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Prof. Amar Flood publishes latest work in Science magazine on cages that catch chloride


May 23, 2019

Professor Amar Flood led a team of IU chemists to create molecular cages that display the strongest chloride binding from the weakest hydrogen bonds. The cage has the potential to be used for improving technologies that make clean water. Read more about it in the article, and in IU’s press release. For the story of the cage’s creation, read on.

The cage was made in response to an offhand challenge. The lead author, Yun Liu, was a second-year student in the Fall of 2012 when his wife Jie Fu (IU PhD graduate from the Skrabalak Lab), then fiancee, suggested he make the cage. Jie had spotted Kristin Bowman-James’s cage being used by Dan Nocera and Chris Cummins to capture anions and thought her partner, Yun, could replace the amides in the Bowman-James cage with triazoles.

Yun needed a little push but in the end he designed the cage and set about making it in over the winter break. It took another 6 months to grow an X-ray quality crystal for the determination of the cage’s structure by co-author Dr. Josh Chen October 2013. Yet, the full characterization of the cage took time.

The cage simply would not let go of the chloride it had captured and that was a problem. With the chloride binding strength so high, it took a while to work out the right way to measure its affinity. It did not help that Yun was distracted by other projects. By 2016, and nearing the end of his PhD, Yun picked up the cage again. This time he was successful. The trick was to have a competition for the chloride. Thus, a tug-of-war was set up between the strong-binding cage dissolved in some oil and an equal measure of water where chloride prefers to be dissolved. With a little bit of the chloride in the cage and a little in the water, Yun was able to get an actual measurement of the cage’s strength. Before too long, Yun had made good progress on defining the power of the cage’s chloride binding strength and the cage’s ability to selectively recognize chloride from a veritable sea of other anions.

When the manuscript was being drafted, Yun and Amar both came to the realization that the cage holds the world record for the strongest chloride binding strength. Experience teaches us that extraordinary claims require extraordinary evidence. Thus, co-author Dr. Wei Zhao was asked to remake the cage and to retest the cage’s ability to extract chloride from water. With the result independently verified and the story settled, the paper was submitted New Year’s Eve of 2018 and finally accepted late Spring to come out in Science May 23, 2019.