‘Dynamic bonds’ reshape the rules of aromaticity and chirality

 New discoveries in ‘dynamic bonds’ could reshape our fundamental understanding of key chemical concepts, including aromaticity and chirality. A team at the University of York in the UK has synthesised a polycyclic molecule whose aromaticity can be switched on and off, as well as a carbon cage where chiral carbon atoms interconvert without breaking bonds at the stereocentre. This ‘subverts our view of carbon-based molecules as fixed objects’, according to lead author Paul McGonigal. In the future, these new concepts could one day underpin ‘new applications for dynamic molecular materials’.

The researchers started by studying fluxional molecules.1 In these species, different functional groups interchange positions but, depending on the velocity of the process and the timescales of the observations, they may appear identical. An example is the extremely fast interconversion between cyclohexane chair and boat conformations. In an attempt to control and condition the interconversion rates of a range of fluxional molecules, researchers at York started overcrowding the structures with bulky and highly crowded systems. ‘We were lucky to observe both phenomena while exploring the effects of bond strain in fluxional molecules,’ explains McGonigal. Previously, the team had used this strategy to create unusual luminescence in strained structures, such as molecular rotors. Now, the results demonstrate dynamism is more common in organic molecules than previously thought.

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A New Experiment Casts Doubt on the Leading Theory of the Nucleus

 

Excited helium nuclei inflate like balloons, offering physicists a chance to study the strong nuclear force, which binds the nucleus’s protons and neutrons.

Anew measurement of the strong nuclear force, which binds protons and neutrons together, confirms previous hints of an uncomfortable truth: We still don’t have a solid theoretical grasp of even the simplest nuclear systems.

To test the strong nuclear force, physicists turned to the helium-4 nucleus, which has two protons and two neutrons. When helium nuclei are excited, they grow like an inflating balloon until one of the protons pops off. Surprisingly, in a recent experiment, helium nuclei didn’t swell according to plan: They ballooned more than expected before they burst. A measurement describing that expansion, called the form factor, is twice as large as theoretical predictions.

“The theory should work,” said Sonia Bacca, a theoretical physicist at the Johannes Gutenberg University of Mainz and an author of the paper describing the discrepancy, which was published in Physical Review Letters. “We’re puzzled.”

The swelling helium nucleus, researchers say, is a sort of mini-laboratory for testing nuclear theory because it’s like a microscope — it can magnify deficiencies in theoretical calculations. Physicists think certain peculiarities in that swelling make it supremely sensitive to even the faintest components of the nuclear force — factors so small that they’re usually ignored. How much the nucleus swells also corresponds to the squishiness of nuclear matter, a property that offers insights into the mysterious hearts of neutron stars. But before explaining the crush of matter in neutron stars, physicists must first figure out why their predictions are so far off.

Bira van Kolck, a nuclear theorist at the French National Center for Scientific Research, said Bacca and her colleagues have exposed a significant problem in nuclear physics. They’ve found, he said, an instance where our best understanding of nuclear interactions — a framework known as chiral effective field theory — has fallen short.

“This transition amplifies the problems [with the theory] that in other situations are not so relevant,” van Kolck said.

The Strong Nuclear Force

Atomic nucleons — protons and neutrons — are held together by the strong force. But the theory of the strong force was not developed to explain how nucleons stick together. Instead, it was first used to explain how protons and neutrons are made of elementary particles called quarks and gluons.

For many years, physicists didn’t understand how to use the strong force to understand the stickiness of protons and neutrons. One problem was the bizarre nature of the strong force — it grows stronger with increasing distance, rather than slowly dying off. This feature prevented them from using their usual calculation tricks. When particle physicists want to understand a particular system, they typically parcel out a force into more manageable approximate contributions, order those contributions from most important to least important, then simply ignore the less important contributions. With the strong force, they couldn’t do that.

Then in 1990, Steven Weinberg found a way to connect the world of quarks and gluons to sticky nuclei. The trick was to use an effective field theory — a theory that is only as detailed as it needs to be to describe nature at a particular size (or energy) scale. To describe the behavior of a nucleus, you don’t need to know about quarks and gluons. Instead, at these scales, a new effective force emerges — the strong nuclear force, transmitted between nucleons by the exchange of pions.

