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article imageLighting up the lab improves medicines Special

By Tim Sandle     Apr 25, 2014 in Science
In a new study, researchers have detailed a way to use sunlight and two catalysts to create molecules that are difficult to make with conventional techniques. The finding may have implications for drug making.
To explain more, Digital Journal spoke with Tehshik Yoon, the lead scientist behind the project. Dr. Yoon is a chemistry professor at University of Wisconsin-Madison.
Yoon explained that in chemistry, heat and ultraviolet (UV) light are commonly used to drive reactions. Although light can power reactions that heat cannot, UV has disadvantages. The UV often used in industry carries so much energy that “it's dangerous to use, unselective, and prone to making unwanted byproducts.”
Many chemicals exist in two forms that are mirror images of each other, and Yoon is interested in reactions that make only one of those images.
“It's like your hands,” Yoon says. “They are similar, but not identical; a left-hand glove does not fit the right hand. It's the same way with molecules in biology; many fail unless they have the correct 'handedness,' or 'chirality.'“
The pharmaceutical industry, in particular, is concerned about controlling chirality in drugs, but making those shapes is a hit-or-miss proposition with UV light, Yoon says.
Yoon began to explore powering reactions compounds with metals that are used to capture the sun's energy in solar cells. In a solar cell, these metals release electrons to make electricity.
“We are taking the electrons that these metals spin out and using their energy to promote a chemical reaction,” Yoon says.
Plants do the same thing during photosynthesis: absorb light, release high-energy electrons, and use those electrons to bond water and carbon dioxide into sugars. That reaction is the basis of essentially all of agriculture and all food chains.
Once the solar-cell metal supplied electrons, Yoon thought about using a second catalyst to control chirality. To control chirality, the second catalyst held the chemicals under transformation in the correct orientation so the electrons could create only the desired chirality. Yoon then “made an interesting discovery: if you make a really small tweak to the chiral control catalyst, you get a completely different shape to the product molecules.”
Introducing a second catalyst allowed much greater control, Yoon says. “One reason this field has failed is that a single catalyst had to both absorb light and control the chirality. If you tweak the single catalyst, you change its effects. By separating the two roles, you can make all kind of changes to chirality without messing up the photochemical catalyst. To get this to work, two stars have to be aligned.”
The experiments to date have made square structures with four carbons that would be difficult to make with UV or heat. Ultimately, Yoon says, the technique may interest material scientists, who are focusing more on chirality, and especially drug makers.
“Drug companies need compounds with well-defined chirality, and they want structures that nobody has made, and we have structures that are really strained, exotic, with controlled chirality. These are part of an unexplored space in molecular diversity. Now that we have a platform for using these catalysts in tandem, we are looking more broadly to see what else we can do.”
Ultimately, Yoon would like to move back to the future and to make chemicals as plants do: powered by sunlight, with all the environmental advantages that implies.
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