Communicating Chemistry (2020w209)

A study led by the University of British Columbia reveals the potential of applying short-lived radioactive species in ordinary chemical reactions.

Stable isotopes–––atoms of lifetime longer than the age of the universe (t1/2> ~10^20 years)–––predominate the every-day chemistry labs. Introducing radioactive isotopes in chemical reactions can greatly improve the diversity of available reaction mechanisms. But most of these isotopes have an extremely short lifetime, decaying into the void before any reaction will occur. 

An Imaginary Muon Source: Scorge (Deviantart)

The team led by Fleming and Macfarlane successfully introduced a highly unstable hydrogen isotope, muonium, in reactions with gold nanoparticles (AuNPs) and benzene molecules. Theoretical calculations on relevant reaction rates and quantum mechanical variables are now realized in real-life experiments.

In this study, short-lived muonium nuclides were produced from a particle accelerator at TRIUMF. It had a mean lifetime of only 2.2 microseconds, and possess only one-tenth the mass of stable hydrogen. 

TRIUMF particle accelerator at the University of British Columbia

The first reaction took place inside a cell that contained porous silica-coated AuNPs. Here, Bz vapor of known pressure monitors the study of reaction rate. Interestingly, upon introducing the Bz vapor into the reaction cell, the pressure decreased dramatically to near-zero, and its constant changes were good indications of changes in the reaction rate.

The muonium is then reacted with benzene, creating a muonated benzene radical. Radicals are highly reactive species with one unpaired electron, and they are synthesized in organic labs in limited ways. The radioisotope may build onto current techniques in creating a radical.

The researchers also analyzed the catalyzing effect of AuNPs on the muonium benzene reaction. The results show considerable weaker binding energy of benzene on AuNPs than on bulk gold metal surfaces, which indicates that the presence of AuNPs increased the reaction rate.

Gold nanoparticle
Source: Equinox Graphics (Flickr)

In Chemistry, AuNPs are great heterogeneous catalysts––––reaction accelerators that differ in phase from the reactants. These solid catalysts all together account for 90% of the catalysts used worldwide. The successful catalysis in muonium reactivity demonstrates the possible use of heterogeneous catalysts in radioisotopic reactions that await to be explored.

Besides, the reaction of AuNPs with radioisotopes would inspire more detailed studies into their biomedical benefits as cancer detectors and drug deliverers.

Unique optical properties of various sized gold. Source: Sigma-Aldrich

This avant-garde reaction will inspire the development of chemical reactions into the unstable nuclides, where new reaction pathways lie.

For the most part, sleep is an inherently simple process. You close your eyes and wake up 8 hours later feeling refreshed. However, there is still a possibility that it can become inexplicably difficult. For instance, consider insomnia, a condition where falling asleep can feel practically impossible. One study has even estimated that the prevalence of the condition can be as high as 30%. Fortunately, there is a solution.

Man sitting on bed with head in hands
Source: Getty Images

With the FDA’s recent approval of a new drug called DAYVIGO^TM, which contains the active ingredient lemborexant, there is a possibility of improving the current insomnia treatment methods.

The billion-dollar question

The first thought that tends to arise is: Is the drug actually worth it? A study conducted by the Tufts Center for the Study of Drug Development estimated the cost of producing novel compounds at 1.395 billion (2013 USD). Even without factoring in inflation, that number is staggering. Furthermore, there are a number of drugs currently available to treat insomnia. Be that as it may, DAYVIGO^TM  displays a number of advantages.

It has been shown that DAYVIGO^TM is one of the first FDA approved substances to report safety data for 12 months. Furthermore, the drug is rapidly absorbed and has a high affinity for the binding receptors. Another advantage is its extreme selectivity. In my opinion, the drug could be the key to providing an over-the-counter pharmaceutical that is both potent and safe. But how is it capable of achieving all of these advantages?

DAYVIGO^TM and the role of Lemborexant

It is important to mention that the effects produced by this drug are attributed to lemborexant, which is an orexin receptor antagonist. The official IUPAC name is (1R, 2S)-2-{[(2,4-dimethylpyrimidin-5-yl)oxy]methyl}-2-(3-fluorophenyl)-N-(5-fluoropyridin-2-yl) cyclopropanecarboxamide, and the molecular formula is C₂₂H₂₀F₂N₄O₂.

Structure of Lemborexant Chem Spider ID:34500836, (accessed 03:33, Jan 28, 2020)

Rather interestingly, this compound contains a number of nitrogens. While this is a common theme in most substances, this particular substance has fluorine attached to a pyridine. Although, the most important nitrogen-containing structure in this drug is a pyrimidine. What is particularly interesting about pyrimidine is that it shows a range of biological activity. For instance, it is used in depressants, anti-inflammatory drugs, analgesics and more.

A number of tests conducted with this compound illustrate the efficacy of the drug in the onset and sustenance of sleep. For instance, the molecule selectively binds to the orexin receptors, which are thought to be involved in the regulation of sleep.⁷ Essentially what the drug does is it interacts with the receptors, preventing the “wake-promoting neuropeptides orexin A and orexin B” from interacting and therein suppressing the feeling of wakefulness.

-Akash Panjabi