Weinberg’s work helped physicists understand how the strong nuclear force emerges from the strong force. It also made it possible for them to perform theoretical calculations based on the usual method of approximate contributions. The theory — chiral effective theory — is now widely considered the “best theory we have,” Bacca said, for calculating the forces that govern the behavior of nuclei.

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To Battle Climate Change, Scientists Tap Into Carbon-Hungry Microorganisms for Clues

 

Scientists at Lawrence Berkeley National Laboratory (Berkeley Lab) have demonstrated a new technique, modeled after a metabolic process found in some bacteria, for converting carbon dioxide (CO2) into liquid acetate, a key ingredient in “liquid sunlight” or solar fuels produced through artificial photosynthesis.

The new approach, reported in Nature Catalysis, could help advance carbon-free alternatives to fossil fuels linked to global warming and climate change.

The work is also the first demonstration of a device that mimics how these bacteria naturally synthesize acetate from electrons and CO2.

“What’s amazing is that we learned how to selectively convert carbon dioxide into acetate by mimicking how these little microorganisms do it naturally,” said senior author Peidong Yang, who holds titles of senior faculty scientist in Berkeley Lab’s Materials Sciences Division and professor of chemistry and materials science and engineering at UC Berkeley.

“Everything we do in my lab to convert CO2 into useful products is inspired by nature. In terms of mitigating CO2 emissions and fighting climate change, this is part of the solution.”

– Peidong Yang, Berkeley Lab senior faculty scientist, Materials Sciences Division

For decades, researchers have known that a metabolic pathway in some bacteria allows them to digest electrons and CO2 to produce acetate, a reaction driven by the electrons. The pathway breaks CO2 molecules down into two different or “asymmetric” chemical groups: a carbonyl group (CO) or a methyl group (CH3). Enzymes in this reaction pathway enable the carbons in CO and CH3 to bond or “couple,” which then triggers another catalytic reaction that produces acetate as the final product.

Researchers in the field of artificial photosynthesis have wanted to develop devices that mimic the pathway’s chemistry – called asymmetric carbon-carbon coupling – but finding synthetic electrocatalysts that work as efficiently as bacteria’s natural enzymatic catalysts has been challenging.

“But we thought, if these microorganisms can do it, one should be able to mimic their chemistry in the lab,” Yang said.

Advancing artificial photosynthesis with carbon-hungry copper

Copper’s talent for converting carbon into various useful products was first discovered in the 1970s. Based on those previous studies, Yang and his team reasoned that artificial photosynthesis devices equipped with a copper catalyst should be able to convert CO2 and water into methyl and carbonyl groups, and then turn these products into acetate. So for one experiment, Yang and team designed a model device with a copper surface; then, they exposed the copper surface to liquid methyl iodide (CH3I) and CO gas, and applied an electrical bias to the system.

The researchers hypothesized that CO would stick to the copper surface, triggering the asymmetric coupling of CO and CH3 groups to produce acetate. Isotope-labelled CH3I was used in the experiments in order to track the reaction pathway and final products. (An isotope is an atom with more or fewer neutrons (uncharged particles) in its nucleus than other atoms of an element.)

And they were right. Chemical analytical experiments conducted in Yang’s UC Berkeley lab revealed that copper’s pairing of carbonyl and methyl groups produced not only acetate but other valuable liquids, including ethanol and acetone. The isotopic tracking allowed the researchers to confirm that the acetate was formed through the combination of the CO and CH3.

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Biosensor Versatility; From Analytical Chemistry to Diagnostics

 Classical diagnostic methods tend to be better suited for on-the-spot blood measurement. This form of method is conducted periodically, such as twice a year, and it gives you a single measurement. However, some circumstances require biochemical information to be monitored as it fluctuates, such as diabetics.

In diabetics, we have what we call CGM, Continuous Glucose Monitor, which monitors glucose fluctuation. This type of fluctuation must also be monitored for cardiac, stress, wellness, and nutrition.

