Making molecules that self-assemble.

Making molecules that self-assemble

Filling a bag with toy car parts -- little chassis, wheels, windows, engines and seats -- and then shaking it up yields a bag of jumbled car parts. Imagine the possibilities if the parts could actually find each other, arrange themselves properly and then self-assemble into little cars.

Though self-assembling vehicles remain fantasy, chemists at the State University of New York at Buffalo have carefully designed molecular subunits that automatically snap together into a "threaded molecular loop" and into more complex, oxygen-binding assemblies that the scientists hope to develop into the basis for artificial blood.

"The self-assembly process is a theme found throughout biology," notes project leader David S. Lawrence. In nature, enzymes, organs and even entire human beings self-assemble from smaller constituents. "It's a neat process in which all of the pieces find another, like a jigsaw puzzle coming together giving the correct picture."

In the April 25 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, Lawrence and Tata Venkata S. Rao report making a four-piece, self-assembling complex. A long, flat molecule -- a diammonium salt with a central, hydrophobic (water-avoiding) region flanked by charged ammonium groups -- serves as a template for the complex. One end of the salt threads through the interior of a starch-like molecule called a cylodextrin, which looks likes an empty lampshade, until its hydrophobic center finds itself inside the molecular lampshade. A boron-centered molecule that looks like a four-bladed wing nut then caps each of the salt's two ends.

In the March 14 issue of the same journal, Lawrence and John S. Manka report pulling off an 11-piece self-assembly effort. They used a pair of cyclodextrins, six of the wing-nut molecules, a couple of sodium atoms and a square porphyrin molecule with one ammonium-tipped group jutting from each of its four sides. Tow of these groups thread through their own cyclodextrin lampshades. The other two sneak sideways through a groove formed by the two bottom-to-bottom lampshades. A wing-nut molecule sticks to each of the four ammoniums. Charged sodium ions hover above and below the plane of the porphyrin, and a wing nut associates with each of these as well. In all, 11 pieces come together to make the complex.

Lawrence told SCIENCE NEWS he has also made complexes with cobalt-centered porphyrins. Like the iron-centered porphyrin (heme) molecules in the blood protein known as hemoglobin, these complexes bind oxygen. With further development, such complexes might serve in artificial blood formulations, Lawrence says.

He points out that none of the parts of these assemblies link up via covalent chemical bonds, which connect the atoms that make up the individual pieces. Rather, by timing the addition of successive pieces, the researchers rely on hydrophobic and electrostatic interactions to get the pieces to stick together in just the right way.