Microscopic vessels merge to mix molecules.

A new technique that blends minuscule amounts of chemicals can help researchers study the biochemical reactions that occur inside cells.

Richard N. Zare of Stanford University and his colleagues encapsulate solutions in tiny spheres, or vesicles, less than 5 micrometers in diameter. By fusing the vesicles, the researchers allow the solutions to combine and react.

Clyde F. Wilson of Stanford described the new technique last week at the Pittsburgh Conference in Orlando, Fla. The Stanford researchers and their collaborators from Goteborg University in Sweden and Pomona College in Claremont, Calif., also report their results in the March 19 Science.

"This technique comes closer to mimicking what goes on in cells as opposed to in free solution," says Zare. In a cell, molecules repeatedly bump into the cell membrane, which affects the reaction rate. "These [vesicles] are the tiniest test tubes you've ever heard of," he says. Each can hold as little as a billionth of a trillionth of a liter ([10.sup.-21] I).

The researchers make the microscopic containers out of phospholipids, long molecules that assemble into a double-layer membrane. With a laser beam or a thin glass tube, Zare and his colleagues trap the vesicles and move them around. If they bring a pair close together and then zap them with several electric pulses, the two vesicles unite into one.

To demonstrate this fusion, the researchers made containers containing green and red fluorescent dyes and joined them. The contents mixed together, and the resulting single sphere glowed bright orange. They also combined calcium ions in one vesicle with an organic compound in another.

"I think the work is truly amazing," says Zeev Rosenzweig, a chemist at the University of New Orleans. In a standard test tube, what limits the reaction rate is how fast the molecules can drift toward each other, he notes. In vesicles, however, the reactants don't have far to travel, so other factors influence the rate.

Rosenzweig thinks that this technique could eventually be used to study reactions between individual molecules. "Try to take two molecules and react them in a beaker, and they will never make it," he says. In the microscopic test tube, however, one single molecule would have no problem meeting another.