Measuring blood monthly or quarterly is an insufficient method, as you are unable to get an idea of the overall picture and identify any trends. This is not only the case in medicine, but also in fitness; you want to continuously monitor your hydration or lactate – all of which fluctuate.

There are a lot of temporal variations in kidney and cardiac diseases. Thus, taking just one measurement will not provide useful information.

If you look at the glucose market, it is dominated by electrochemical devices. The beauty of an electrochemical sensor is that it is a small, compact, portable device that is easy to mass produce with a low power requirement, making them very attractive technologies.

For instance, the finger stick blood test, a mobile self-testing or wearable device, relies on electrochemistry because of these unique properties.

We are creating wearable alcohol or opioid sensors to help prevent drunk driving or drug abuse. A sensor that can monitor cortisol levels would also be highly advantageous when it comes to determining stress levels. Other useful targets would be vitamins to aid in monitoring personal nutrition.

We are also hoping to develop a sensor to help identify trace elements and minerals in food supplements. Nerve agents for monitoring the body’s surroundings – such as electrolytes, metabolites, and hormones – are other viable targets.

The beauty of these sensors is that they are non-invasive, as you do not need to physically take a sample of blood. However, everything needs to be validated by comparing to blood, which is the gold standard, so we must also validate without controlled conditions. Other challenges include changes in the surrounding temperature, e.g., when running around in the summer compared to in winter. Common bioreceptor, such as enzymes are not so stable in uncontrolled extreme conditions.

There is also the issue of bio-fouling. In terms of mobility, it is easy to measure your steps, calories, or ECG artery as these are physical characteristics. However, when it comes to chemical sensing, you need a bioreceptor, and you need to immobilize it to make it stable. This is why we do not have many of these except glucose.

It is a great honor to have been presented with this award, especially as I knew Ralph Adam personally. Ralph Adam passed away in 2002; he was a fantastic scientist and, more importantly, a wonderful person. He had a vision and shared my idea to make receptors simple but creative.

He was the first to put electrodes in the brain of small animals and gain insight into the neurochemicals of the brain. This was back in the 70s. At present, I am now putting electrodes in the skin. The progression in this field is astounding, but it would not have been possible without great scientists like Ralph.

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New Unsaturated Fatty Acids Discovered in Human Samples

 

QUT researchers have discovered 103 new unsaturated fatty acids in human derived samples. These findings have doubled the number of these fundamental building blocks of life previously reported in human blood plasma.

In an article in the prestigious journal Nature Communications, QUT researchers and their colleagues in Adelaide and Prague have described their findings and the new analytical technique that enabled the discoveries.

Professor Stephen Blanksby, from the QUT Centre for Materials Science said the human body made its own fatty acids but also took up fatty acids from food that were then modified to make them fit for purpose.

“Lipids play many roles in the body - some form cell membranes, others are precursors for signalling molecules that regulate how the body copes with inflammation and the resolution of inflammation,” Professor Blanksby said.

“This means changes in fatty acids and other lipids (complex fats made from fatty acids) in the body can provide critical clues for health and disease.”

“We know that blood tests report on lipids like cholesterol and triglycerides that are linked to our health status and, with further research, these new molecules could provide critical information about our bodies’ responses to diet or disease.”

Professor Blanksby said QUT researchers developed advanced analytical technology to probe the human lipidome (all lipids in a cell) more deeply than was previously possible.

“The discovery of new lipids and new lipid metabolism using this approach paves the way for more sensitive and selective diagnostic tests,” he said.

Dr Jan Philipp Menzel, a postdoctoral fellow in the QUT School of Chemistry and Physics, said the discoveries were enabled by a combination of liquid chromatography with a mass spectrometer modified to enable a gas-phase reaction with ozone that broke down the carbon-carbon double bonds in unsaturated fatty acids.

Dr Menzel developed custom software to trawl the complex datasets the team obtained to identify the novel lipids.

“It was an innovative approach that allowed us to characterise the structure of unsaturated fatty acids,” Dr Menzel said.

