Carbon dioxide can help dissolve proteins.

Take a deep breath, then sigh. That exhaled carbon dioxide (CO2), the ubiquitous gas enjoyed by plants and blamed for the greenhouse effect, has just proved helpful in laboratory biochemistry. For the first time, researchers have demonstrated that heated, pressurized carbon dioxide can dissolve proteins.

Known for adding effervescence to mineral water and soft drinks, the gas also finds industrial use in processing beer, removing nicotine from tobacco, and pumping oil from wells. Under pressure, it serves as a powerful, environmentally friendly solvent, replacing many of the hazardous, toxic organic solvents used widely by industry.

Now, there's likely to be a demand for carbon dioxide in biotechnology laboratories as well, says Eric J. Beckman, a chemical engineer at the University of Pittsburgh.

"Not only is CO2 more environmentally acceptable and naturally abundant than organic solvents," says Keith P. Johnston, a chemical engineer at the University of Texas at Austin, "but it is also nonflammable, essentially nontoxic, and the least expensive solvent after water."

At normal atmospheric pressure, carbon dioxide freezes directly from a gas into a solid-dry ice. Only above 31 degrees C and 73.8 atmospheres does it exist as a fluid. At the transition point, it becomes a supercritical fluid with industrial uses that include decaffeinating coffee, spray paint propulsion, fiber manufacture, and soil cleanup.

Decaffeinating coffee beans with carbon dioxide shows why the new method has great potential, says Alan J. Russell, a biochemist at Pittsburgh. "If a technique can selectively pull molecules of caffeine out of a mixture as complex as a coffee bean, then perhaps the same technique could be used to pull a specific protein out of a group of cells.

"There aren't many techniques capable of doing that," he adds. "This one could prove to have great utility."

Johnston's team describes in the Feb. 2 Science a method for employing carbon dioxide to dissolve proteins, making it potentially useful in biotechnology. Mixing supercritical carbon dioxide with pressurized water, the researchers made "water-in-CO2 microemulsions," which they used to dissolve a protein called bovine serum albumin.

This work may help to "pave the way for use of CO2 in extraction and purification of biomolecules, an exciting application that had previously been thought untenable," says Beckman in an accompanying article in Science. The way microemulsions dissolve proteins resembles the way soapy water breaks up dirt during laundering. "When you dissolve soap in water, the mixture forms tiny [droplets], called micelles, which will solubilize oil and grease," Beckman explains. "In the water-in-CO2 microemulsions, the micelles function as tiny chambers for dissolving molecules."

Industrial advantages of carbon dioxide over standard solvents come from its ease of use, says Beckman. No government agency regulates use of the gas, it can easily be removed from and returned to the environment, and at 4 cents per pound, it is "probably the least expensive organic solvent one can obtain commercially."

Although carbon dioxide remains a weaker solvent than its volatile cousins, it could replace hazardous, toxic solvents in processes where residues could prove harmful to people-for instance, in purifying food or preparing pharmaceuticals.