Click reaction under scrutiny

It is amazing how Huisgen azide-alkyne cycloaddition, once resurrected and reinvented by K. Barry Sharpless, generated so many application papers while we are still digging to understand its copper-catalyzed mechanism.

A recent JACS paper from Don Tilley lab sheds some more light on the catalytic cycle. And it’s not that easy as setting up the reaction itself. Disrupting catalytic cycle in step-by-step fashion is a tricky business but it was beautifully done in this study. First, trapping elusive cycloadduct within dinuclear copper complex:

fig1

Then displacing it with a fresh alkyne to get the product and the complex ready for another catalytic cycle:

fig2

But then look at the scheme below, which is arguably the least dramatic change one can imaging from a chemical transformation (involving organic compounds).

fig3

For those still scratching their head about what did actually happen here, it’s a one-electron oxidation of Cu(I)-Cu(I) dicopper complex into mixed-valence Cu(I)-Cu(II). That slight tilt of the bridging alkyne ligand seems to be the only indicator that reaction indeed took place without much of competing disproportionation or whatever could happen to the complex. Yes, it takes X-ray crystallography to prove that, and I wonder if Micah Ziegler, the first author, a priori knew what to look for to monitor the reaction success. EPR? Cyclic voltammetry? Maybe color change was enough?[1]

Anyway, attempts to get the mixed complex 3 to react with tolylazide didn’t succeed. So authors concluded that CuAAC does not involve mixed-valence dicopper complexes. In addition, they excluded a bunch of alternatively proposed intermediates. This was in fact contradicting with earlier results published by Jin et al of Bertrand lab. The key difference between papers was that in the latest study authors forced two copper atoms to sit close to each other, while Jin et al used mononuclear copper complex to initiate the reaction [2]. This may bring up a discussion of what study is more relevant for ‘real world’ cycloaddition. I wonder more, however, if knowing the exact mechanism will help one to improve the reaction in any way. It already is pretty well-optimized and reliable (to a point).

More generally, it seems like the dissociation of practical application from (strictly unambiguous) theoretical explanation is a genuine feature of science. Take for instance CRISPR-Cas9, which leading experts are still trying to understand how it works in native systems (i.e. bacteria) while other scientists are ready to tweak human embryos with it.


[1] 19F NMR had enough difference due to different stoichiometry of triflimide anions.

[2] It still might be that both results are ‘right’ and one intermediate can turn into the other.

Remote cell reprogramming for diabetes treatment

Since I’m not that long in diabetes business, two new Cell papers from Collombat and Kubicek labs looked quite sensational for me. Both are the products of multi-centered collaborations, and both report regeneration of insulin-producing beta-cells in vivo with small molecules.

As I learned from introductions, reprogramming of pancreatic alpha cells (glucagon-secreting) into beta cells is a sort of a Holy Grail of regenerative medicine for diabetes treatment. Naturally, first attempts to reprogramming were performed with aid of transcription factors. But pretty soon small molecules kicked in. These were kinase inhibitors and chromatin-altering probes from Stuart Schreiber lab, resveratrol (of course!), and peptide hormone betatrophin. OK, the last one doesn’t count, and it’s not a small molecule anyway. What’s unusual about the latest Cell papers, is that they describe reprogramming by small molecules acting pretty high upstream from direct gene regulation [1]. Both papers involve messing with GABAA receptor signaling.

Let’s start with the Kubicek lab paper, which found that common (yet Nobel-winning) malaria drug, artemisinin, can make pancreatic alpha cells to secret insulin. The authors identified artemisinin and its metabolite dehydroartemisinin from a library of 280 existing drugs [2]. After they found that the drugs induce insulin secretion, they identified gephyrin as the most likely target. Then, via electrophysiology and a series of inhibitory tests, they linked gephyrin-mediated activity to GABAA receptor signaling. Known agonists of GABAA, however, didn’t increase insulin secretion as much as artemisinin (after 72 h treatment of cells). The drug then increased mass of beta cells islets in zebrafish, healthy and diabetic mice (while reducing basal glucose level in the last ones). Finally, it altered gene expression in human alpha cells and increased insulin secretion by the islets. Frankly, the figure 7A-C, which is supposed to convince in the last effect, raises some questions as data look cherry-picked from different donors. But authors do address that by briefly mentioning donor-to-donor variability. And it’s not surprising at n = 6 sample size.

