Feb. 8, 1999 CHAMPAIGN, Ill. -- One of Mother Nature's most important talents is the ability to distinguish one molecule from another. Like locks accepting keys, proteins function by being able to let one particularly shaped molecule inside while barring other shapes from entering.
The idea of molecular locks and keys goes back a hundred years, but it is only now that chemists have made progress in making artificial receptors with high selectivity. Professor Ken Suslick and his colleagues at the University of Illinois recently have designed and built synthetic locks that can bind straight, skinny molecules but block out bent or fat ones. These synthetic receptors are intended for use as molecular sensors and selective catalysts.
"Molecules are the messengers between cells and even between whole organisms," said Suslick, the William H. and Janet Lycan Professor of Chemistry at the U. of I. "The cells understand these messages by recognizing these messenger molecules, often on the basis of their shape."
At the heart of Suslick's synthetic lock is a metalloporphyrin (a metal ion bound in the middle of a molecular ring), much like the iron compound found in the hemoglobin in blood. To this core are attached dendrimers (branching polymers) that spread out like limbs on a tree.
"Depending on where the dendrimers are placed, the resulting receptor has either a big round hole or a narrow chimney above and below the metal ion," Suslick said. "The shape of these pockets determines the shape of molecules that can bind to the metal. The chimney-shaped pocket only lets in straight, narrow molecules, while the round hole lets in fat or bent-shaped molecules."
As reported in the Jan. 12 issue of the Journal of the American Chemical Society, Suslick and his colleagues have been able to change the binding strength of various molecules to the metal at the center of these artificial receptors.
"Just the shape of an incoming molecule can change its binding by almost a million-fold," Suslick said. "Such exquisite selectivity is very much the way that living systems control chemical reactions."
It is also the way that industrial chemists manipulate many chemical feedstocks. For example, the improvement of octane ratings in gasoline often makes use of shape-selective catalysts called zeolites that can select for the formation of high-octane molecules during the reforming of crude oil based on the molecular shape.
Suslick and his research group are interested in using such synthetic receptors as shape-selective catalysts and sensors. For example, by controlling access of molecules by their shape, oxidation can be carefully directed to specific sites in complex molecules. As another example, the ability to detect molecules on the basis of their shapes is extremely important in the development of artificial noses and other chemical sensors.
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