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New process sifts 'mirror image' molecules

By
CHARLES CHOI, UPI Science News

NEW YORK, Oct. 9 (UPI) -- Chemists have developed a new process that could help scientists manufacture drugs without creating unwanted, potentially toxic, mirror-image molecules inadvertently.

If large-scale experiments prove this novel, simple technique is effective, experts say it could have a major impact on the $100-billion pharmaceutical industry.

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The advance, developed by researchers at the University of Missouri-Rolla, is rooted in straightforward geometry -- just as human hands come in left and right varieties, so do many molecules. This handedness property is known as chirality, which is derived from the Greek word for hand.

Perhaps surprising, but chirality is quite common in nature. For example, all terrestrial life uses only right-handed sugars and left-handed amino acids. More than 50 percent of the world's top 100 drugs are chiral, including familiar brand names such as Lipitor, Paxil, Zoloft and Nexium, each of which yields sales of more than $1 billion a year.

A mirror image version of a molecule is known as its enantiomer. Organic molecules often work in an extraordinarily specific, lock-and-key manner in the body. Just as Britons are used to driving on the left-hand side of the road and face potential disaster if they fail to adjust to the American preference for traveling on the right side, so can right-handed and left-handed enantiomers "have radically different effects," National Science Foundation program officer and chemistry expert Michael Clarke told United Press International.

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The classic, tragic example is the sedative thalidomide, made infamous during the 1960s. Though thalidomide's right-handed enantiomer helped fight morning sickness in pregnant women, its left-handed version caused severe birth defects. In the case of the common pain-reliever ibuprofen, the molecule's right hand is simply 100 times less powerful than its left.

Currently, most industries create desired chiral molecules en masse by mixing solutions. One solution contains the components for the desired molecule, while the other contains right- and left-handed molecules. The components assemble around the handed molecules, much as one might take threads and weave them into a right-hand or left-hand glove, as NSF program officer and chemist Katherine Covert explained.

"But separating the useful 'gloves' from the chemicals that assembled them can be a difficult process," Covert told UPI.

Eric Bohannon, an inorganic chemist at UM-Rolla, said there are now whole industries based on separating the left- and right-handed versions of molecules. "It can be a time-consuming and expensive step," he told UPI.

Instead, his colleague, chemist Jay Switzer, and others have created a way to make solid coppery films into "hands," on which solutions of chiral molecules are manufactured and from which they are easily separated.

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"Once we figured out what was going on, it only took us about two months to work things out," Switzer told UPI from Yokohama, Japan.

In earlier experiments, Switzer explained, scientists would produce the desired chirality on a material's surface by using a chirality modifying agent. If the agent washed off, however, the surface no longer could be effective.

"We've made a material that is entirely chiral, not just the surface," Switzer said. "In our new research, the film itself is chiral. The effectiveness remains even after many chemical reactions."

In an article appearing in a recent issue of the British journal Nature, Switzer and colleagues described how they created the new catalyst by immersing gold in a copper-loaded liquid tainted with tartrate, an organic chiral substance that often crystallizes on the bottom of wine corks. Louis Pasteur introduced the concept of molecular handedness after studying chiral tartrate crystals when he was 26 in 1848.

When the investigators applied electric current to the system, the electricity caused copper oxide to bond, layer-by-layer, in a thin film to the gold. The tartrate crystals guide the growth of the copper oxide film in a chiral manner. To make a purely right-handed film, the researchers used left-handed crystals, and vice versa.

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"Like most interesting discoveries, we found this pretty much by accident," Switzer told UPI. "We didn't really look for this because copper oxide is not intrinsically chiral." Pending experiments will test whether the process can be applied to essential organic molecules such as sugars and amino acids.

"What's most surprising about this is how they do it so simply," Clarke said. "It's a very simple method that can be extended to other sorts of surfaces."

If this method works with other metal-laden films possessing different chemical properties -- such as iron oxides -- it could be incorporated into existing solid-state technologies and "could be revolutionary," Clarke predicted. "If other people jump on this and it turns out to be generalizable to other materials -- and that's the big 'if' -- I would imagine seeing it in the market very quickly."

The hope is this technique can lead to sensor films that can tell one enantiomer from another, or surfaces that can trigger the formation of one enantiomer exclusively. Switzer said another potential application is in medical implants to monitor the bloodstream for enantiomers.

"Some of these handed molecules -- thalidomide being one -- when in a biological system, can switch back and forth between left- and right-handed versions," Bohannon said. "That can be a really bad thing if one version can do some damage. So what you could potentially do is take a material and use it as a sensor for each different version."

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