s c i e n c e / t e c h n o l o g y

Combinatorial Encoding Technique: A Lucky Charm

I n the movie "Austin Powers, Interna­tional Man of Mystery," Dr. Evil, Austin Powers' nemesis, introduces

assassin Patty O'Brien, one of his evil henchmen: "A superstitious man, he leaves a tiny keepsake from his good luck bracelet on every victim he kills. Scotland Yard would love to get their hands on that piece of evidence."

"Yeah, they're always after me lucky charms," Patty replies. The others laugh. "Why does everyone always laugh when I say that? They are after me lucky charms."

Evil henchwoman Frau Farbissina then explains to Patty that she and others were laughing because Patty's reply had reminded them of a certain television commercial, "with this cartoon lepre­chaun. And all of these children are try­ing to chase him—'Hey leprechaun man, leprechaun man, I want to get your Lucky Charms.' Oh, and there's all these tiny bits of marshmallows just stuck right in the ce real, so that when the kids eat them they think—Oh, this is candy. I'm having fun!' "

Knowing that combinatorial chem­ists like to have fun too, a group at Scripps Research Institute has devised a new combinatorial encoding technique based on tiny bits of solid-phase materi­al that resemble the aforementioned ce­real bits. The technique seems to work like—well, a lucky charm. It was devel­

oped by postdoc Andrew R. Vaino and chemistry professor Kim D. Janda at Scripps [Proc. Natl Acad. Sci. USA, 97, 7692 (2000)].

"It is a new way to do combinatorial chemistry," Janda explains. "We have dubbed it a lucky charms' approach af­ter the cereal, which comes in different shapes and sizes."

In combinatorial chemistry experi­ments, the identity of library com­pounds that are found to exhibit desired activity must be determined. This can be difficult because compounds are of­ten present in libraries in vanishingly small amounts.

A number of methods are already available to identify library compounds, including deconvolution strategies (fig­uring out the identity of a compound by tracing it back through the synthetic mixtures used to synthesize it) and en­coding of compounds with molecular, radio frequency, or isotopic tags. But Vaino and Janda have now designed a strategy in which a polymeric support serves as an encoding element.

In the strategy they developed, a lightly cross-linked gel polymer is pro­cessed into an array of pieces of unique shape by cutting them up with a razor blade and stencil. "We make the lucky charms like you make different shaped cookies," Janda says. "You could also

make them in an injectable mold, like a molded cookie sheet. It is as simple as child's play, and that's why we like it."

A combinatorial library is synthesized right on the shapes. In the paper, Vaino and Janda synthesized a library of 25 urea compounds with a split, mix, and re-combine synthetic technique. The com­pounds were identified by noting which of five reaction vessels they ended up in

Vaino (left) and Janda with one of their favorite breakfast cereals.

and which of five charm shapes they were found on.

"Using our shape method and split-and-mix synthesis takes a total of 11 re­actions," Janda says. "If you made this library by parallel synthesis, it would have taken 30 reactions. Split synthesis provides a savings in reaction number, and the shape of the lucky charms pro­vides you with the history of the reac­tion scheme."

The new technique is also inexpen-

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Real Lucky Charms (left) and combinatorial lucky charms (right).

sive. "You don't have to buy expensive things like radio frequency tags, and you don't have to analyze with expen­sive equipment like a mass spectrome­ter, " Janda says.

"Another plus is that each charm or shape is quite large, so you can load a lot of compound on it," he adds. "It is al­most like each charm is a minireactor. Each fully loaded charm holds as much as 4,000 beads of a typical synthesis res­in. So now you can make much more compound more readily, and it is much easier to get analytical analysis on more compound/'

The main drawback of the method, Janda says, "is the number of com­pounds you can encode and the number of different reaction steps you can per­form. This is all limited to the number of shapes you can think up. You can in­crease the number of compounds you can encode by using a fluorescence tag in conjunction with shape. Being realis­tic, using our method you would not make and thus code for a library of more than 1,000 members. But today most libraries are focused, so their sizes typically don't exceed 1,000."

Asked to evaluate the significance of the work, one researcher in the field, commenting anonymously, says, "This is yet another sort-and-combine method, very much like the 'tea bags' Houghten [President Richard A Houghten of Tor-rey Pines Institute for Molecular Studies, San Diego] introduced in the early 1990s. Instead of bags of resin with names or barcodes, Janda is using polystyrene discs cut into five shapes with a razor blade. Mimotopes [a company in Clay­ton, Australia] did something similar with colored plastic pins. K. C. Nicolaou [chemistry professor at Scripps Research Institute and the University of California, San Diego] did something similar with I

radio frequency tags that could be sorted robotically."

Janda replies that the research­er "needs to recognize that while shape tags, color tags, and radio frequency tags can all encode, they are very different beasts. An impor­tant difference is the cost of using each encoding technique, including the cost of analysis in the decoding process. The underlying theme with our work is simplicity. All the other techniques are more com­plicated and/or more expensive. What we have devised is something the common 'combijoe,' who does not have all the money in the world,

can take advantage of. Mark my words: Solid-phase chemistry is moving toward higher loading materials that can be molded into a variety of shapes."

Assistant professor Tingyu Li of Vanderbilt University, whose research interests include combinatorial chemis­try, says of the Scripps study, "By com­bining syntheses using large beads with shape coding, the method offers two po­tential advantages—convenient resin sorting in split-pool synthesis and ready structural identification of the resulting library members. Coding with shapes appears completely new in combinatori­al libraries. It has the advantage of not disturbing the surface chemical proper­ties of the solid support."

The method's simplicity "makes it novel," Li says, "but whether it will be of general applicability remains to be seen. Of concern is the synthetic ef­ficiency on these large beads. The yields reported by the authors are mod­erate. Therefore, additional studies are necessary to demonstrate the practicali­ty of this approach."

Janda replies that his group is cur­rently working to improve reaction yields on the shapes by using a second-generation polymer—a technique they are calling "frosted lucky charms."

Janda notes that he actually got the idea for shape-based encoding "from my youngest child, Christopher. A couple of years ago, when he was four, I came to the breakfast table and I asked what he was doing. He told me he was having Lucky Charms to eat because they come in different shapes and sizes and are mag­ically delicious. I think he was watching too much TV and was brainwashed. But I started thinking about the shapes and siz­es—and the rest is history. As Austin Powers would say, Yeah, baby!' "

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