Transient dominance in a central African rain forest

The large-crowned emergent tree Microberlinia bisulcata dominates rain forest groves at Korup National Park, Cameroon, along with two codominants, Tetraberlinia bifoliolata and T. korupensis. M. bisulcata has a pronounced modal size frequency distribution around 110 cm stem diameter: its recruitment potential is very poor. It is a long-lived light-demanding species, one of many found in African forests. Tetraberlinia species lack modality, are more shade tolerant, and recruit better. All three species are ectomycorrhizal. M. bisulcata dominates grove basal area, even though it has similar numbers of trees (≥50 cm stem diameter) as each of the other two species. This situation presented a conundrum that prompted a long-term study of grove dynamics. Enumerations of two plots (82.5 and 56.25 ha) between 1990 and 2010 showed mortality and recruitment of M. bisulcata to be very low (both rates 0.2% per year) compared with Tetraberlinia (2.4% and 0.8% per year), and M. bisulcata grows twice as fast as the Tetraberlinia. Ordinations indicated that these three species determined community structure by their strong negative associations while other species showed almost none. Ranked species abundance curves fitted the Zipf-Mandelbrot model well and allowed “overdominance” of M. bisulcata to be estimated. Spatial analysis indicated strong repulsion by clusters of large (50 to <100 cm) and very large (≥100 cm) M. bisulcata of their own medium-sized (10 to <50 cm) trees and all sizes of Tetraberlinia. This was interpreted as competition by M. bisulcata increasing its dominance, but also inhibition of its own replacement potential. Stem coring showed a modal age of 200 years for M. bisulcata, but with large size variation (50–150 cm). Fifty-year model projections suggested little change in medium, decreases in large, and increases in very large trees of M. bisulcata, accompanied by overall decreases in medium and large trees of Tetraberlinia species. Realistically increasing very-large-tree mortality led to grove collapse without short-term replacement. M. bisulcata most likely depends on climatic events to rebuild its stands: the ratio of disturbance interval to median species' longevity is important. A new theory of transient dominance explains how M. bisulcata may be cycling in abundance over time and displaying nonequilibrium dynamics.

EphA2 signaling following endocytosis: Role of Tiam-1

Eph receptors and their membrane-bound ligands, the ephrins, represent a complex subfamily of receptor tyrosine kinases (RTKs). Eph/ephrin binding can lead to various and opposite cellular behaviors such as adhesion versus repulsion, or cell migration versus cell-adhesion. Recently, Eph endocytosis has been identified as one of the critical steps responsible for such diversity. Eph receptors, as many RTKs, are rapidly endocytosed following ligand-mediated activation and traffic through endocytic compartments prior to degradation. However, it is becoming obvious that endocytosis controls signaling in many different manners. Here we showed that activated EphA2 are degraded in the lysosomes and that about 35% of internalized receptors are recycled back to the plasma membrane. Our study is also the first to demonstrate that EphA2 retains the capacity to signal in endosomes. In particular, activated EphA2 interacted with the Rho family GEF Tiam1 in endosomes. This association led to Tiam1 activation, which in turn increased Rac1 activity and facilitated Eph/ephrin endocytosis. Disrupting Tiam1 function with RNA interference impaired both ephrinA1-dependent Rac1 activation and ephrinA1-induced EphA2 endocytosis. In summary, our findings shed new light on the regulation of EphA2 endocytosis, intracellular trafficking and signal termination and establish Tiam1 as an important modulator of EphA2 signaling.

Large π-Conjugated Chromophores Derived from Tetrathiafulvalene

A large π-conjugated chromophore composed of two dipyrido[3,2-a:2′,3′-c]phenazine units directly fused to the central tetrathiafulvalene core has been prepared as a bridging ligand and its strong binding ability to Ru2+ to form a new dinuclear complex is presented. The electronic absorption and luminescence spectra and the electrochemical behavior of the free ligand and the Ru2+ complex have been investigated in detail. The free ligand shows a very strong band in the UV region consistent with ligand-centered π–π* transitions and an intense broad band in the visible region that corresponds to an intramolecular charge-transfer (ILCT) transition. Upon coordination, a metal-to-ligand charge-transfer band appears at 22520 cm−1, and the ILCT band is bathochromically shifted by 1620 cm−1. These electrochemically amphoteric chromophores have also been characterized by spectro-electrochemical methods. The oxidized radical species of the free ligand show a strong tendency to undergo aggregation, in which long-distance attractive interactions overcome the electrostatic repulsion. Moreover, these two new chromophores reveal an ILCT fluorescence with large solvent-dependent Stokes shifts and quantum efficiencies of 0.052 for the free ligand and 0.016 for its dinuclear Ru2+ complex in CH2Cl2.

Toward Cove-Edged Low Band Gap Graphene Nanoribbons

Graphene nanoribbons (GNRs), defined as nanometer-wide strips of graphene, have attracted increasing attention as promising candidates for next-generation semiconductors. Here, we demonstrate a bottom-up strategy toward novel low band gap GNRs (E-g = 1.70 eV) with a well-defined cove-type periphery both in solution and on a solid substrate surface with chrysene as the key monomer. Corresponding cyclized chrysene-based oligornerS consisting of the dimer and tetramer are obtained via an Ullmann Coupling followed by oxidative intramolecular cyclodehydrogenation in solution, and much higher GNR homologues via on-surface synthesis. These oligomers adopt nonplanar structures due to the isteric repulsion between the two C-H bonds at the inner cove position. Characterizations by single crystal X-ray analysis, UV-vis absorption spectroscopy, NMR spectroscopy, and scanning tunneling microscopy (STM) are described. The interpretation is assisted by density functional theory (DFT) calculations.

