NEWS OF THE WEEK MOLECULAR
ELECTRONICS
SEEING IS BELIEVING
PICTURE THIS With atomic-scale dexterity, UC Irvine scientists have used a scanning tunneling microscope to place a copper phthalocyanine molecule between tiny gold leads and to image the bridge.
STM technique lets single-molecule junction be prepared and imaged
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FAPICTURE IS WORTH ATHOU-
sand words, then scientists' grasp of molecular electronics has just been expanded by agrand
quantity Researchers at the University of California, Irvine, have recorded the first direct image of a small molecule confined between two metal contacts and have probed its electronic structure systematically, revealing much information about the microscopic junction. The potential of extremely fast and ultradense electronics based on single molecules as circuit components has motivated many research groups to study the electronic properties of individual molecules. Several teams have created tiny structures that hold a single molecule in a nanometersized gap between electrical leads and probed the trapped molecule's electronic conductivity
BIOCHEMISTRY
E COLI KEEPS TIGHT REIN ON COPPER Discriminating protein binds Cu+ with high selectivity and sensitivity
CLUTCHED A copper ion (blue) is held in an unusual linear coordination between two cysteine residues of the protein CueR.
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N ESCHERICHIA COLI, COPPER
rarely gets a moment to itself: A metalloregulatory protein called CueR senses and binds free copper in the zeptomolar (10~21 mol per L) range. Such extraordinary sensitivity and selectivity, Northwestern University chemists Thomas V. O'Halloran and Alfonso Mondragon conclude, are due to CueR's unique binding site [Science, 301, 1383(2003)}. CueR is one member of a family of proteins that
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2003
choreograph the movement of metals within E. coll CueR binds copper, MerR binds mercury, ZntRbinds zinc, and so on. Once metal-bound, the proteins, already attached to D N A , turn genes that code for other metalprocessing proteins on or off. To discover how CueR "binds just one type of metal tightly," O'Halloran's group worked with chemist James E. Penner-Hahn at the University of Michigan. O'Halloran compared two X-ray crystal structures: CueR bound to Cu+ and ZntRattached to Zn2+. His group found that ZntR has a
But typically, such studies cannot answer unambiguously various questions about the structure and nature of the junction. For example, how does the molecule bond to the metal contacts? Is it attached to both leads? What is its orientation? And what effect does the metal have on the molecule's properties? UC Irvine physics and chemistry professor Wilson Ho and coworkers have now used a scanning tunneling microscope to position a copper phthalocyanine molecule in the gap between two gold contacts that they constructed one atom at a time. They then recorded high-resolution images of the junction [Science, published online Sept. 4, http://www sciencemag.org/cgi/content/abstr act/1088971vl}. In addition, by altering the length of the gold leads on an atomic scale, the Irvine team was able to study the effect of the metal on the system's electronic density of states. — MITCH JACOBY
site well suited to divalent ions, with four coordinating ligands. CueR's binding setup, however, is more unusual: It coordinates to Cu+ with just two ligands—cysteine thiolates—in a recessed, hydrophobic binding pocket. This arrangement forces copper to bind as a monovalent ion, making the site more specific for copper. In addition, a net negative charge is stabilized by a "helix-dipole interaction as well as a couple of hydrogen bonds and a lysine about 5 A away." These interactions, O'Halloran says, give the complex "exceptional stability" Those subtle structural differences "reveal the nuances necessary for biological control," says David L. Huffman, chemistry professor at Western Michigan University. And studying them, O'Halloran says, helps scientists "decode what it takes to build a mercury-specific, copper-specific, or silver-specific sensor."— LOUISA DALTON
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