Science Concentrates

Total synthesis of thiopeptide antibiotic achieved

The first total synthesis of the natural product amythiamicin D could pave the way for syntheses of other thiopeptide natural products, according to a team led by chemistry professor Christopher J. Moody of Exeter University, in England [Chem. Commun., published online March 16, http://dx.doi.org/10.1039/b401580k]. Thiopeptide antibiotics are a family of sulfur-containing macrocyclic peptides that inhibit protein synthesis in bacteria and have recently been shown to exhibit potent antimalarial activity. The family includes the amythiamicins, which are isolated from a strain of Amycolatopsis bacteria. The team's total synthesis of amythiamicin D unambiguously confirms the structure (shown) of the natural product. The keystone of the synthetic strategy is a biosynthesis-inspired hetero-Diels-Alder reaction that gives the key 2,3,6-trisubstituted pyridine core of the thiopeptide antibiotic, Moody says.

Mass spec flies high for ozone

The amount of ozone that drifts down from the stratosphere into the upper troposphere can be precisely determined, thanks to a new experi- mental technique developed by the National Oceanic & Atmospheric Administration's T. P. Marcy and colleagues [Science, 304, 261 (2004)]. Ozone in the upper troposphere (UT)--the upper part of the atmospheric layer directly above Earth--affects global warming, agriculture, and human health. Determining how much of that ozone comes from the stratosphere could help scientists develop better models of global chemical transport. Marcy's group used a chemical ionization mass spectrometer (CIMS) flown in a high-altitude aircraft to get in situ measurements of HCl levels in the atmosphere. HCl is an indicator of stratospheric ozone because the two chemicals are both produced in the stratosphere. All the HCl in the UT comes from the stratosphere, so the scientists can use the CIMS data to distinguish the ozone that has drifted down from the stratosphere from ozone that was already in the UT.

Micelles that last, sans water

Seven T-shaped amphiphiles based on a derivatized calixarene form a spherical micelle in the manner shown. In the amphiphile, hydrocarbons emerge like tails from the bottom of the calixarene bowl and hydrophilic dendrimer-like decorations sit on two opposing sides of the bowl's rim. The micelles are unusually persistent, maintaining structural integrity even when the associated water is removed. The persistence has enabled Andreas Hirsch at the University of Erlangen-Nürnberg, in Germany, Christoph Böttcher at the Free University of Berlin, and coworkers to visualize the micelles at molecular resolution through cryogenic transmission electron microscopy and other image-processing techniques [Angew. Chem. Int. Ed., published April 2, http://www3.interscience.wiley.com/cgi-bin/fulltext/107642998/PDFSTART]. The micelle's cavity can take high loadings of nonpolar molecules such as porphyrin and fullerene derivatives and can transfer the load to an aqueous phase. Hirsch says the micelles have potential for drug delivery.

Greenland's ice may disappear

If Greenland's average annual temperature rises by more than about 3 °C, its ice sheet is likely to be eliminated, raising sea levels by 7 meters. Unless drastic reductions are made in greenhouse gas emissions, the temperature in Greenland is likely to exceed this threshold by 2100. These are the primary conclusions of computer simulations by meteorologist Jonathan M. Gregory of the University of Reading, in England, and his colleagues [Nature, 428, 616 (2004)]. Gregory finds that, for a warming of 3 °C, the ice sheet loses mass slowly, and over thousands of years, it might reach a steady state in a much smaller form. For temperature increases of more than 3 °C, the ice sheet is likely to melt away almost entirely in 1,000 years or more. Warming is likely to exceed 3 °C in Greenland unless much more substantial CO₂ reductions are made than those considered by the United Nations Intergovernmental Panel on Climate Change, Gregory writes. With the ice sheet gone, the climate of Greenland would be much warmer because the exposed land would reflect less sunlight, he notes. Modeling also shows that once the ice sheet is gone, it is unlikely to return.

Enzyme relies on membrane distortions

The activity of a photosynthesis enzyme requires the presence of a special type of distortion in the neighboring lipid membrane, according to a new report, suggesting that such membrane distortions play an important physiological role. Certain lipids tend to form inverted hexagonal structures (shown) in the membrane rather than the more common lipid bilayers. The activity of a handful of enzymes has been shown to depend on such non-bilayer-forming lipids, but it has remained unclear whether it's the lipids' chemical properties or their ability to form inverted hexagonal structures that is essential for enzyme activity. Now, a team led by Kazimierz Strzalka of Jagiellonian University, in Kraków, Poland, has shown that violaxanthin de-epoxidase requires hexagonal inverted lipid structures for its activity [Biochemistry, published online March 24, http://dx.doi.org/10.1021/bi049652g]. The chemical identity of the lipid doesn't seem to matter as long as it forms such structures, the team reports. The researchers are now investigating whether the enzyme's activity requires such membrane distortions in vivo, too.