artemisiningaba

The paper from Collombat lab branches from the screening results of the first one. Once researchers noticed that activation of GABA signaling correlates with alpha-to-beta conversion, they thought “why not injecting plain ol’ GABA into mice?” And miraculously this simple idea worked. Just look at the jaw-dropping figures 1B-D! Figure S7C,G (below) can somewhat give you the feeling, but go check out the main paper, you won’t be disappointed.

insulin
Figure S7 fragment showing increase in insulin-producing cells from rat pancreas. I assume scale bars, if they were present, would be equal (Ben-Othman et al paper)

Here are the main results: daily injections of GABA at 250 μg/kg over three months convert pancreatic alpha cells into beta. But what’s even more exciting is that the new alpha cells are continuously being produced to compensate for those that were converted into beta! They even caught small fraction of cells in some transitional state, where they secret both glucagon and insulin. I particularly liked the discussion section where authors warn that before you, all excited, rush to inject diabetic patients with GABA think why there’s not enough beta cells in the first place. Yes, it is patient’s immune system that attacks her own beta cells. So before this approach makes into clinic one needs to figure out that autoimmune component of type 1 diabetes.

In a sum we have two great papers with rock-solid mouse data and some exciting preliminary results in human beta cells. Let’s see where it will end up. Regardless of the future success, isn’t it amazing how small, simple, and seemingly well-known molecules like GABA (and artemisinin for that matter) can upturn human cells identity?


[1] OK, authors do not strictly claim reprogramming as the identity of cells doesn’t change completely from alpha to beta, but their secretory activity is definitely flipped.

[2] Side note: check out this sexy acoustic liquid handler they used.

 

β2AR: old horse’s new tricks

It’s almost four years since the Nobel prize in chemistry went to Brian Kobilka and Robert Lefkowitz for their contribution in our understanding of G protein-coupled receptor (GPCR) signaling. They did their most exciting work by studying β2 adrenergic receptor (β2AR). Yet, despite the titanic efforts, the receptor still holds lots of secrets from us. Continue reading “β2AR: old horse’s new tricks”

No fail no gain

While astronomers recently have been celebrating great discoveries (gravitational waves from colliding black holes keep rolling), at the opposite end of matter size ruler things don’t look as brightly. For instance, the search for ‘sterile’ neutrinos didn’t spot anything, despite the IceCube, a massive 2-km long neutrino detector buried in the Antarctic ice.

Continue reading “No fail no gain”

Molecular tribology

It’s hard to imagine more intriguing title in Angewandte Chemie International Edition than “Astringent Mouthfeel as a Consequence of Lubrication Failure“. The first impression doesn’t deceive, and the paper is really interesting and fun to read. Somehow manifestations of molecular interactions in the macroscopic world never stop amusing me. And this communication is exactly about such emergent effect. Continue reading “Molecular tribology”

Pheromone puzzle

A couple of weeks ago I attended a presentation given by Lisa Stowers from Scripps, entitled “Decision-making in the nose: a molecular rationale for the unpredictable nature of female behavior”. The catchy title did its job, so I was there, learning that reaction of female mice on male pheromones depends on (suprise-surprise!) phase of their estrous cycle. The neurobiology behind it is quite amazing, definitely worth reading about. However, my interest was triggered by one question asked after the presentation.

Continue reading “Pheromone puzzle”

Chemical Panoptikum #1

Creating new substances, sometimes just for the sake of the creation act itself, is an undeniable part of chemists’ nature. Having flawless analytical data of a newly prepared sample always fills one with mystical joy and feeling of omnipotence. And some chemical creatures are so bizarre that the very fact of their isolation and characterization causes reverence of fellow chemists. So welcome to the chemical Panoptikum, a collection of all sorts of weird structures from the recent literature. Don’t be surprised to meet boron very often here, it’s a really weird element. Continue reading “Chemical Panoptikum #1”