Finding chemical tools in a crystal forest.

Finding chemical tools in a crystal forest

Crystal made of the same kinds of chemicals that enable blood proteins to carry oxygen and chlorophyll molecules to capture solar energy appear suited for such nonbiological roles s pollution control and catalyzing chemical reactions in the lab, and even as exotic research materials that turn liquids into solids without lowering the liquid's temperature.

At the core of these crystals lie flat molecular structures known as tetraarylporphyrins (TPPs), each of which consists of a square-shaped porphyrin molecule with a benzene molecule attached to each of its sides. Chemists at the University of California, Los Angeles, have discovered that TPP molecules self-assemble into channel-ridden crystals.

By modifying the benzene compoents, Charles E. Strouse and his co-workers say they can control the shape, size and orientation of the channels within the crystals. Such control might enable them to "program" the crystal lattice to "preferentially incorporate guest molecules of a predetermined size, shape, handedness and charge," the chemists suggest in the Feb. 28 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY.

To get a feel for what the chemists have in mid, press the bottoms of your palms and the ends of your fingers together. By further pushing against your fingertips, you can alter the size and shape of the cage defined by your hands, and thus whether you can trap a tennis ball or short stack of crackers.

"That's kind of what these things can do," Strouse says. When TPP molecules crystallize in solutions that also contain dissolved molecules such as toluene (a solvent used in paint), the dissolved molecules get trapped within the regularly spaced channels of the TPP crystals, which the chemists also call "TPP sponges." The sponges resemble zeolites, an already important class of channel-containing crystals, but their flixibility enables them to accomodate a wider variety of guests, Strouse says.

By examining the crystal structures of about 70 TPP molecules that differ in the metal atom that sits at the center of their porphyrin components or in the way their benzene components have been chemically modified, the chemists discovered common structural features that others had not observed, Strouse says.

"They saw the forest despite the trees," comments longtime porphyrin researcher W. Robert Scheidt of the University of Notre Dame (Ind.). Others who study porphyrin crustals focus primarily on just a few structures, and so have not been well positioned to perceive structural similiarities across the extended family of TPP crystals, Scheidt says.

Strouse imagines using TPP sponges to sop up organic contaminants from liquids or gases. He also envisions filling the channels with monomers and then using the natural light-harvesting powers of porphyrin to catalyze the linking of monomers into polymers. A more esoteric use involves trapping molecules of a hard-to-crystallize liquid inside the sponges' regularly arranged channels, thus forcing the liquid into a more solid-like form amenable to structural analyses such as X-ray crystallography. "We haven't demonstrated in gruesome detail how effectively one can do that in general," Strouse stresses.