Science Concentrates

Al₁₃, a cluster of 13 aluminum atoms, acts in chemical reactions like a single halogen atom, suggesting that it can be used as a building block for a new class of nanoscale materials composed of similar element-like clusters [Science, 304, 84 (2004)]. Experimental work by Denis E. Bergeron and A. Welford Castleman Jr. of Pennsylvania State University and theoretical studies by Tsuguo Morisato and Shiv N. Khanna of Virginia Commonwealth University, Richmond, revealed that the Al₁₃ group maintains its integrity as an intact metal cluster when it reacts with HI in the gas phase to form the anionic cluster Al₁₃I^– (shown, Al = blue and I = red). In the cluster, the Al₁₃ group has higher electron affinity than iodine and behaves like bromine. This, along with its single-entity behavior in reactions, demonstrates that Al₁₃ has "pseudohalogen" or "superhalogen" character, and it is the first metal cluster found to act in this manner. Castleman says the group hopes the work will lead to a "periodic table, not of elements but rather of clusters simulating the properties of elements," that could be used to design and construct nanoscale materials with tailored properties.

Improving DNA detection

An enzymatic method improves the detection limits of DNA microarray analysis by surface plasmon resonance (SPR) a millionfold, making SPR competitive with other DNA detection techniques, according to a new report. SPR is attractive for DNA analysis because it doesn't require labeling of DNA. Chemistry professor Robert M. Corn of the University of Wisconsin, Madison, and his coworkers detect genomic DNA without polymerase chain reaction amplification by using RNA microarrays and the enzyme RNase H, which selectively and irreversibly destroys the RNA in RNA-DNA heteroduplexes [J. Am. Chem. Soc., 126, 4086 (2004)]. The DNA in the sample binds to the RNA probe, which is then hydrolyzed by RNase H. The released DNA can bind to another RNA probe, so a single DNA molecule can initiate the destruction of many RNA probes. The loss of RNA probes from the surface results in a significant reduction in the percent reflectivity of a specific portion of the array that can be integrated over time, and RNA loss can conceivably continue until all the probes are destroyed. Lower DNA concentrations take longer to reach a given change in reflectivity. Corn and his colleagues demonstrate a detection limit of 1 femtomolar DNA.

Boron-doped diamond superconducts

Boron-doped diamond, which is normally a semiconductor, turns into a superconductor at low temperature, according to scientists from the Russian Academy of Sciences and Los Alamos National Laboratory. Russian physicist Vladimir A. Sidorov and his colleagues report that synthetic diamond containing about 3% boron becomes a superconductor at about 4 K [Nature, 428, 542 (2004)]. Because it can carry electricity without resistance, superconducting diamond might be useful for making improved power storage devices or electrical motors. Although superconducting semiconductors are rare, "the discovery of superconductivity in diamond-structured carbon suggests that silicon and germanium, which also form in the diamond structure, may similarly exhibit superconductivity under the appropriate conditions," the authors write. They suggest that previous characterizations of boron-doped diamond missed this intriguing property because the material has rarely been studied at such low temperatures.

Handy catalyst for C–C coupling

The Suzuki-Miyaura coupling is a palladium-catalyzed reaction that joins aryl boronates to aryl halides. It is one of the most powerful ways to form carbon-carbon bonds, but it is not always straightforward to execute. A step toward an "operationally simple and general" procedure has been taken by chemists Shawn D. Walker, Timothy E. Barder, Joseph R. Martinelli, and Stephen L. Buchwald at Massachusetts Institute of Technology. They have designed, prepared, and tested the ligand shown (Cy = cyclohexyl) to meet the following criteria: high activity with a broad range of substrates, including hindered aryl halides and boronates; ability to operate at room temperature; and ease of use, that is, it does not require the aid of a glove box. Buchwald and coworkers find the new catalyst system highly active for coupling of heteroaryl halides and unactivated aryl chlorides and aryl bromides [Angew. Chem. Int. Ed., 43, 1871 (2004)].

Crystallographic warning issued

About 90% of all protein crystal structures are determined at temperatures of 90 to 120 K. The biological relevance of such "cryostructures" hinges on the assumption that flash-cooling is fast enough to trap the protein in its room-temperature equilibrium state. Although this appears to be true for the global backbone fold, a new report shows that even the fastest flash-cooling protocols induce local structural changes [Proc. Natl. Acad. Sci. USA, 101, 4793 (2004)]. Model calculations by Bertil Halle, biophysical chemistry professor at Lund University, in Sweden, indicate that the positions of side chains, weakly bound ligands, and hydration water observed in low-temperature structures may not be the same as those under physiological conditions. Such "cryoartifacts" are most pronounced in the functional parts of proteins, such as recognition, binding, and catalytic sites. Halle's analysis shows that these cryoartifacts have the same molecular origin as does the dynamical glass transition in proteins, namely, the dramatic slowing down of water motions near 200 K.