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Analytical Chemistry

Shifting Sensibilities In Molecular Sensors

Molecular recognition arrays offer an alternative to lock-and-key approach

by Bethany Halford
January 30, 2006 | A version of this story appeared in Volume 84, Issue 5

Sensor Array
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This cartoon shows how Anslyn's sensor array can detect a single analyte or multiple analytes via pattern recognition.
This cartoon shows how Anslyn's sensor array can detect a single analyte or multiple analytes via pattern recognition.

Historically, organic chemists designing sensors based on molecular recognition have basically been molecular locksmiths. For each molecule to be detected, they would design a locklike receptor that, through physical and electronic interactions, recognizes only that molecule.

This "lock and key" strategy works well for small molecules, but when it comes to recognizing larger biological molecules, receptor design becomes considerably more daunting. Speaking at last month's 5th International Chemical Congress of Pacific Basin Societies (Pacifichem) in Honolulu, University of Texas, Austin, chemistry professor Eric V. Anslyn described an alternative approach to molecular recognition inspired by our senses of taste and smell.

Although nature employs many "exquisitely specific" binding interactions for molecular recognition, such as in enzymes and antibodies, Anslyn noted that nature uses other kinds of molecular recognition as well. The sense of taste, he said, is the perfect example of sensing that doesn't rely on precise lock-and-key binding.

People don't have separate receptors for each flavor they encounter. There's no chocolate receptor or banana receptor. Instead, on the molecular level, we taste via chemical interactions between flavorants and five different types of receptors—sweet, salty, sour, bitter, and a savory flavor known as umami—clustered within the taste buds on our tongues.

"All we've got are these five different tastes, yet we differentiate thousands of different flavors," Anslyn explained. We are able to make these thousands of distinctions with just five types of receptors, he continued, because for each flavor, our brains recognize a pattern of multiple binding interactions to these receptors. The combined response of several taste receptors, along with the sense of smell, essentially gives each flavor its own fingerprint.

Anslyn calls this phenomenon differential binding. A group of generalized receptors that have different binding characteristics, none of which are necessarily specific or even very selective, create a composite signal or pattern. The signal can then be interpreted by pattern recognition protocols.

The analytical chemistry community has known about this type of molecular recognition for decades, Anslyn pointed out. Pattern recognition of volatile chemicals adsorbed onto an array of semiconducting polymers, for example, is the basis of an electronic nose.

Anslyn and his collaborators have used arrays of synthetic differential receptors along with pattern recognition in a similar way to simultaneously identify multiple analytes in water. Although the electronic nose systems are limited to detecting volatile compounds, Anslyn's solution-based system can identify a much wider range of compounds (Chem. Soc. Rev. 2006, 35, 14).

One of the group's first sensor arrays was designed to be a rudimentary mimic of the tongue. The researchers created an array of nine resin beads, wherein each bead was synthetically derivatized to act as a taste bud. One bead responded to pH, one to fructose, one to Ca2+, and so on. The group used pattern recognition analysis to decode the colorimetric or fluorescent responses triggered by binding to those receptors. John T. McDevitt, Anslyn's collaborator and colleague at the University of Texas, Austin, has since improved and refined the technology into a "taste chip."

"The senses of smell and taste are very powerful sources of inspiration because they take us into a new direction in analytical chemistry away from the one-pony show" of lock-and-key sensing molecular recognition, McDevitt told C&EN. He likened differential sensor arrays to chameleons because they change in response to each analyte.

With the success of the first differential receptor arrays, Anslyn set out to apply the technique to more elaborate analytes. "I wanted to see how complex a guest one could target," he said. Figuring that a large globular protein was a suitably complex target, Anslyn designed an array to differentiate proteins and glycoproteins as a proof-of-concept experiment (Angew. Chem. Int. Ed. 2005, 44, 6375).

Loosely Bound
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A library of resin-bead-bound receptors based on this structure can differentiate proteins and glycoproteins.
A library of resin-bead-bound receptors based on this structure can differentiate proteins and glycoproteins.

Anslyn based the receptors in this array on a benzene scaffold having two arms that contain both a boronic acid and a combinatorially synthesized tripeptide. Anslyn noted that this receptor design was important to creating an effective array. The peptide arms on the receptor would provide molecular recognition sites for the proteins via ion pairing, hydrogen bonding, and hydrophobic interactions, while the boronic acids could bind sugars, Anslyn explained. The benzene scaffold creates the binding cavity. As in the other arrays, the receptor molecules are attached to resin beads.

Using all the standard natural amino acids (with the exception of cysteine, which was excluded to avoid unwanted disulfide bond formation), Anslyn's group created a library of 6,859 receptors. From these, they randomly chose 29 to create an array.

Anslyn's group then used the array to see if they could differentiate the proteins elastin, lysozyme, and bovine serum albumin from the glycoproteins ovalbumin and fetuin. Not only did the array readily differentiate the proteins from the glycoproteins, but it also subtly differentiated individual proteins of the same class. This differentiation, Anslyn said, wasn't based purely on molecular weight or isoelectric point but resulted from contacts between the receptors and protein surfaces.

In terms of applications, Anslyn hopes to use this approach to perform qualitative analyses of complex protein mixtures. Identifying and differentiating proteins, he explained, is important for medical diagnostics, proteomics, and detecting bioterrorism threats.

"Clinical chemistry is a very good match" for this technology, McDevitt added. The "taste chip" is already making strides in medical diagnostics. The technology has been adapted to create a biochip that determines the immune function of HIV patients.

Once the immune function of an HIV patient drops below a certain level, that patient will likely die within months without intervention. According to McDevitt, the equipment currently used to determine immune function is a refrigerator-sized instrument that costs about $100,000. It's not practical for use in Africa, where HIV has reached pandemic levels. McDevitt says the biochip does the same analysis in a toaster-sized instrument at a fraction of the cost.

Following successful pilot studies in Boston and Botswana, the Austin-based company LabNow, where McDevitt is a scientific adviser, plans to introduce the biochip in Africa by the end of 2006.

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