Science Concentrates

Light sends flies through hoops

Cell biologists at Yale University can control fruit fly behavior with flashes of light (Cell 2005, 121, 141). Gero Miesenböck and Susana Q. Lima genetically engineer flies to express a particular kind of ion channel in specific groups of neurons that normally lack them. These ion channels can be opened by adenosine triphosphate (ATP), which activates the neurons. The researchers want to control when the ion channels open. So they inject light-sensitive, chemically "caged" ATP molecules (shown) into the flies' central nervous system. When the flies are subsequently exposed to a flash of light, the cages release the ATP. The liberated ATP opens the ion channels and activates the target neurons. Depending on the type of neurons, this causes a fly to walk more or to jump, beat its wings, and fly. Researchers will be able to use this technique to determine which neural circuits are involved in particular behaviors. The method might also eventually be used to compensate for neural circuits damaged by injury or disease.

I⁺ is catalyst for oxidation of CH₄ to CH₃OH

Gas-phase studies using iodine cations to catalyze C–H activation of methane to form methanol have pinpointed I⁺ as the active species in the reaction (J. Phys. Chem. A 2005, 109, 3433). Gustavo E. Davico of the University of Idaho selectively generated I₂⁺ or I⁺ cations and reacted them with methane, finding that the reaction proceeds only with I⁺ to form the necessary CH₃IH⁺ intermediate. Reaction of the intermediate with H₂SO₄ would lead to CH₃OH. The results are supported by theoretical studies that indicate I⁺ inserts into a C–H bond the same way as a transition-metal catalyst. "The results are significant, as they help clarify the actual activation species in one of the simplest systems reported for selective methane activation," notes Roy A. Periana of the University of Southern California. In 2002, Periana's group reported that I₂ can convert CH₄ to CH₃OH in oleum (SO₃ in H₂SO₄) and proposed that I₂⁺ or I⁺ might be the active catalyst, ruling out other iodine species. If a practical, high-yield methanol synthesis under mild conditions without the need for a transition-metal catalyst could be developed, it would be an economic and environmental advantage for petrochemical companies. A patent has been filed to cover Davico's research.

Mid-IR spectra of naked protein

Structural information about naked proteins in the gas phase can help reveal the role that water plays in determining protein structure in solution. Gert von Helden at Max Planck Institute in Berlin, Jos Oomens at FOM Institute for Plasma Physics in the Netherlands, and coworkers obtained gas-phase infrared spectra for several different charge states of the 104-amino acid protein bovine heart cytochrome c. The charge states were isolated by Fourier transform ion cyclotron resonance mass spectrometry (Phys. Chem. Chem. Phys. 2005, 7, 1345). The amide I and amide II vibrational bands in the IR spectra are slightly shifted relative to the spectrum of the solution structure, as is expected for an environment with reduced hydrogen bonding. The positions of those bands suggest that the protein is predominantly an -helix in the gas phase, as is the case in solution. An unexpected and not-yet-identified band appears in the gas-phase IR spectra of higher charge states, suggesting that the protein's secondary structure may change at higher charge states.

Bacteria coaxed into producing just one protein

A bacterial toxin can be used to turn an Escherichia coli bacterium into a factory focused on making a single protein of interest, according to a new study (Mol. Cell 2005, 18, 253). Masayori Inouye and coworkers at Robert Wood Johnson Medical School in Piscataway, N.J., engineered E. coli containing MazF, a bacterial toxin that typically degrades all protein-encoding messenger RNAs in a cell. They then introduced a target protein gene that's been engineered to be resistant to cleavage by MazF. This combination resulted in high-level production (up to 90%) of the engineered target protein against a background of virtually no other protein production in living cells. The team has used the technique for high-level expression of bacterial, yeast, and human proteins, including a membrane protein usually expressed at low levels. In addition to being a highly effective method for producing recombinant proteins, the technique allows for nearly exclusive isotopic labeling of the target protein in the virtual absence of background protein synthesis. This "may enable structural and functional studies of proteins in intact, living cells using nuclear magnetic resonance," the authors note.

Shape-shifting polymers respond to light

A new family of polymers that can change shape when illuminated with ultraviolet light may offer advantages over temperature-responsive "shape-memory" polymers in certain medical and other applications. The light-responsive materials were developed by Andreas Lendlein of GKSS Research Center, in Teltow, Germany; Robert Langer of MIT; and their colleagues (Nature 2005, 434, 879). Key to the work are "molecular switches"--photoresponsive groups such as the one shown--that are grafted onto a copolymer backbone. When the copolymer film is stretched and then illuminated with UV light, the molecular switches cross-link, maintaining the polymer chains in their elongated state long after the external stress has been released. When this elongated film is subsequently exposed to a different UV wavelength, the cross-links are cleaved and the material springs back to its original shape. Other temporary shapes also can be produced. For example, when the researchers irradiate only the top side of a stretched-out polymer film, a corkscrew spiral having two layers is formed. The top layer, they explain, is locked into its elongated shape while the bottom layer remains elastic. Thus, when the external stress is released, the bottom layer contracts much more than the top layer, forming a circular shape.