Glycans have diverse physiological functions, ranging from energy storage and structural integrity to cell signalling and the regulation of intracellular processes
. Although biomass-derived ...carbohydrates (such as D-glucose, D-xylose and D-galactose) are extracted on commercial scales, and serve as renewable chemical feedstocks and building blocks
, there are hundreds of distinct monosaccharides that typically cannot be isolated from their natural sources and must instead be prepared through multistep chemical or enzymatic syntheses
. These 'rare' sugars feature prominently in bioactive natural products and pharmaceuticals, including antiviral, antibacterial, anticancer and cardiac drugs
. Here we report the preparation of rare sugar isomers directly from biomass carbohydrates through site-selective epimerization reactions. Mechanistic studies establish that these reactions proceed under kinetic control, through sequential steps of hydrogen-atom abstraction and hydrogen-atom donation mediated by two distinct catalysts. This synthetic strategy provides concise and potentially extensive access to this valuable class of natural compounds.
Plastid-derived signals are known to coordinate expression of nuclear genes encoding plastid-localized proteins in a process termed retrograde signaling. To date, the identity of retrograde-signaling ...molecules has remained elusive. Here, we show that methylerythritol cyclodiphosphate (MEcPP), a precursor of isoprenoids produced by the plastidial methylerythritol phosphate (MEP) pathway, elicits the expression of selected stress-responsive nuclear-encoded plastidial proteins. Genetic and pharmacological manipulations of the individual MEP pathway metabolite levels demonstrate the high specificity of MEcPP as an inducer of these targeted stress-responsive genes. We further demonstrate that abiotic stresses elevate MEcPP levels, eliciting the expression of the aforementioned genes. We propose that the MEP pathway, in addition to producing isoprenoids, functions as a stress sensor and a coordinator of expression of targeted stress-responsive nuclear genes via modulation of the levels of MEcPP, a specific and critical retrograde-signaling metabolite.
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► The MEP pathway produces plastidial isoprenoids and serves as a plant stress sensor ► Excess light or wounding elevates the levels of isoprenoids intermediate, MEcPP ► Accumulation of MEcPP induces expression of selected stress genes in the nucleus ► MEcPP is a retrograde-signaling metabolite coordinating stress-response pathways
A metabolite produced in plastids in response to stress in turn activates stress-response genes in the nucleus, providing insight into the signaling mechanisms that link plastids and nuclei.
High T_{c} superconductors show a rich variety of phases associated with their charge degrees of freedom. Valence charges can give rise to charge ordering or acoustic plasmons in these layered ...cuprate superconductors. While charge ordering has been observed for both hole- and electron-doped cuprates, acoustic plasmons have only been found in electron-doped materials. Here, we use resonant inelastic x-ray scattering to observe the presence of acoustic plasmons in two families of hole-doped cuprate superconductors (La_{1.84}Sr_{0.16}CuO_{4} and Bi_{2}Sr_{1.6}La_{0.4}CuO_{6+δ}), crucially completing the picture. Interestingly, in contrast to the quasistatic charge ordering which manifests at both Cu and O sites, the observed acoustic plasmons are predominantly associated with the O sites, revealing a unique dichotomy in the behavior of valence charges in hole-doped cuprates.
Terahertz electromagnetic radiation is extremely useful for numerous applications, including imaging and spectroscopy. It is thus highly desirable to have an efficient table-top emitter covering the ...1-30THz window that is driven by a low-cost, low-power femtosecond laser oscillator. So far, all solid-state emitters solely exploit physics related to the electron charge and deliver emission spectra with substantial gaps. Here, we take advantage of the electron spin to realize a conceptually new terahertz source that relies on three tailored fundamental spintronic and photonic phenomena in magnetic metal multilayers: ultrafast photoinduced spin currents, the inverse spin-Hall effect and a broadband Fabry-Perot resonance. Guided by an analytical model, this spintronic route offers unique possibilities for systematic optimization. We find that a 5.8-nm-thick W/CoFeB/Pt trilayer generates ultrashort pulses fully covering the 1-30THz range. Our novel source outperforms laser-oscillator-driven emitters such as ZnTe(110) crystals in terms of bandwidth, terahertz field amplitude, flexibility, scalability and cost.
