rearrangement of septin assemblies from hourglass-shaped collars into ring-like structures. Using a chemical genetic approach we demonstrate that the germinal centre kinase Don3 triggers this septin reorganization during secondary septum formation. Although chemical inhibition of an analogue-sensitive version of Don3 prevented septation, a stable septin collar was assembled at the presumptive septation site. Interestingly, the essential light chain of type II myosin, Cdc4, was already associated with this septin collar. Release of Don3 kinase inhibition triggered immediate dispersal of septin filaments and concomitant incorporation of Cdc4 into a contractile actomyosin ring, which also contained the F-BAR domain protein Cdc15. Inhibition of actin polymerization or deletion of the cdc15 gene, did not affect assembly of the initial collar consisting of septin and myosin light chain. However, reassembly of septin filaments into a ring-like structure was prevented in the absence of either F-actin or Cdc15, indicating that septin ring formation in U. maydis depends on a functional contractile actomyosin ring.

Cdc42 and the Ste20-like kinase Don3 act independently in triggering cytokinesis in Ustilago maydis.

In the dimorphic fungus Ustilago maydis the Rho-family GTP-binding protein Cdc42 and the Ste20-like kinase Don3 are both essential for triggering cell separation during cytokinesis. Since Don3 does not contain a Cdc42/Rac interaction and binding domain (CRIB), it is unclear how Cdc42 and Don3 cooperate in the regulation of cytokinesis. To analyse the regulatory network we generated an analogue-sensitive Don3 variant (Don3-as) that allows specific inhibition in vivo. The engineered kinase Don3(M157A) is fully active in vivo and can be specifically inhibited by low concentrations of the ATP-analogue NA-PP1. Inhibition of the Don3-as kinase activity immediately blocked cell separation resulting in the formation of clusters of nonseparated cells. Covalent labelling of cell wall proteins showed that, upon release of inhibition, cytokinesis was resumed instantaneously in all cells. By sequentially activating Don3 and Cdc42 we were able to demonstrate that both proteins act independently of each other and that Don3 activity precedes that of Cdc42. We provide evidence that Don3 and Cdc42 are crucial for the assembly of a contractile actomyosin ring, which is a prerequisite for secondary septum formation. We propose, that Don3 is involved in establishing a landmark, at which the Cdc42-dependent actomyosin ring formation will occur.

Proteomic analysis of dimorphic transition in the phytopathogenic fungus Ustilago maydis.

In the corn smut fungus Ustilago maydis, the dimorphic transition from budding to filamentous growth is intrinsically associated with the switch from a saprophytic to a pathogenic lifestyle. Both pathogenicity and filament formation are triggered by a heterodimeric homeodomain transcription factor encoded by the b mating type locus. Here, we present a reference map of the proteome of this dimorphic phytopathogenic fungus. Using 2-DE in combination with MALDI-TOF-MS and ESI-MS/MS, we were able to identify 250 distinct proteins obtained from soluble protein samples. In addition, we determined the abundance of cytosolic proteins in filamentous U. maydis cells and compared it with that of budding cells. Filamentous growth was induced by two independent regimes, either by overexpression of the bW2/bE1-heterodimer or by overexpression of the small GTP binding protein Rac1. By comparison of expression profiles, we have identified 13 protein spots that were significantly enhanced during filamentous growth induced by bW2/bE1. Rac1 only up-regulates a subset of four of these protein spots. None of these proteins have previously been associated with filamentous growth. Comparison of Rac1- and b-regulated protein sets supports the hypothesis that filament formation during pathogenic development occurs via stimulation of a Rac1-containing signalling module.

Dual function of the germinal centre kinase Don3 during mitosis and cytokinesis in Ustilago maydis.

Septum formation is a crucial step of cytokinesis in fungi. In the basidiomycete Ustilago maydis, the germinal centre kinase Don3 triggers initiation of a secondary septum necessary for cell separation after cytokinesis. Here we show that oligomerization of Don3 via a putative coiled-coil domain is critical for secondary septum formation. Within the Don3 sequence we detected a characteristic C-terminal sequence motif (T-motif), which determines the subcellular localization of Don3 but is not required for regulation of cell separation. This motif defines a novel family of fungal protein kinases including Sid1p, an essential component of the septation initiation network (SIN) in Schizosaccharomyces pombe. Using the yeast two-hybrid system we isolated the Don3-interacting protein Dip1, which is similar to S. pombe Cdc14p, another member of the SIN. Remarkably, deletion of dip1 did not interfere with cytokinesis in U. maydis, but both dip1 and don3 mutants were affected in nuclear envelope breakdown (NEBD) during mitosis. This phenotype has already been described for mutants, which lack the small GTPase Ras3, the U. maydis homologue of the SIN component Spg1p. We propose that the Don3 kinase exerts a dual function in the regulation of cell separation and NEBD.