Conspectus We describe here a family of coordination cages with interesting structural, guest-binding, and catalytic properties. Flexible bridging ligands containing two bidentate pyrazolylpyridine ...termini assemble with transition-metal dications to afford coordination cages containing a metal ion at each vertex, a bridging ligand spanning each edge, and a 2:3 metal:ligand ratio. This stoichiometry is expressed in structures ranging from M4L6 tetrahedra to M16L24 tetracapped truncated tetrahedra, which are stabilized by the formation of π-stacked arrays between electron-rich and electron-poor ligand segments that form around the cage periphery. In some cases concentration- and/or temperature-dependent equilibria between multiple cage structures occur, arising from a balance between entropy, which favors the formation of a larger number of smaller assemblies, and enthalpy, which maximizes both interligand aromatic stacking and solvophobic effects in the larger assembles. The cages are hollow and can accommodate guestsoften anions or solvent moleculesin the central cavity. For one cage family, M8L12 species with an approximately cubic structure and a ca. 400 Å3 cavity, the guest binding properties have been studied extensively. This cage can accommodate a wide range of neutral organic guests, with binding in water being driven principally by the hydrophobic effect, which leads to binding constants of up to 108 M–1. The accumulation of a large amount of empirical data on guest binding in the M8L12 cage in water provided the basis for a predictive tool for in silico screening of potential guests using the molecular docking program GOLD; this methodology has allowed the identification of numerous new guests with accurately predicted binding constants and provides a transformative new approach to exploring the host/guest chemistry of cages. Binding of benzisoxazole inside the M8L12 cage results in substantial rate enhancementsby a factor of up to 2 × 105of the Kemp elimination, in which benzisoxazole reacts to give 2-cyanophenolate. Catalysis arises because the 16+ cage cation accumulates anions around the surface by ion pairing, leading to a high effective concentration of hydroxide ions surrounding the guest even when the bulk pH is modest. Thus, the catalysis relies on the operation of two orthogonal interactions that bring the reaction partners together: hydrophobic guest binding in the cavity, which is lined with CH groups from the ligands, and ion pairing around the highly cationic cage surface. A consequence of this is that under some conditions the product of the cage-catalyzed Kemp elimination (the 2-cyanophenolate anion) itself accumulates around the cage surface and deprotonates another benzisoxazole guest, perpetuating the reaction in an autocatalytic manner. Thus, different anions accumulating around the cage can act as partners for reaction with a cavity-bound guest, opening up the possibility that the M8L12 cage can act as a general catalyst for reactions of electrophilic guests with surface-bound anions.
Abstract
Terrestrial ecosystems play a significant role in the global carbon cycle and offset a large fraction of anthropogenic CO
2
emissions. The terrestrial carbon sink is increasing, yet the ...mechanisms responsible for its enhancement, and implications for the growth rate of atmospheric CO
2
, remain unclear. Here using global carbon budget estimates, ground, atmospheric and satellite observations, and multiple global vegetation models, we report a recent pause in the growth rate of atmospheric CO
2
, and a decline in the fraction of anthropogenic emissions that remain in the atmosphere, despite increasing anthropogenic emissions. We attribute the observed decline to increases in the terrestrial sink during the past decade, associated with the effects of rising atmospheric CO
2
on vegetation and the slowdown in the rate of warming on global respiration. The pause in the atmospheric CO
2
growth rate provides further evidence of the roles of CO
2
fertilization and warming-induced respiration, and highlights the need to protect both existing carbon stocks and regions, where the sink is growing rapidly.
The hollow cavities of coordination cages can provide an environment for enzyme-like catalytic reactions of small-molecule guests. Here, we report a new example (catalysis of the Kemp elimination ...reaction of benzisoxazole with hydroxide to form 2-cyanophenolate) in the cavity of a water-soluble M8L12 coordination cage, with two features of particular interest. First, the rate enhancement is among the largest observed to date: at pD 8.5, the value of kcat/kuncat is 2 × 10(5), due to the accumulation of a high concentration of partially desolvated hydroxide ions around the bound guest arising from ion-pairing with the 16+ cage. Second, the catalysis is based on two orthogonal interactions: (1) hydrophobic binding of benzisoxazole in the cavity and (2) polar binding of hydroxide ions to sites on the cage surface, both of which were established by competition experiments.
Regulation by gene-distal enhancers is critical for cell type-specific and condition-specific patterns of gene expression. Thus, to understand the basis of gene activity in a given cell type or ...tissue, we must identify the precise locations of enhancers and functionally characterize their behaviors. Here, we demonstrate that transcription is a nearly universal feature of enhancers in
and mammalian cells and that nascent RNA sequencing strategies are optimal for identification of both enhancers and superenhancers. We dissect the mechanisms governing enhancer transcription and discover remarkable similarities to transcription at protein-coding genes. We show that RNA polymerase II (RNAPII) undergoes regulated pausing and release at enhancers. However, as compared with mRNA genes, RNAPII at enhancers is less stable and more prone to early termination. Furthermore, we found that the level of histone H3 Lys4 (H3K4) methylation at enhancers corresponds to transcriptional activity such that highly active enhancers display H3K4 trimethylation rather than the H3K4 monomethylation considered a hallmark of enhancers. Finally, our work provides insights into the unique characteristics of superenhancers, which stimulate high-level gene expression through rapid pause release; interestingly, this property renders associated genes resistant to the loss of factors that stabilize paused RNAPII.