Radical-mediated transformations have emerged as powerful methods for the synthesis of rare and unnatural branched, deoxygenated, and isomeric sugars. Here, we describe a radical-mediated ...axial-to-equatorial alcohol epimerization method to transform abundant glycans into rare isomers. The method delivers highly predictable and selective reaction outcomes that are complementary to other sugar isomerization methods. The synthetic utility of isomer interconversion is showcased through expedient glycan synthesis, including one-step glycodiversification. Mechanistic studies reveal that both site- and diastereoselectivities are achieved by highly selective H atom abstraction of equatorially disposed α-hydroxy C–H bonds.
The selective manipulation of carbohydrate scaffolds is challenging due to the presence of multiple, nearly chemically indistinguishable O–H and C–H bonds. As a result, protecting-group-based ...synthetic strategies are typically necessary for carbohydrate modification. Here we report a concise semisynthetic strategy to access diverse 2- and 4-deoxygenated carbohydrates without relying on the exhaustive use of protecting groups to achieve site-selective reaction outcomes. Our approach leverages a Mn2+-promoted redox isomerization step, which proceeds via sugar radical intermediates accessed by neutral hydrogen atom abstraction under visible light-mediated photoredox conditions. The resulting deoxyketopyranosides feature chemically distinguishable functional groups and are readily transformed into diverse carbohydrate structures. To showcase the versatility of this method, we report expedient syntheses of the rare sugars l-ristosamine, l-olivose, l-mycarose, and l-digitoxose from commercial l-rhamnose. The findings presented here validate the potential for radical intermediates to facilitate the selective transformation of carbohydrates and showcase the step and efficiency advantages attendant to synthetic strategies that minimize a reliance upon protecting groups.
Tardigrades are renowned for their ability to survive a wide array of environmental stressors. In particular, tardigrades can curl in on themselves while losing a significant proportion of their ...internal water content to form a structure referred to as a tun. In surviving varying conditions, tardigrades undergo distinct morphological transformations that could indicate different mechanisms of stress sensing and tolerance specific to the stress condition. Methods to effectively distinguish between morphological transformations, including between tuns induced by different stress conditions, are lacking. Herein, an approach for discriminating between tardigrade morphological states is developed and utilized to compare sucrose- and CaCl2-induced tuns, using the model species Hypsibius exemplaris. A novel approach of shadow imaging with confocal laser scanning microscopy enabled production of three-dimensional renderings of Hys. exemplaris in various physiological states resulting in volume measurements. Combining these measurements with qualitative morphological analysis using scanning electron microscopy revealed that sucrose- and CaCl2-induced tuns have distinct morphologies, including differences in the amount of water expelled during tun formation. Further, varying the concentration of the applied stressor did not affect the amount of water lost, pointing towards water expulsion by Hys. exemplaris being a controlled process that is adapted to the specific stressors.
Obesity rates have increased dramatically in recent decades, and it has proven difficult to treat. An attentional bias towards food cues may be implicated in the aetiology of obesity and influence ...cravings and food consumption. This review systematically investigated whether attentional biases to food cues exist in overweight/obese compared with healthy weight individuals. Electronic database were searched for relevant papers from inception to October 2014. Only studies reporting food‐related attentional bias between either overweight (body mass index BMI 25.0–29.9 kg m⁻²) or obese (BMI ≥ 30) participants and healthy weight participants (BMI 18.5–24.9) were included. The findings of 19 studies were reported in this review. Results of the literature are suggestive of differences in attentional bias, with all but four studies supporting the notion of enhanced reactivity to food stimuli in overweight individuals and individuals with obesity. This support for attentional bias was observed primarily in studies that employed psychophysiological techniques (i.e. electroencephalogram, eye‐tracking and functional magnetic resonance imaging). Despite the heterogeneous methodology within the featured studies, all measures of attentional bias demonstrated altered cue‐reactivity in individuals with obesity. Considering the theorized implications of attentional biases on obesity pathology, researchers are encouraged to replicate flagship studies to strengthen these inferences.
Enzymes are a longstanding source of inspiration for synthetic reaction development. However, enzymatic reactivity and selectivity are frequently untenable in a synthetic context, as the principles ...that govern control in an enzymatic setting often do not translate to small molecule catalysis. Recent synthetic methods have revealed the viability of using small molecule catalysts to promote highly selective radical-mediated transformations of minimally protected sugar substrates. These transformations share conceptual similarities with radical SAM enzymes found in microbial carbohydrate biosynthesis and present opportunities for synthetic chemists to access microbial and unnatural carbohydrate building blocks without the need for protecting groups or lengthy synthetic sequences. Here, we highlight strategies through which radical reaction pathways can enable the site-, regio-, and diastereoselective transformation of minimally protected carbohydrates in both synthetic and enzymatic systems.
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