LIGHT IT UP Fluorescence occurs only in the presence of chloride (8.0 µM, left, and 0.8 µM, right). In the absence of chloride (center), the vial remains dark.
	© JACS

"Can we take the ability to mimic biological systems through supramolecular chemistry to create detection systems similar to either PCR [polymerase chain reaction] or ELISA [immunoassays] that allow one to detect an analyte, trigger some catalytic event that amplifies the signal, and provide some sort of readout?" asks Mirkin, a chemistry professor at Northwestern University.

Mirkin believes that the answer is yes, and now his group brings these two lines of research together in a new type of chemical sensor using supramolecular allosteric catalysts [J. Am. Chem. Soc., published online, http://dx.doi.org/10.1021/ja0437306]. Other members of the research team include associate chemistry professor SonBinh T. Nguyen and graduate student Nathan C. Gianneschi.

Two rhodium centers in the catalyst serve as allosteric regulatory sites that open the cavity when chloride binds to them, allowing acetic anhydride and pyridyl carbinol to enter and undergo a catalyzed acyl transfer reaction. This reaction in binuclear macrocycles was discovered and developed in the laboratories of Nguyen and Joseph T. Hupp, another Northwestern chemistry professor. When the cavity remains closed in the absence of the chloride analyte, the reaction occurs very slowly. "The ideal system is one that is completely dead--catalytically inactive--until that allosteric regulator binds and turns the catalyst on," Mirkin says. Acetic acid, a by-product of the acyl transfer reaction, protonates a pH-sensitive fluorophore, causing it to shine.

The use of allosteric regulation is a general concept, so several aspects of the sensor could be modified to create other detection systems. For example, the transition metals and ligands could be changed to provide selectivity for other analytes. "Even more intriguing, we can create systems where the macrocyclic pocket recognizes a molecule and turns on a catalytic reaction at the periphery," Mirkin says. "Then one can begin to use macrocycle design principles to control what [the sensor] recognizes and what turns on the catalytic event."

Mirkin would like to begin designing sensors for molecules that can't be detected easily using conventional detection systems, including chiral pharmaceuticals. The approach "takes supramolecular coordination chemistry into a new area, one that is not just fundamentally interesting, but one that could ultimately be technologically significant," he says.