Building up better synthetic receptors.

Building up better synthetic receptors

From a small molecule's point of view, they're creatures with jellyfish-like tentacles and gaping, sticky mouths. From a chemist's perspective, they're fascinating molecular assemblies -- synthetic receptors known as cavitands -- suited for basic research, but also promising for medical and agricultural uses.

"You start with oddities, and if you're lucky you end up with something practical," says chemist Donald J. Cram of the University of California, Los Angeles. In the May 10 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, he and three UCLA colleagues report making a new type of cavitand that has two differently shaped cavities. Earlier versions had only one, bowl-like cavity. Last year, Cram reported linking two bowl assemblies to make the first "molecular cells," which he also calls carcerands because small molecules get trapped inside when the bowl rims meet.

The new cavitands contain two outward-opening cavities fused bottom to bottom, one shaped like a box and the other like a bowl. The researchers can fine-tune the shapes and dimensions of the cavities by linking various chemical groups to their rims. In the same way, they can "dial in" the degree of solubility of the cavitands, and they hope to adorn the rims with catalytic groups to trigger specific chemical reactions.

Some of the two-cavity cavitands host the same molecular "guests" in each cavity; others host different guests in the bowl and box. One version remains empty despite the availability of guests. "It's very rare to have a vacuum," remarks Cram. "We're puzzled by it."

Cram expects next to build new carcerands by linking a pair of box cavities to make an elongated box with a bulge in the middle. Compared to the carcerands Cram described last year, the box-shaped interiors of carcerands made with the new cavitands should be inviting to a different variety of guests, he says.

"All of this is related to how enzymes work," remarks chemist Ronald Breslow of Columbia University. Cram envisions using cavitands and carcerands to shuttle drugs to diseased cells, as slow-release pesticide delivery systems, and as novel liquid crystals in which the carcerands crystallize in place while their molecular prisoners are free to respond to electric fields.