Click chemistry is a favorite tool of mine. I’ve developed close to a dozen patented technologies over the past few years, and a third of them rely in some fashion upon click chemistry. It’s a type of organic reaction in which there is minimal waste, fast reaction time, and almost 100% conversion to the desired product. One of the most popular “click” reactions utilizes a copper catalyst as the critical reagent. It can come in a variety of forms, and it doesn’t have to be present in huge amounts, but it has to be there. Without it, the reactions take longer, require more heat, and form byproducts. Normally the presence of copper is not an issue – it’s inexpensive, and it’s rare that the presence of copper leads to undesirable side products. I simply add a small amount of copper salt to the reaction and it works beautifully.
However, there are huge differences between doing reactions in a flask and doing reactions in living cells. Primary among these is the need to avoid toxic reagents. Copper can quickly kill cells when present at even small concentrations. Click chemistry has therefore been limited for in vivo applications, as the crucial catalyst is missing. It’s a shame, because click chemistry is so powerful and leads to so many interesting structures in a very facile, straight-forward approach. Up until now there really hasn’t been a way around this limitation. Copper is needed for the reaction to proceed in a fast manner (60 seconds versus several hours without the catalyst), but copper ends up killing the cell being investigated.
This drawback is felt especially severely in the area of fluorescent labeling. Reagents have been available for years which substitute glycans (an important structure found in biological samples) with either azides or alkynes. These are the two components needed for the [3+2] cycloaddition reaction – the reaction that is accelerated by copper. A successful linkage between these two fragments could lead to new fluorescent labels, which would make microscopic imaging of cell structures much simpler. The reaction won’t proceed without the presence of the catalyst; the catalyst can’t be added, because it’ll kill the cells. It was painstaking for me, as an organic chemist, to see a marriage made in heaven between these two molecules; I knew full well that they could not be brought together unless a new method was found to alleviate the toxicity of the catalyst.
That’s why I was particularly excited to read about a recent development published in the German journal Angewandte Chemie: International Edition. Angewandte is one of the most prestigious journals in chemistry circles. The authors of the paper discussed how they have developed a new ligand for the click reaction which vastly diminishes the toxicity of the copper. Ligands are spectator molecules who don’t produce a product on their own, but work in concert with a metal (such as copper) to produce an interesting result. An analogy would be taping a plastic bag over your feet before walking out into snow. Your feet will stay dry and you can get where you need to go. The bag doesn’t do any of the work – it’s just a fragile sheet of thin plastic – but it plays a critical role: it protects your feet so they don’t get soaked in freezing cold water. The bags aren’t consumed in the process (they can be reused), but without them you probably wouldn’t get very far in the snow.
Ligands play a similar role. In the case of the paper under discussion, the molecules the authors added as ligands wrapped around the copper atom, partially shielding it from the cellular components. The copper was still accessible to the azide and alkyne groups, meaning that the click reaction could proceed. However, the ligand:copper complex was much less toxic to the cell than bare copper by itself. This discovery means that click chemistry is now wide open for use in fluorescence labeling. As someone who uses click chemistry everyday and who is eager to make a contribution towards new microscopy solutions, I’m extremely happy to read about this new advance, and I’m hoping that researchers will be able to use it to make new strides in molecular studies.
The source of this article can be found at:
Besanceney-Webler, C., et al. “Increasing the efficacy of bioorthogonal click reactions for bioconjugation: a comparative study”. Angew. Chem. Int. Ed. Engl. 2011, 50, 8051-8056.