“Using this process we studied human blood plasma, cancer cells, and vernix caseosa, a white layer covering newborns, and found new and different fatty acids in each.

“Some of the newly found fatty acids may not originate from human metabolism but are likely present in blood plasma, for example, after being consumed in food whereas most fatty acids found in vernix caseosa are likely to be a product of human metabolism.

“Our investigation of cancer cell lines included the addition of an enzyme inhibitor to one cell line that helped to assign which fatty acids were formed in increased amounts in laboratory conditions.

“Some of our results show the same trends established in several recent publications and add to the body of evidence that fatty acid metabolism is an important aspect of the metabolism of cancer cells.

“It will take a concerted effort by many scientists around the world to unravel the full biological significance of all the fatty acids that were identified in this study. For example, some new omega-3 fatty acids found in vernix caseosa have unusual patterns of double bonds.

“Fish and seafood, walnuts and flaxseed are well known for essential fatty acids (omega-3 polyunsaturated fatty acids) and their health benefits. However, we currently know very little about the new omega-3 fatty acids we detected on the skin of newborns.”

“The exact structure of a biomolecule determines its biological function, a principle used extensively in biochemistry and biomedical research. Finding biomolecules with new structures (here, differences in the position of double bonds along a fatty acid chain) could be a first step towards studying new metabolic pathways or even develop diagnostic methods or treatments.”

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This one-atom chemical reaction could transform drug discovery

 Researchers from Osaka University have stabilized atomic carbon for common reaction conditions in organic chemistry. This nearly unprecedented work could simplify and lower the cost of pharmaceutical synthesis

Osaka, Japan – Pharmaceutical synthesis is often quite complex; simplifications are needed to speed up the initial phase of drug development and lower the cost of generic production. Now, in a study recently published in Science, researchers from Osaka University have discovered a chemical reaction that could transform drug production because of its simplicity and utility.

Pharmaceuticals generally contain a few tens of atoms and a similar number of chemical bonds between the atoms. Thus, designing complex drug architectures from simple precursors using the techniques of organic chemistry usually requires careful planning and tedious, incremental steps. The gold standard in drug synthesis is to create, in one step, as many chemical bonds as possible. In principle, adding one carbon atom—by forming four bonds in one step—to a drug precursor can be a means of doing so. Unfortunately, atomic carbon is generally too unstable for use in common chemical reaction conditions. This is the problem that the researchers sought to address.

"Because atomic carbon is too unstable for use in organic synthesis, reagents such as dihalocarbenes are basically all that's available as atomic carbon equivalents," explains Miharu Kamitani, the lead author of the study. "We have expanded the toolkit for such reactions and have applied our technique to an established pharmaceutical."

The Osaka University researchers' discovery is based on a class of molecules known as N-heterocyclic carbenes. By a chemical process known as resonance, these molecules contain a stabilized version of a carbon atom equivalent. By a straightforward reaction with alpha, beta-unsaturated amides (an important molecule in cancer progression), various gamma-lactams (cyclic molecules that are common in antibiotics) were produced in one step, often in greater than 60% yield. Particularly noteworthy is a one-step chemical modification of aminoglutethimide—a drug for treating seizures and other conditions—in 96% yield. Thus, even complex drugs can be modified for drug targeting and activity studies, as well as a myriad of other procedures that are otherwise synthetically complex aspects of drug discovery.

"Pharmaceutical companies are always on the lookout for straightforward reactions that achieve complex chemical transformations," says Mamoru Tobisu, senior author of the study. "We envision that our single carbon atom doping reaction will be broadly useful in this context."

This work succeeded in using an atomic carbon equivalent to form four chemical bonds in one step, synthesize pharmaceutically useful chemical architectures, and fundamentally transform the chemical nature of an established drug molecule. The Osaka University researchers' approach will be useful for quickly preparing potential pharmaceuticals, which will speed up drug research and development as well as production of currently established drugs—especially if the approach is extended to additional classes of transformations in organic chemistry.

