Science Concentrates

Structure of drug-breakdown enzyme is determined

The high-resolution crystal structure (shown) of human cytidine deaminase (CDA) with a bound potent inhibitor (gold) has been determined for the first time by Gregory L. Verdine and coworkers at Harvard University [J. Med. Chem., 48, 658 (2005)]. CDA plays an essential catalytic role in a key cytidine and deoxycytidine breakdown pathway. It also breaks down nucleoside-type anticancer and antiviral agents (such as gemcitabine and cytarabine), shortening the drugs' duration of action and sometimes inducing unpleasant side effects. Researchers have thus been trying to design inhibitors of CDA as therapeutic additives, but they've been hampered by insufficient structural data on the enzyme. Now, Verdine's structure reveals that several moderate inhibitors bind CDA by coordinating to its zinc ion but that a strong inhibitor binds differently. The strong inhibitor doesn't bind zinc and makes up for this lack of metal coordination by participating in a / edge-to-face interaction instead. The findings could aid in the design of CDA inhibitors that enhance the efficacy of CDA-sensitive drugs.

Magnet deforms self-assembled nanocapsules

Three decades ago, it was predicted that magnetic fields could be used to deform spherical phospholipid bilayer vesicles into elongated shapes, but this anticipated deformation hasn't been experimentally observed until now. Peter C. M. Christianen of Radboud University Nijmegen and Albertus P. H. J. Schenning of Eindhoven University of Technology, both in the Netherlands, and colleagues report that thiophene-based nanocapsules can be deformed into flattened spheres in a magnetic field and that the flattened shape can be retained when the nanocapsules are removed from the magnetic field if they are fixed in an organogel matrix [J. Am. Chem. Soc., 127, 1112 (2005)]. The team makes the nanocapsules from a nonpolar sexithiophene oligomer containing polar ethylene oxide chain substituents mixed in 2-propanol. These molecules align parallel to one another to form spherical aggregates. The researchers use electron microscopy to observe the nanocapsules slowly flatten as the molecules realign in a magnetic field of up to 20 teslas. Magnetic manipulation of the nanocapsules might be used to encapsulate or release guest species for many applications.

Martian NO glows at night

An international team of scientists has detected a night-glowing blanket of nitric oxide surrounding Mars that could be used to track circulation in the martian atmosphere. Jean-Loup Bertaux at the Service d'Aéronomie du Centre National de la Recherche Scientifique, in France, and coworkers recorded the ultraviolet glow with a spectrometer aboard the European Space Agency's Mars Express spacecraft, which orbits the planet [Science, 307, 566 (2005)]. On the day side of Mars, extreme ultraviolet light from the sun breaks down N₂, O₂, and CO₂ into N and O. The atoms then migrate to the night side of the planet, where they combine to form NO. The NO glow is particularly bright in the winter night at Mars's south pole, where the atmosphere is transported downward; as much as 30% of the atmosphere freezes at the surface.

A peek at how nature turns CH₄ into CH₃OH

The first peek at the structure of a membrane-spanning metalloenzyme capable of converting methane into methanol has been provided by crystallographers at Northwestern University. Amy C. Rosenzweig and Raquel L. Lieberman have solved a 2.8-Å-resolution structure of particulate methane monooxygenase (pMMO), the dominant methane-oxidation enzyme in methane-consuming bacteria [Nature, published online Jan. 26, http://dx.doi.org/10.1038/nature03311]. They show that the enzyme is made up of three monomers that cluster to form a membrane-spanning barrel. Each monomer contains two metal centers in its soluble region (one, a single copper ion and the other, a pair of copper ions) and a third (a single metal ion of yet-to-be-confirmed identity in vivo) within its membrane-spanning region. They are now trying to figure out which of these centers is catalytic and where the electrons needed to oxidize CH₄ come from. This information may facilitate the development of synthetic catalysts capable of converting CH₄ into CH₃OH, Rosenzweig says. Such catalysts might make natural gas a realistic alternative to petroleum for the synthesis of useful fuels and chemicals, she suggests.

Bowl-shaped aromatic anion

A convex, nitrogen-containing aromatic polycycle has been prepared and characterized by chemists Mark Mascal and Jordi Cerón Bertran at the University of California, Davis [J. Am. Chem. Soc., 127, 1352 (2005)]. This new aromatic heterocycle, called the azaacepentalenide anion (shown, N is blue), is derived from oxidation of 10-azatriquinacene, a hemispherical N-containing tricycle. "The synthesis of [the new] molecule opens up lots of questions about p system reactivity and metal binding," among others, comments Jay S. Siegel of the University of Zurich. Other scientists have predicted the aromaticity of the azaacepentalenide anion from theoretical calculations. The UC Davis work comes in the wake of another group's earlier synthesis of the acepentalene dianion, which is an aromatic bowl-shaped fragment of C₂₀ fullerene and is the hydrocarbon analog of the new anion. Spectroscopic and other measurements indicate that the aromaticity of the anion forces the fused framework to become much less bowl-shaped than fused triquinacenes prefer to be.