SPECTROSCOPY
▸ Soft X-rays probe buried interfaces
BIOTECHNOLOGY
Second-harmonic generation (SHG) is a nonlinear optical process in which two photons of a given energy interact with select types of materials and combine to form a single photon with double the original energy. The SHG process, and a closely related one known as sum frequency generation, lie at the heart of a number of spectroscopy methods based on infrared, visible, and ultraviolet laser light. As a result of spectroscopy selection rules, these nonlinear processes are particularly adept at probing interfaces, even ones hidden by many layers of molecules, as is the case for a solid catalyst in contact with high-pressure gas or an electrode in contact with a liquid-electrolyte solution. X-rays with photons in the 100-to-1,000-eV energy range, socalled soft X-rays, can provide valuable information about chemical bonding and structure with elemental specificity. But because of the lack of available light sources with the required intensity and coherence, researchers have been unable to develop an SHG version of soft X-ray interface spectroscopy—until now. In a proof-of-concept study, Richard J. Saykally and a large team of researchers working at the FERMI facility in Trieste, Italy, have demonstrated that the method can selectively probe layers of graphene inside a graphite sample (Phys. Rev. Lett. 2018, DOI: 10.1103/physrevlett.120.023901). The new technique may eventually enable researchers to use X-rays to track chemical reactions at interfaces with femtosecond resolution.—MITCH JACOBY
OH The industrial process for making and OH HO using indigo—the dye that gives blue denO O OH im its distinctive color—has two strikes against it in terms of environmental N friendliness. First, the synthesis involves H hazardous chemicals, including aniline, Indican formaldehyde, and strong bases. Second, because indigo isn’t water soluble, the Add Remove synthesized compound must be treated glucose glucose with reducing agents before it can be used as a dye. A team led by John E. Dueber OH of the University of California, Berkeley, has devised a microbial fermentation that results in a more sustainable process for N using indigo (Nat. Chem. Biol. 2018, DOI: H 10.1038/nchembio.2552). They engineered Indoxyl Escherichia coli bacteria to produce indoxyl—a precursor to indigo—from tryptoDimerize, oxidize phan. They also engineered the bacteria to produce a glycosyltransferase, an enzyme that adds glucose to indoxyl as a protectO H ing group to stabilize it. The glycosylated N compound, indican, is sufficiently stable for long-term storage. When the dye is N H needed, the glucose can be enzymatically O removed with a β-glucosidase to regenerIndigo ate indoxyl, which spontaneously dimerizes, yielding the reduced form of indigo, the form that crystallizes directly onto cotton fibers. The researchers dyed cotton fabric by spraying it with an indican solution, dipping it in a β-glucosidase solution, and oxidizing the dye compound in air.—CELIA ARNAUD
STRUCTURAL BIOLOGY
▸ Palmitoyltransferase structures could aid inhibitor discovery
C R E D I T: S CI E NC E
Palmitoyltransferases (PATs) are a bit of a mystery. These membrane-embedded enzymes add lipid groups of various
Human DHHC20 (yellow), in membrane of a cell organelle, has just transferred a palmitoyl group (white) to a protein (blue, right) and has another palmitoyl group ready for an incoming protein (blue, left). Gray spheres are Zn2+ ions.
Green route to a blue dye
lengths to more than 10% of human proteins, enhancing the proteins’ abilities to associate with cell and cell-organelle membranes. PATs also play important roles in cancer and other diseases. One called DHHC20 adds lipids to epidermal growth factor receptors like HER2, which is overexpressed in some breast cancer
cells. But the PAT family was discovered only in 2002, and no one had succeeded in analyzing the structure of anything other than just a fragment of one of the enzymes. Now, by screening to find a PAT amenable to crystallization, mutating another to boost its tendency to crystallize, and laboriously optimizing the crystals that formed, Anirban Banerjee of the National Institute of Child Health & Human Development and coworkers have obtained key structural details on two PATs, DHHC20 from humans and DHHC15 from zebrafish (Science 2018, DOI: 10.1126/science.aao6326). In addition, the team’s characterization of PAT mutants enabled them to propose that PATs with larger active-site cavities catalyze the addition of longer lipids to protein substrates. Banerjee and coworkers hope to structurally analyze additional family members and screen for PAT inhibitors that might have anticancer or other therapeutic effects.—STU BORMAN JANUARY 15, 2018 | CEN.ACS.ORG | C&EN
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