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"Weber, P."
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The photochemical ring-opening of 1,3-cyclohexadiene imaged by ultrafast electron diffraction
The ultrafast photoinduced ring-opening of 1,3-cyclohexadiene constitutes a textbook example of electrocyclic reactions in organic chemistry and a model for photobiological reactions in vitamin D synthesis. Although the relaxation from the photoexcited electronic state during the ring-opening has been investigated in numerous studies, the accompanying changes in atomic distance have not been resolved. Here we present a direct and unambiguous observation of the ring-opening reaction path on the femtosecond timescale and subångström length scale using megaelectronvolt ultrafast electron diffraction. We followed the carbon–carbon bond dissociation and the structural opening of the 1,3-cyclohexadiene ring by the direct measurement of time-dependent changes in the distribution of interatomic distances. We observed a substantial acceleration of the ring-opening motion after internal conversion to the ground state due to a steepening of the electronic potential gradient towards the product minima. The ring-opening motion transforms into rotation of the terminal ethylene groups in the photoproduct 1,3,5-hexatriene on the subpicosecond timescale.
The photochemical electrocyclic ring-opening of 1,3-cyclohexadiene is a textbook organic chemistry reaction. Now, using ultrafast electron diffraction its reaction pathway has been resolved on the level of atomic distances and on its natural femtosecond timescale. Furthermore, coherent isomerization dynamics of the photoproduct 1,3,5-hexatriene were observed.
Journal Article
First impressions : a reader's journey to iconic places of the American Southwest
This unique guide for literate travelers in the American Southwest tells the story of fifteen iconic sites across Arizona, New Mexico, southern Utah, and southern Colorado through the eyes of the explorers, missionaries, and travelers who were the first non-natives to describe them. Noted borderlands historians David J. Weber and William deBuys lead readers through centuries of political, cultural, and ecological change. The sites visited in this volume range from popular destinations within the National Park System--including Carlsbad Caverns, the Grand Canyon, and Mesa Verde--to the Spanish colonial towns of Santa Fe and Taos and the living Indian communities of Acoma, Zuni, and Taos. Lovers of the Southwest, residents and visitors alike, will delight in the authors' skillful evocation of the region's sweeping landscapes, its rich Hispanic and Indian heritage, and the sense of discovery that so enchanted its early explorers. Publisher description
Book
Gene Transfer from Bacteria and Archaea Facilitated Evolution of an Extremophilic Eukaryote
Some microbial eukaryotes, such as the extremophilic red alga Galdieria sulphuraria, live in hot, toxic metal-rich, acidic environments. To elucidate the underlying molecular mechanisms of adaptation, we sequenced the 13.7-megabase genome of G. sulphuraria. This alga shows an enormous metabolic flexibility, growing either photoautotrophically or heterotrophically on more than 50 carbon sources. Environmental adaptation seems to have been facilitated by horizontal gene transfer from various bacteria and archaea, often followed by gene family expansion. At least 5% of protein-coding genes of G. sulphuraria were probably acquired horizontally. These proteins are involved in ecologically important processes ranging from heavy-metal detoxification to glycerol uptake and metabolism. Thus, our findings show that a pan-domain gene pool has facilitated environmental adaptation in this unicellular eukaryote.
Journal Article
Force sensitivity of multilayer graphene optomechanical devices
Mechanical resonators based on low-dimensional materials are promising for force and mass sensing experiments. The force sensitivity in these ultra-light resonators is often limited by the imprecision in the measurement of the vibrations, the fluctuations of the mechanical resonant frequency and the heating induced by the measurement. Here, we strongly couple multilayer graphene resonators to superconducting cavities in order to achieve a displacement sensitivity of 1.3 fm Hz
−1/2
. This coupling also allows us to damp the resonator to an average phonon occupation of 7.2. Our best force sensitivity, 390 zN Hz
−1/2
with a bandwidth of 200 Hz, is achieved by balancing measurement imprecision, optomechanical damping, and measurement-induced heating. Our results hold promise for studying the quantum capacitance of graphene, its magnetization, and the electron and nuclear spins of molecules adsorbed on its surface.
Graphene is a promising material for the design of mechanical resonators. Here, the authors fabricate a multilayer graphene resonator coupled to a superconducting cavity, to achieve efficient readout of mechanical vibrations and quantitatively investigate its force sensing performance.
