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August 15, = 2005 Vol. 83, Iss.=20 33
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August 15,=20 2005
Volume 83, = Number=20 33
p. 38

Unearthing=20 rare treasures
SF₆=20 is reactive fluorinator
Cornflower=20 pigment pictured
Cholera=20 toxin's harmful handshake=20
Frogs=20 eat ants for poisonous protection =





	COURTESY OF FRANK=20 SCHROEDER/CORNELL

Unearthing rare treasures=20

The characterization of a crude mixture of natural = products=20 can be difficult, particularly when the sample is = small and=20 composed of a complex brew of low-concentration = compounds such=20 as steroids. But researchers continue to find ways to = extract=20 more information from ever-smaller samples. Frank C. = Schroeder=20 of Cornell University and colleagues now offer a = powerful=20 demonstration of the sensitivity that can be achieved = by=20 capillary NMR spectroscopy of biological samples = (J.=20 Am. Chem. Soc. 2005,=20 127, 10810). The researchers collected and = combined=20 whole-body extracts from just 50 members of a rare = firefly=20 species. They partially fractionated the mixture and = analyzed=20 each fraction by capillary NMR. The resulting spectra = led to=20 the identification of 13 new steroids, including the = one shown=20 here. Most of the steroids "could not have been = characterized=20 in a reasonable amount of time" without capillary NMR, = according to Schroeder. "Crude or semipurified natural = products extracts" can be characterized "without = exhaustive=20 purification," he adds. The approach should "greatly = extend=20 the range of accessible natural products research."=20

SF₆ is reactive=20 fluorinator

Sulfur hexafluoride (SF₆), a gas known = for its=20 extreme inertness and extraordinary stability, has now = been=20 shown to have promise as a selective fluorinating = agent. Richard D.=20 Ernst and coworkers at the University of Utah have = discovered that SF₆ is quite reactive = toward=20 low-valent organometallic compounds of titanium and = zirconium=20 at room temperature and even below (J.=20 Am. Chem. Soc. 2005, 127, = 11924). And surprisingly, they report, the gas's=20 reactivities "can rival or exceed those of some = commonly=20 employed fluorinating agents." For example, when they = treated=20 a titanium complex having three different ligands (one = of them=20 being a disubstituted cyclopentadienyl, R) with=20 SF₆, they produced a cyclic tetrameric = complex,=20 [Ti(R)F₂]₄. The chemists believe = that=20 the SF₆ reactions leading to oxidation of = the metal=20 center occur when one or more of the fluorine atoms = coordinate=20 and then transfer to the metal center. They suggest = that=20 SF₆ may prove useful for preparing = compounds "not=20 normally obtained through other reactions."=20

Cornflower pigment pictured =

My love is like a ... blue cornflower? Maybe not, = but the=20 same anthocyanin pigment that makes roses red also = makes=20 cornflowers blue. It's just packaged differently in = the=20 cornflower. A team of Japanese researchers led by = Kosaku=20 Takeda of Tokyo Gakugei University has obtained the = crystal=20 structure of the blue pigment in cornflowers. They = find that=20 the pigment is a complex of six anthocyanin molecules, = six=20 flavone molecules, one ferric ion, one magnesium ion, = and two=20 calcium ions (Nature 2005, = 436,=20 791). The ions line up along the pseudo-three-fold = axis, with=20 each Fe³⁺ and Mg²⁺ coordinated = to three=20 anthocyanin molecules and each Ca²⁺ = coordinated to=20 three flavone molecules. In the structure, stacking of = the=20 anthocyanin and flavone is evident. The C-C and C-O = bond=20 lengths indicate that the anthocyanin is in the=20 4=B4-keto-quinoidal form. The chelation of the = Fe³⁺=20 and the Mg²⁺ with the 4=B4-keto-quinoidal = base is=20 important for the blue color, and the Ca²⁺ = ions=20 help stabilize the complex. The authors think that the = tetranuclear metal complex may represent a new type of = supramolecular pigment.=20

Cholera toxin's harmful=20 handshake=20

3D""=20

COURTESY OF RANDALL HOLMES = AND WIM=20 HOL

A new study gives a = peek at how=20 cholera-causing bacteria trigger devastating bouts of = diarrhea=20 that can lead to severe dehydration and, if untreated, = death=20 (Science 2005, 309, 1093). = Cholera=20 is ca used by infection of the human intestine by = Vibrio=20 cholerae. When activated by its human host, the = toxin=20 produced by this bacterium catalyzes the transfer of = an=20 adenosine diphosphate ribosyl group from the cofactor=20 nicotinamide adenine dinucleotide (NAD⁺) to = the=20 human signaling protein G_sα. This = chemical=20 modification triggers a chain of events that result in = the=20 intestine being flooded with water and salts, = generating the=20 watery diarrhea characteristic of cholera. Wim G. = J.=20 Hol and Claire J. O'Neal of the University of = Washington,=20 Seattle, and Michael G. Jobling of the University of = Colorado=20 Health Sciences Center used X-ray crystallography to = probe how=20 a ubiquitous human G protein activates the cholera = toxin to=20 catalyze this crucial protein modification. They find = that=20 binding of the human G protein (yellow) induces a = dramatic=20 conformational change in the toxin (gray) that allows=20 NAD⁺ (ball-and-stick model) to bind to the = toxin's=20 active site (green). The regions thought to bind=20 G_sα are shown in red and brown.=20

Frogs eat ants for = poisonous=20 protection=20

The brightly colored Mantella frogs of Madagascar = seem like=20 they'd be an easy-to-spot snack. But for most = predators, those=20 vivid hues translate to "keep away" rather than "eat = me."=20 That's because the frogs' skin contains poisonous = alkaloids.=20 Despite the advantageous presence of the alkaloids, = the frogs=20 don't make the chemicals themselves. Instead, the = frogs=20 sequester the alkaloids from a steady diet of = alkaloid-rich=20 ants, according to a new report from Valerie C. = Clark=20 of Columbia University and colleagues (Proc. Natl. = Acad.=20 Sci. USA, published online Aug. 8, dx.doi.org= /10.1073/pnas.0503502102).=20 Scientists had previously identified ants as the = source of=20 alkaloids in the neotropical poison dart frogs of = South and=20 Central America, but Clark's is the first study to = identify=20 the source of the Madagascar frogs' alkaloids. Clark = notes=20 that the finding is a striking example of convergent=20 evolution--the phenomenon in which different species = end up=20 appearing superficially similar because they have = taken the=20 same approach to overcoming a challenge in their = environment.=20

SCIENCE=20 CONCENTRATES
Chemical & Engineering = News
ISSN=20 0009-2347
Copyright =A9 2005





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