NEWS OF THE WEEK

SILICON TURNS INTO SUPERCONDUCTOR

ELEMENT OFCHANGE Electrons won't

go far in this softball-sized

sphere of silicon. But load a silicon film with boron, cool it close to absolute zero,

and it becomes a superconductor.

MATERIALS SCIENCE: Extreme boron doping transforms archetypal

semiconductor at ambient pressure

W ITH A MASSIVE BLAST of boron, mild-man­nered silicon, best known for its mediocre ability to conduct electricity, can be strong-

armed into shuttling electrons virtually resistance-free (Nature 2006,444,465). Scientists have been trying to get silicon to make this semiconductor-to-supercon­ductor transformation for 60 years.

Inciting a metamorphosis reminiscent of comic­book superheroes, a group led by Grenoble, France-based researchers Etienne Bustarret of the National Center for Scientific Research (CNRS) and Christophe Marcenat of the French Atomic Energy Commission (CEA) immersed a silicon film in BC13 gas and then blasted it with high-intensity laser light. With each blast, more and more boron was forced into the silicon

until the boron concentration reached several percent. The researchers then cooled the film until they ob­served superconductivityjust above absolute zero—at about 0.35 K.

The extreme temperature "raises questions as to how useful this turncoat-silicon might be," notes Princeton University solid-state chemist Robert J. Cava in a commentary that accompanies the report. "But its existence is impressive in its own right," he continues, adding that the successful transformation itself is a "breakthrough."

The authors, Cava explains, "are motivated by the possibility that, if silicon could be made superconduct­ing—even under conditions too extreme to be useful in practical devices—the integration of superconducting silicon into the sophisticated world of microelectronics processing might uncover new electronic functions."

Cava wonders what would happen if, instead of superdoping with electron-poor boron, the team used electron-rich arsenic or phosphorus. Could they make an electron-rich silicon superconductor? "That would allow the gamut of microelectronics concepts and processing to be applied to superconductors," he says, "but is far from an obvious extension of the present work."—BETHANY HALFORD

First atomic-level analysis of an organelle reveals a dense protein cover on synaptic vesicles.

ORGANELLE ANATOMY

CELL BIOLOGY: Atomic-level scrutiny of neurotransmitter vesicles is a first

T HE FIRST DEPICTION of an organelle right down to its molecular minutia has finally come to light. Using a combination of biophysical and proteomic techniques, European and Japanese

scientists examined quantitatively the rela­tive ratios of the lipids and 180 or so proteins that adorn synaptic vesicles, the compartments that

house and traffic neu­rotransmitters essential for brain function (Cell 2006,127,671).

It's a "technical tour-de-force," comments Thomas G. Sudhof, a medical researcher at

the University of Texas Southwestern Medical Cen­

ter. The group's 10-year endeavor has culminated "in the construction of the first atomic model for any organelle."

The "big surprise" was the overall density of protein on the membrane of these vesicles, says author Rein-hard Jahn, a neurobiologist at Max Planck Institute for Biophysical Chemistry in Gottingen, Germany. "Al­most a quarter of the entire membrane is taken up by the transmembrane domains of the vesicular proteins, and the surface of the organelles is almost completely covered by proteins.

"We all thought of membranes as lipid bilayers in which proteins float like icebergs in the sea," Jahn says. Instead, the membrane appears as a heavily packed "cobblestone pavement" of protein.

It turns out that so-called SNARE (soluble N-ethyl-maleimide-sensitive factor-activating protein recep­tor) proteins are the most abundant in the vesicle. When a synaptic vesicle approaches another mem­brane, SNARE proteins on both membranes interact to create a bridge required for the vesicle to fuse with the membrane. This is essential for neural function: Neu-rotransmitter-filled vesicles in a neuron must fuse with the cell's membrane to release their contents into the space that separates one neuron from the next, a region commonly called the synapse. Once released into the synapse, the neurotransmitters trigger an electrical sig­nal in nearby neurons. The high abundance of SNARE proteins in synaptic vesicles may facilitate speedy fu­sion and, consequently, rapid synaptic transmission, Jahn says.

Next, the researchers will focus on how vesicles that carry different neurotransmitters—such as glutamate or y-aminobutyric acid—differ from one another.— SARAH EVERTS

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