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Detecting endogenous cyanide in common foodstuffs In the U.S., cyanide poisonings are encountered almost exclusively in news stories about industrial accidents or in intricately diabolical schemes played out on one of TV’s police procedurals. But for hundreds of millions of people worldwide, cyanide-related ailments can be the all-natural result of eating an everyday food. In a recent AC article (2009, DOI 10.1021/ac901977u), Felix Zelder and a team at the University of Zürich report a new means of detecting cyanide within the biological matrix. Because the presence of the cyanide is almost immediately visible to the naked eye when food is prepared, the approach could help diminish the frequency of some all-too-common illnesses. The cassava root is just one of many foods that can serve as a natural source of cyanide, but it’s a particularly important one because of its extensive use as a food source in South America and parts of Africa. (Its one common manifestation on U.S. menus is tapioca.) “Cassava is a staple food of about 600 million people,” says Zelder, “and according to a recent review [J. Sci. Food Agric. 2008, DOI 10.1002/jsfa.3337], cassava production is increasing faster than the population, due to its agricultural advantages.” Cassava and similar plants, such as sorghum, flax, and bamboo, can contain cyanogenic glycosides, which produce hydrogen cyanide enzymatically after cell rupture. With proper handling (soaking and washing, for example), the cyanide can be removed, leaving behind a nutritious starch. However, incomplete cyanide removal contributes to various illnesses, including konzo, a paralysis of the legs that afflicts ⬃100,000 people in Africa. The heart of the Zürich team’s sensing technology is the corrin ring, which is related structurally to the porphyrin ring of hemoglobin. Cobyrinic acid, a corrin ring with Co(III) in its center and two axial cyano substituents, was readily
prepared from its commercially available heptamethyl ester by alkaline hydrolysis. After a nonselective displacement of one cyano group with a water molecule in the laboratory, the resulting complex
(Top) The (a) orange reagent turns (b) violet in the presence of cyanide, as on the surface of (e) freshly cut cassava or (c) ground cassava. (d) With proper washing to remove cyanide, ground cassava no longer elicits a color change. (Bottom) The cyano-aqua complex, in (2) water-soluble and (3) a more lipophilic form.
can serve as an effective chemosensor for cyanide, because “substitution of Co(III)-bound water by cyanide alters the ⫺* transitions of the corrin macrocycle, inducing a visible color change,” says Zelder. Although the system was described previously, in this paper the researchers “show that it may be applied to the detection of biological cyanide samples,” says Jonathan Sessler of the University of Texas Austin. “This is a big step forward.” Franc¸ois Gabbaı¨ of Texas A&M University agrees. “One of the most attractive facets of these results,” he says, “is the fact that the chemosensors can be used directly on the biological sample
10.1021/AC9024279 2009 AMERICAN CHEMICAL SOCIETY
Published on Web 11/04/2009
without any preprocessing.” He notes that “the chemosensors are closely related to vitamin B12. Presumably, the robustness of their structures makes them compatible with the biological matrix.” One of the most challenging aspects of the project was refining “the surface experiments, because the methods were new for us,” says Zelder. Through the use of diffuse reflectance UV⫺vis spectroscopy, quantitative measurements were taken of surface reactions, which could then be compared with stoppedflow kinetic experiments in solution. “My co-worker Christine Ma¨nnel-Croise´ had to work very precisely to set up and standardize these experiments,” says Zelder. The heptamethyl ester of cobyrinic acid was also used, providing an analog readily partitioned from the biological matrix into organic solvents for examination by NMR. The technology represents a promising means of enhancing the safety of cyanogenic foods. “It would be nice to see the method help reduce health problems related to cassava consumption in developing countries,” says Zelder. “It could also improve manufacturing techniques to remove cyanide from cassava.” Both Zelder and Gabbaı¨ envision utility in scientific research as well. “While such sensors might be useful to increase consumer safety, they could also become useful research tools for the discovery, via rapid colorimetric screening, of other cyanide-producing species,” says Gabbaı¨. —Steven C. Powell
DECEMBER 1, 2009 / ANALYTICAL CHEMISTRY
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