Identifying DNA by the speed of electrons.

Ordinarily, a line represents the shortest distance between two points. Chances are, the straightest path also provides the quickest route.

Not surprisingly, this simple geometric truth holds sway in the world of biological molecules, where intricately twisted shapes sometimes obscure the shortest path between two positions.

Exploiting this principle, chemist Thomas J. Meade and molecular biologist Jon F. Kayyem, both at the California Institute of Technology in Pasadena, report that they are developing a new biosensing system for fast, precise detection of single DNA strands or their fragments.

The method rests on the simple premise that an electron careering down the center of a DNA helix needs less time for its journey than an electron spiraling along the molecule's edge. The new technique uses a single strand of known DNA. If a complementary (matching) piece of DNA binds to it, an electron's speed through the molecule increases, revealing the presence of the DNA being sought.

"If you know at least 20 base pairs of the DNA fragment you're looking for, you could use this method to detect it," Meade said last week in Atlanta at the annual meeting of the American Association for the Advancement of Science. "You don't even need to purify the sample. This method is selective enough that impurities can be present in the sample. All that matters is whether or not the DNA you're looking for is there."

"What we've stumbled across is a difference in two rates of electron transfer in DNA," Meade says. "One rate is for a path along the phosphate backbone of a single strand of DNA, while the other rate is for a path down the middle of the molecule. We've been able to measure that difference."

"That difference in the two rates is the key to detecting complementary strands of DNA," Meade adds.

By measuring the difference between those two electron transfer rates, the new system immediately detects whether a sample contains complementary DNA strands. A report of the electron transfer method also appears in the February issue of the German journal Angewandte Chemie.

Next, the researchers want to fit their system onto disposable microchips. "We believe this technique could lead to quick, accurate, and inexpensive tests for pathogens, infectious agents, or bacteria," Meade says.

Physicians could quickly screen blood samples for genetically based illnesses or disease organisms, including HIV, the AIDS-causing virus. Meat packers could test for contamination -- by toxic Escherichia coli bacteria, for instance --at the slaughterhouse. Agricultural researchers could monitor disease resistance in plants in the field. In environmental monitoring, this method could hasten assessments of waste sites and aid bioremediation.

"You could perform these tests on the spot without having to send samples back to a lab," Meade says. "There's a great need for fast, portable sensors like this."