Making hydrogenation greener: Using iron as catalyst for widely used chemical process, replacing heavy metals

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Widely used filtering material adds arsenic to beers

The mystery of how arsenic levels in beer sold in Germany could be higher than in the water or other ingredients used to brew the beer has been solved, scientists announced here today at the 245th National Meeting & Exposition of the American Chemical Society, the world's largest scientific society. The meeting, which features almost 12,000 reports and other presentations, continues through Thursday.

Mehmet Coelhan, Ph.D., and colleagues said the discovery could be of importance for breweries and other food processors elsewhere that use the same filtering technology implicated in the elevated arsenic levels in some German beers. Coelhan's team at the Technische Universität in Munich set out to solve that riddle after testing 140 samples of beers sold in Germany as part of a monitoring program. The monitoring checked levels of heavy metals like arsenic and lead, as well as natural toxins that can contaminate grain used in brewing beer, pesticides and other undesirable substances.

Coelhan explained that the World Health Organization uses 10 micrograms per liter of arsenic in drinking water as a limit. However, some beers contained higher arsenic levels. "When arsenic level in beer is higher than in the water used during brewing, this excess arsenic must come from other sources," Coelhan noted. "That was a mystery to us. As a consequence, we analyzed all materials, including the malt and the hops used during brewing for the presence of arsenic."

They concluded that the arsenic was released into the beer from a filtering material called kieselguhr, or diatomaceous earth, used to remove yeast, hops and other particles and give the beer a crystal clear appearance. Diatomaceous earth consists of fossilized remains of diatoms, a type of hard-shelled algae that lived millions of years ago. It finds wide use in filtering beer, wine and is an ingredient in other products.

"We concluded that kieselguhr may be a significant source of arsenic contamination in beer," Coelhan said. "This conclusion was supported by analysis of kieselguhr samples. These tests revealed that some kieselguhr samples release arsenic. The resulting arsenic levels were only slightly elevated, and it is not likely that people would get sick from drinking beers made with this filtration method because of the arsenic. The arsenic is still at low levels—the risk of alcohol poisoning is a far more realistic concern, as stated in previous studies on the topic."

Coelhan pointed out that beers produced in at least six other countries had higher arsenic amounts than German beers, according to a report published four years ago. He said that breweries, wineries and other food processors that use kieselguhr should be aware that the substance can release arsenic. Substitutes for kieselguhr are available, he noted, and simple measures like washing kieselguhr with water can remove the arsenic before use.

More information: Abstract

The German Brewing industry has more than 1000 members and produces annually 100 million hectoliter beer. Although beer consumption in Germany per capita has been stagnating for many years, it is relatively high (around 100 liter) compared to many other countries. A large part of beer produced is exported. Brewers take a great deal of care to ensure that the beers they produce are entirely safe. Heavy metals are subject to stringent legislation under German and EU law. In addition to these requirements, contaminants are subject to monitoring organized by the Association of German Beer Brewers, in order to check for any adverse effects on malt or beer quality or other effect on processing before they are accepted for use on malting barley. Hence, a malt monitoring program was started 2011 in Germany to explore levels of heavy metals, mycotoxins, dioxins and dioxine-like PCBs, and a large number of pesticides. In the present study results for arsenic levels are presented. Analyses revealed that particularly kieselguhr used for filtration of beers may be a significant source of arsenic contamination in beer.

Provided by American Chemical Society

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Chemists illuminate elusive mechanism of widely used click reaction

Scientists at The Scripps Research Institute (TSRI) have illuminated the mechanism at the heart of one of the most useful processes in modern chemistry. A reaction that is robust and easy to perform, it is widely employed to synthesize new pharmaceuticals, biological probes, new materials and other products. But precisely how it works had been unclear since its invention at TSRI more than a decade ago.

"These new findings allow us to exert greater control of the reaction and make it faster and more efficient under the most challenging conditions," said chemist Valery Fokin, an associate professor at TSRI, who was principal investigator for the new study. "The reaction-tracking techniques we developed here also can be applied to the study of other complex processes, both chemical and biological."

The report, which sheds light on the reaction known as copper-catalyzed azide–alkyne cycloaddition (CuAAC), on April 4 in Science Express, the advance online edition of the journal Science, and in the April 18, 2013 issue of the journal.

Classic Click Reaction

Fokin and his laboratory, and the laboratory of K. Barry Sharpless, a Nobel laureate and the W.M. Keck Professor of Chemistry at TSRI, reported the discovery of the CuAAC reaction in 2002. Danish researchers independently reported a similar reaction in the same year. The reaction involves the use of copper compounds to catalyze the linkage of two functional groups, a nitrogen-containing azide and a hydrocarbon alkyne, to make a stable five-membered heterocycle, 1,2,3-triazole. Azides and alkynes are small functional groups that can be easily introduced into a wide variety of structures using chemical or biological methods without interfering with normal biological processes.

The experimental simplicity and reliable performance of CuAAC under virtually all conditions, including in water and in the presence of oxygen, has made it a "go-to" method whenever covalent stitching of small man-made molecules or large biopolymers is needed, exemplified by protein and nucleic acid labeling, in vitro and in vivo imaging, drug synthesis and the forging of complex molecular architectures with surgical precision.

"Despite its many uses, the nature of the copper-containing reactive intermediates that are involved in the catalysis had not been well understood, in large part due to the promiscuous nature of copper, which rapidly engages in dynamic interactions with other molecules," said Fokin.

Previous studies had hinted that in the swirl of short-lived bondings and partings that occur during a given CuAAC reaction, not one but two copper-containing catalytic units—"copper centers"—are needed to help build the new triazole structure. To confirm this, Fokin and two of his graduate students, Brady Worrell and Jamal Malik, tried to reproduce key steps of the CuAAC catalytic cycle with either one or two copper atoms available. Analysis of the reaction course by tracking the heat given off by each reaction as well as product yield indicated whether it worked efficiently. "By monitoring the reaction in real time, we showed that both copper atoms are needed and established the involvement of copper-containing intermediates that could not be isolated or directly observed," said Worrell, who was the paper's first author.

In a second set of experiments, Worrell, Malik and Fokin introduced a pure isotope of copper—which differs slightly in mass from the isotope blend found in natural copper—as one of the two copper centers so that they could track their respective fates during the reaction. "We hypothesized that the two copper centers would have distinct roles, but found unexpectedly that their functions during key steps in the reaction are effectively interchangeable," said Malik.

New Linkages

The research reveals the popular CuAAC reaction in unprecedented detail. In addition to the fundamental insights into the chemistry of copper and its interactions with organic molecules, the techniques will lead to better understanding of many chemical and biological processes involving copper. The current study also enables development of new reactions that exploit weak interactions of copper catalysts with carbon-carbon triple bonds. In fact, based on the new findings, Fokin and his team have begun to devise new reactions in which one copper center can be replaced with a different element, to pursue complementary biocompatible and efficient techniques.

More information: "Direct Evidence of a Dinuclear Copper Intermediate in Cu(I)-Catalyzed Azide–Alkyne Cycloadditions," Science Express, 2013.

Journal reference: Science Science Express

Provided by Scripps Research Institute

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