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Luminous Molecules – A New Concept in Synthesis

 

A team of chemists at the University of Basel have pioneered a new synthesis method for creating chiral, or “twisted,” helicenes—compounds critical to the advancement of organic light-emitting diodes (OLEDs)—which will open the door to better light sources.

Twisted molecules play an important role in the development of organic light-emitting diodes. A team of chemists has managed to create these compounds with exactly the three-dimensional structure that they wanted. In so doing, they are smoothing the path for new and better light sources.

They flash as a warning, glow red on standby mode, and light up your dinner table; light-emitting diodes (LEDs) have become indispensable in our daily lives. Somewhat less well-known, but just as ubiquitous, are organic light-emitting diodes, or OLEDs for short. This technology is used in the screens of smartphones, tablets, and monitors. It is cheaper to produce in the form of a thin-film component, but cannot yet compete with conventional LEDs in some ways, such as in light output and lifespan.

In the search for new molecules that possess the characteristics needed for OLEDs, compounds known as helicenes are playing a central role. Helicenes are a group of substances in which rings made up of six carbon atoms (benzene rings) are joined together in a helical structure. When synthesizing these compounds, it was previously difficult to control the direction in which the molecules twist – their “chirality.” It was only possible with certain types of helicenes and to a very limited extent.

A new concept in synthesis

Professor Olivier Baudoin, Dr. Shu-Min Guo, and Soohee Huh from the University of Basel’s Department of Chemistry have just made an important step forward. In the latest issue of Nature Chemistry, they describe a new concept in the synthesis of these important chiral molecules.

In their synthesis route, the Basel researchers make use of a reaction that can split a carbon-hydrogen and carbon-bromine bond and create a carbon-carbon bond. This is called C-H activation, and over the past few years, it has developed into a valuable tool in synthesis. This method allows chemists to create helicenes with the desired chirality, and could also be suitable for longer chains of benzene rings.

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Discovering evidence of superradiance in the alpha decay of mirror nuclei

 

Scientists refer to atomic nuclei as "quantum many-body systems" because they are formed by many particles (nucleons, which include neutrons and protons) that interact with each other in complex ways. Nuclei can absorb energy, placing them into excited states. These states then lose energy through decay and may emit different particles. The various processes of decay and particle emission are called decay channels. The interplay between the internal characteristics of the excited states and the different decay channels gives rise to interesting phenomena.

One of these phenomena is superradiance. This occurs when a nucleus reaches a high excitation energy. According to the nuclear shell model, nuclei get excited by promoting nucleons to higher shells. These configurations are called excited states. As the excitation energy available increases, the number of ways the nucleons can be promoted increases, therefore the number of excited states increases. Superradiance can take place when excited states are so close to each other that neighboring excited states overlap with each other. If it happens, instead of observing many states, we see only one "superradiant" state.

To find evidence of superradiance in nuclei, nuclear physicists look for two systems that have the same internal structure but different decay channels. Mirror nuclei have the same total number of protons and neutrons, but the number of protons in one equals the number of neutrons in the other. The internal structure of mirror nuclei is the same since the nuclear force is the same whether between two protons, two neutrons, or a proton and a neutron. This makes the nuclear force "charge independent." However, the decay channels are different due to the different electric charge repulsion in the two systems because of the difference in each system's number of protons.

In a new study published in Physical Review C, scientists from Texas A&M University, the CEA research institute in France, the University of Birmingham, UK, and Florida State University have found evidence of the superradiance effect in the differences between the alpha decaying states in Oxygen-18 and Neon-18.

The research team studied the structure of Neon-18 by scattering a radioactively unstable beam of Oxygen-14 on a thick Helium-4 gas target. The gas target allowed the experimentalists to measure the tracks of the incoming and outcoming particles and produce a complete reconstruction of the nuclear events. The structure of Oxygen-18 had been previously studied at Florida State University by scattering Carbon-14 on a Helium-4 target using a particle accelerator. This experiment had very good results, allowing the researchers to use the information about the Oxygen-18 excited states to find the initial parameters for the analysis of the Neon-18 data.