From supramolecular sheets to fibers and back: establishing the critical parameters that govern the morphology of pyrene-based self-assembled materials

The precise arraying of functional entities in morphologically well-defined shapes remains one of the key challenges in the processing of organic molecules1. Among various π-conjugated species, pyrene exhibits a set of unique properties, which make it an attractive compound for the utilization in materials science2. In this contribution we report on properties of self-assembled structures prepared from amphiphilic pyrene trimers (Py3) consisting of phosphodiester-linked pyrenes. Depending on the geometry of a pyrene core substitution (1.6-, 1.8-, or 2.7- type, see Scheme), the thermally-controlled self-assembly allows the preparation of supramolecular architectures of different morphologies in a bottom-up approach: two-dimensional (2D) nanosheets3 are formed in case of 1.6- and 2.7-substitution4 whereas one-dimensional (1D) fibers are built from 1.8- substituted isomers. The morphologies of the assemblies are established by AFM and TEM, and the results are further correlated with spectroscopic and scattering data. Two-dimensional assemblies consist of an inner layer of hydrophobic pyrenes, sandwiched between a net of phosphates. Due to the repulsion of the negative charges, the 2D assemblies exist mostly as free-standing sheets. An internal alignment of pyrenes leads to strong exciton coupling with an unprecedented observation (simultaneous development of J- and H-bands from two different electronic transitions). Despite the similarity in spectroscopic properties, the structural parameters of the 2D aggregates drastically depend on the preparation procedure. Under certain conditions extra-large sheets (thickness of 2 nm, aspect ratio area/thickness ~107) in aqueous solution are formed4B. Finally, one-dimensional assemblies are formed as micrometer-long and nanometer-thick fibers. Both, planar and linear structures are intriguing objects for the creation of conductive nanowires that may find interest for applications in supramolecular electronics.

From supramolecular sheets to fibers and back: establishing the critical parameters that govern the morphology of pyrene-based self-assembled materials

The precise arraying of functional entities in morphologically well-defined shapes remains one of the key challenges in the processing of organic molecules1. Among various π-conjugated species, pyrene exhibits a set of unique properties, which make it an attractive compound for the utilization in materials science2. In this contribution we report on properties of self-assembled structures prepared from amphiphilic pyrene trimers (Py3) consisting of phosphodiester-linked pyrenes. Depending on the geometry of a pyrene core substitution (1.6-, 1.8-, or 2.7- type, see Scheme), the thermally-controlled self-assembly allows the preparation of supramolecular architectures of different morphologies in a bottom-up approach: two-dimensional (2D) nanosheets3 are formed in case of 1.6- and 2.7-substitution4 whereas one-dimensional (1D) fibers are built from 1.8- substituted isomers. The morphologies of the assemblies are established by AFM and TEM, and the results are further correlated with spectroscopic and scattering data. Two-dimensional assemblies consist of an inner layer of hydrophobic pyrenes, sandwiched between a net of phosphates. Due to the repulsion of the negative charges, the 2D assemblies exist mostly as free-standing sheets. An internal alignment of pyrenes leads to strong exciton coupling with an unprecedented observation (simultaneous development of J- and H-bands from two different electronic transitions). Despite the similarity in spectroscopic properties, the structural parameters of the 2D aggregates drastically depend on the preparation procedure. Under certain conditions extra-large sheets (thickness of 2 nm, aspect ratio area/thickness ~107) in aqueous solution are formed4B. Finally, one-dimensional assemblies are formed as micrometer-long and nanometer-thick fibers. Both, planar and linear structures are intriguing objects for the creation of conductive nanowires that may find interest for applications in supramolecular electronics.

Cheek Tooth Extraction Via a Minimally Invasive Transbuccal Approach and Intradental Screw Placement in 54 Equids

OBJECTIVE: To describe (1) preoperative findings and surgical technique, (2) intraoperative difficulties, and (3) postoperative complications and long-term outcome of equine cheek tooth extraction using a minimally invasive transbuccal screw extraction (MITSE) technique. STUDY DESIGN: Retrospective case series. ANIMALS: Fifty-four equids; 50 horses, 3 ponies, and 1 mule. METHODS: Fifty-eight MITSE procedures were performed to extract cheek teeth in 54 equids. Peri- and intraoperative difficulties, as well as short- (<1 month) and long-term (>6 months) postoperative complications were recorded. Followup information was obtained through telephone interviews, making specific inquiries about nasal discharge, facial asymmetry, and findings consistent with surgical site infection. RESULTS: Preoperative findings that prompted exodontia included 50 cheek teeth with apical infections, 48 fractures, 4 neoplasia, 2 displacements, and 1 supernumerary tooth. Previous oral extraction was attempted but had failed in 55/58 (95%) animals because of cheek tooth fracture in 28, or insufficient clinical crown for extraction with forceps in 27. MITSE was successful in removing the entire targeted dental structure in 47/58 (81%) procedures. However, MITSE failed to remove the entire targeted dental structure in 11/58 (19%) procedures and was followed by repulsion in 10/11 (91%). Short-term postoperative complications included bleeding (4/58 procedures, 7%) and transient facial nerve paralysis (4/58 procedures, 7%). Owners were satisfied with the functional and cosmetic outcome for 40/41 (98%) animals with followup. CONCLUSION: MITSE offers an alternate for cheek tooth extraction in equids, where conventional oral extraction is not possible or has failed. Overall, there was low morbidity, which compares favorably with invasive buccotomy or repulsion techniques