Journal Article
The single-cell pathology landscape of breast cancer
Single-cell analyses have revealed extensive heterogeneity between and within human tumours
1
–
4
, but complex single-cell phenotypes and their spatial context are not at present reflected in the histological stratification that is the foundation of many clinical decisions. Here we use imaging mass cytometry
5
to simultaneously quantify 35 biomarkers, resulting in 720 high-dimensional pathology images of tumour tissue from 352 patients with breast cancer, with long-term survival data available for 281 patients. Spatially resolved, single-cell analysis identified the phenotypes of tumour and stromal single cells, their organization and their heterogeneity, and enabled the cellular architecture of breast cancer tissue to be characterized on the basis of cellular composition and tissue organization. Our analysis reveals multicellular features of the tumour microenvironment and novel subgroups of breast cancer that are associated with distinct clinical outcomes. Thus, spatially resolved, single-cell analysis can characterize intratumour phenotypic heterogeneity in a disease-relevant manner, with the potential to inform patient-specific diagnosis.
A single-cell, spatially resolved analysis of breast cancer demonstrates the heterogeneity of tumour and stroma tissue and provides a more-detailed method of patient classification than the current histology-based system.
Journal Article
Mechanistic understanding of photorespiration paves the way to a new green revolution
Photorespiration is frequently considered a wasteful and inefficient process. However, mutant analysis demonstrated that photorespiration is essential for recycling of 2-phosphoglycolate in C₃ and C₄ land plants, in algae, and even in cyanobacteria operating carboxysome-based carbon (C) concentrating mechanisms. Photorespiration links photosynthetic C assimilation with other metabolic processes, such as nitrogen and sulfur assimilation, as well as C₁ metabolism, and it may contribute to balancing the redox poise between chloroplasts, peroxisomes, mitochondria and cytoplasm. The high degree of metabolic interdependencies and the pleiotropic phenotypes of photorespiratory mutants impedes the distinction between core and accessory functions. Newly developed synthetic bypasses of photorespiration, beyond holding potential for significant yield increases in C₃ crops, will enable us to differentiate between essential and accessory functions of photorespiration.
Journal Article
How Effective Is Internal Control Reporting under SOX 404? Determinants of the (Non-)Disclosure of Existing Material Weaknesses
We study determinants of internal control reporting decisions under Section 404 of the Sarbanes-Oxley Act (SOX 404) using a sample of restating firms whose original misstatements are linked to underlying control weaknesses. We find that only a minority of these firms acknowledge their existing control weaknesses during their misstatement periods, and that this proportion has declined over time. Further, the probability of reporting existing weaknesses is negatively associated with external capital needs, firm size, non-audit fees, and the presence of a large audit firm; it is positively associated with financial distress, auditor effort, previously reported control weaknesses and restatements, and recent auditor and management changes. These results provide evidence that detection and disclosure incentives play a role in whether existing material weaknesses are reported, which has implications for the effectiveness of SOX 404 in providing investors with advance warning of potential accounting problems.
Journal Article
The Impacts of Fluctuating Light on Crop Performance
Rapidly changing light conditions can reduce carbon gain and productivity in field crops because photosynthetic responses to light fluctuations are not instantaneous. Plant responses to fluctuating light occur across levels of organizational complexity from entire canopies to the biochemistry of a single reaction and across orders of magnitude of time. Although light availability and variation at the top of the canopy are largely dependent on the solar angle and degree of cloudiness, lower crop canopies rely more heavily on light in the form of sunflecks, the quantity of which depends mostly on canopy structure but also may be affected by wind. The ability of leaf photosynthesis to respond rapidly to these variations in light intensity is restricted by the relatively slow opening/closing of stomata, activation/deactivation of C₃ cycle enzymes, and up-regulation/down-regulation of photoprotective processes. The metabolic complexity of C₄ photosynthesis creates the apparently contradictory possibilities that C₄ photosynthesis may be both more and less resilient than C₃ to dynamic light regimes, depending on the frequency at which these light fluctuations occur. We review the current understanding of the underlying mechanisms of these limitations to photosynthesis in fluctuating light that have shown promise in improving the response times of photosynthesis-related processes to changes in light intensity.
Journal Article