As expected from the charge independence of the nuclear force, the researchers found a correspondence between mirror states in the two nuclei, although some differences emerged when comparing the strength of mirror states. If the internal structure of the nuclei is the same, one would expect mirror levels to have the same strength, but in these cases alignment with slightly different decay channels produces observed differences. The researchers interpreted these differences as evidence of the superradiance effect.

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Organometallic chemistry: High-valent iron gets homoleptic

 The diverse roles that iron has in natural and laboratory environments require it to assume a range of oxidation states — each a guise with a distinct personality. In the +IV state, iron can strip hydrogen from C–H bonds, allowing the installation of a halogen or an oxygenic group. Although such base-metal-catalysed direct functionalizations are attractive and intensely studied, our understanding of Fe(IV) complexes that are often proposed as intermediates has yet to catch up. Homoleptic species are particularly elusive, which increases the significance of the [Fe(alkyl)4] complexes — prepared by disproportionation reactions — reported in Angewandte Chemie International Edition by a team led by Alois Firster.

Taming a strong Lewis acid such as Fe(IV) typically calls for anionic ligands that are both σ- and π-basic. Despite this, FeF4 — a D2d-symmetric molecule — only persists within a noble gas matrix. The distorted square-planar ketimido complex [Fe(tBu2CN)4] and salts of [Fe(C5Me5)2]2+ are more robust, although interactions with counterions contribute to the stability of the latter. Alkyl ligands are strong σ-bases, but their lack of π-basicity makes [Fe(alkyl)4] species, with their low formal Fe valence electron count, all the more striking. The first such complex to be isolated was [Fe(1-norbornyl)4], in which the bulky norbornyls crowd the Fe(IV) centre in a tetrahedral geometry. The carbocyclic ligands interdigitate so as to minimize steric strain that might otherwise render the complex unviable. The bulky organic groups participate in London dispersion interactions that, along with the four Fe–C bonds, hold the complex together.

While studying C–C coupling reactions, Fürstner and colleagues considered the reduction of iron catalysts with Grignard reagents. In some cases, Fe(II) or Fe(III) precursors afforded high-valent iron complexes among their products. This is a strange phenomenon: “it was totally unexpected to isolate an oxidized complex from a reaction mixture comprising FeCl2 and cyclohexylmagnesium chloride,” notes Fürstner. The team found that [Fe(cyclohexyl)4] forms alongside Fe metal in a disproportionation reaction that was also used in the synthesis of [Fe(1-norbornyl)4], of which they were unaware at the time. Reflective of the strong donicity of cyclohexyls, the new tetrahedral complex is low-spin (e4t20) because the ligands give rise to a large ligand field splitting (Δt = 40 kcal mol−1). Cyclohexyls are smaller than norbornyls, so the reasonable stability of [Fe(cyclohexyl)4] at −20 °C is interesting given its more exposed Fe(IV) centre and attenuated dispersion interactions. Such interactions appear to tip the Fe(II or III)–Grignard system in favour of disproportionation: [Fe(2-adamantyl)4] could be prepared, but examples with ligands smaller than cyclohexyl could not.

Only then will we know whether this exceptional complex is just a curiosity or an entry into a wider and basically uncharted territory of organometallic chemistry and catalysis

Although similar in nature, [Fe(cyclohexyl)4] differs from [Fe(1-norbornyl)4] in that the former features Fe bound to C atoms that each bear a H atom. It is remarkable that an electrophilic centre such as Fe(IV) does not participate in significant agostic interactions with the C–H groups (let alone abstract an α- or β-hydride), an observation ascribed to these being situated just outside the lobes of the three d orbitals in the t2 set. It remains conceivable that [Fe(cyclohexyl)4] could bind suitably small substrate molecules using these vacant orbitals. Higher levels of theory are required to understand the metastability of [Fe(cyclohexyl)4], which is tentatively rationalized by Fürstner in terms of stereoelectronics and dispersion forces. “Only then,” he adds, “will we know whether this exceptional complex is just a curiosity or an entry into a wider and basically uncharted territory of organometallic chemistry and catalysis.”

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