Changing the length of a laser cavity is a simple technique for continuously tuning the wavelength of a laserbutis rarelyused for broad fractional tuning, with a notable exception of the diode laser ...VCSEL1,2. This is because, to avoid mode hopping, the cavity must be keptopticallyshort to ensure a large free-spectral-range (FSR) compared to the gain bandwidth of the amplifying material. Terahertz quantum-cascade (QC) lasers are ideal candidates for such a short cavity scheme as they demonstrate exceptional gain bandwidths (up to octave spanning) and can be integrated with broadband amplifying metasurfaces. We present such a QC-metasurface based vertical-external-cavity surface-emitting-laser (VECSEL) that exhibits over 20% continuous fractional tuning of a single laser mode. Such tuning is possible because the metasurface has sub-wavelength thickness which allows lasing on low-order Fabry-Pérot cavity modes. Good beam quality and high output power are simultaneously obtained.
Abstract
The replacement of natural lands with urban structures has multiple environmental consequences, yet little is known about the magnitude and extent of albedo-induced warming contributions ...from urbanization at the global scale in the past and future. Here, we apply an empirical approach to quantify the climate effects of past urbanization and future urbanization projected under different shared socioeconomic pathways (SSPs). We find an albedo-induced warming effect of urbanization for both the past and the projected futures under three illustrative scenarios. The albedo decease from urbanization in 2018 relative to 2001 has yielded a 100-year average annual global warming of 0.00014 0.00008, 0.00021 °C. Without proper mitigation, future urbanization in 2050 relative to 2018 and that in 2100 relative to 2018 under the intermediate emission scenario (SSP2-4.5) would yield a 100-year average warming effect of 0.00107 0.00057,0.00179 °C and 0.00152 0.00078,0.00259 °C, respectively, through altering the Earth’s albedo.
Urbanization contributes to the loss of the world's biodiversity and the homogenization of its biota. However, comparative studies of urban biodiversity leading to robust generalities of the status ...and drivers of biodiversity in cities at the global scale are lacking. Here, we compiled the largest global dataset to date of two diverse taxa in cities: birds (54 cities) and plants (110 cities). We found that the majority of urban bird and plant species are native in the world's cities. Few plants and birds are cosmopolitan, the most common being Columba livia and Poa annua. The density of bird and plant species (the number of species per km2) has declined substantially: only 8% of native bird and 25% of native plant species are currently present compared with estimates of non-urban density of species. The current density of species in cities and the loss in density of species was best explained by anthropogenic features (landcover, city age) rather than by non-anthropogenic factors (geography, climate, topography). As urbanization continues to expand, efforts directed towards the conservation of intact vegetation within urban landscapes could support higher concentrations of both bird and plant species. Despite declines in the density of species, cities still retain endemic native species, thus providing opportunities for regional and global biodiversity conservation, restoration and education.
Understanding vegetation recovery after drought is critical for projecting vegetation dynamics in future climates. From 1997 to 2009, Australia experienced a long‐lasting drought known as the ...Millennium Drought (MD), which led to widespread reductions in vegetation productivity. However, vegetation recovery post‐drought and its determinants remain unclear. This study leverages remote sensing products from different sources—fraction of absorbed photosynthetically active radiation (FPAR), based on optical data, and canopy density, derived from microwave data—and random forest algorithms to assess drought recovery over Australian natural vegetation during a 20‐year period centered on the MD. Post‐drought recovery was prevalent across the continent, with 6 out of 10 drought events seeing full recovery within about 6 months. Canopy density was slower to recover than leaf area seen in FPAR. The probability of full recovery was most strongly controlled by drought return interval, post‐drought hydrological condition, and drought length. Full recovery was seldom observed when drought events occurred at intervals of 3 months or less, and moderately dry (standardized water balance anomaly SWBA within −1, −0.76) post‐drought conditions resulted in less complete recovery than wet (SWBA > 0.3) post‐drought conditions. Press droughts, which are long term but not extreme, delayed recovery more than pulse droughts (short term but extreme) and led to a higher frequency of persistent decline. Following press droughts, the frequency of persistent decline differed little among biome types but peaked in semi‐arid regions across aridity levels. Forests and savanna required the longest recovery times for press drought, while grasslands were the slowest to recover for pulse drought. This study provides quantitative thresholds that could be used to improve the modeling of ecosystem dynamics post‐drought.
With analysis of spaceborne optical and microwave remote sensing, we assess the determinants of drought recovery for Australian natural vegetation during a 20‐year period centered on the Millennium Drought (1997–2009). Recovery was most strongly controlled by drought return interval, post‐drought hydrological condition, and drought duration. Press droughts, which are long duration but less extreme, were more likely to cause persistent decline than pulse droughts, which are short duration but extreme.
PDF
