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BIOCHEMISTRY
A novel method for detecting DNA sequences at picomolar concentrations may lead to new types of inexpensive tests with applications in medical diagnostics, genetic mutation studies, gene-delivery procedures, and other areas. "The object is to take advantage of the light-harvesting properties of conjugated polymers to increase signal-to-noise levels in DNA detection methods," says Guillermo C. Bazan, a professor of chemistry and materials at the University of California, Santa Barbara. The homogeneous solution-based technique was developed by Bazan and Santa Barbara coworkers, including graduate student Brent S. Gaylord and physics professor Alan J. Heeger, one of the recipients of the 2000 Nobel Prize in Chemistry (Proc. Nat. Acad. Sci. USA, published online Aug. 7, http://www.pnas.org/cgi/reprint/162375999v1.pdf). When single-stranded DNA (shown as a blue ribbon) is added to a solution containing a peptide nucleic acid labeled with a chromophore (yellow ribbon) and a cationic conjugated polymer (highlighted structure), two outcomes are possible. If the peptide nucleic acid and DNA strands are complementary, as shown at right, then a hybridization reaction joins the nucleic acid strands. Electrostatic attractions between negatively charged DNA and the cationic polymer bring the polymer and the chromophore close together. At short distances, energy transfer from the polymer to the chromophore causes the dye unit to fluoresce efficiently (green) when the solution is irradiated with UV light. If the strands are noncomplementary (left), the units remain too far apart for energy transfer and no signal is detected.
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