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Hidden peanut allergens Nanoparticle size affects enzyme Devices such as sensors, quantum dots, and “smart” materials are currently being developed with conjugated proteins to provide biofunctionality. However, Jonathan Dordick and colleagues at the Rensselaer Polytechnic Institute warn that the dimensions of the device can inadvertently change the properties of the attached proteins. Dordick and colleagues systematically analyzed the effect of a nanoparticle’s size, independent of its surface chemistry, on the properties of adsorbed proteins. They studied the adsorption of chicken egg lysozyme to SiO2 nanoparticles. The nanoparticles had radii ranging from 4 to 100 nm; lysozyme is approximately 4 nm in size. The investigators discovered that lysozyme maintained more of its native structure and function when adsorbed to smaller nanoparticles than it did to larger nanoparticles. The investigators used circular dichroism spectroscopy and colorimetric enzymatic assays to determine how the nanoparticle size affected the enzyme. They found that the enzyme lost more of its -helical content, and hence its activity, when adsorbed to larger nanoparticles compared with smaller ones under the same experimental conditions. Dordick and colleagues speculate that the protein unfolded more on the larger nanoparticles because the nano-
Even trace levels (~200 µg) of peanuts can cause serious allergic reactions in people who have a peanut allergy. Processed foods often contain undeclared amounts of peanuts from crosscontamination, particularly in products made in facilities that also process peanuts. To minimize this cross-contamination and prevent allergic consumers from unintentionally eating peanuts, Stefan Vieths and Oliver Stephan of the Paul-Ehrlich-Institut (Germany) have developed a pair of assays to detect hidden peanut allergens in food. One of the assays detects a peanut protein using a sandwich ELISA, and the other detects peanut-specific DNA by real-time PCR. Although ELISA techniques are commonly used to detect allergenic proteins, this is the first time PCR technology has been directly compared to an ELISA for detecting peanut allergens. The researchers used the two assays to test 33 food products from Germany for the presence of peanuts. Four of the products that did not list peanuts on their ingredients contained peanut protein. Two of them contained enough peanut protein to elicit a reaction in an allergic person. The results from both tests were well correlated; however, realtime PCR gave one more positive result than the sandwich ELISA. Both assays had a peanut detection limit of 10 ppm in whole milk and semisweet chocolate and showed no cross-reactivity with cereals, nuts, and legumes, which suggests that they are highly specific for peanuts. The researchers did not address the differences in thermal degradation of DNA in food products and therefore recommend using the PCR assay only semiquantitatively. They also emphasize the need for standardized reference materials. (J. Agric. Food Chem. 2004, 52, 3754–3760)
particle’s smaller radius of curvature meant there was more available surface area for interaction. However, additional studies are required to see whether the effects of surface curvature can be generalized to interactions with other proteins and biological molecules. (Langmuir 2004, 20, doi 10.1021/la0497200)
4 nm SiO2
100 nm SiO2
Interactions are stronger between lysozyme and a 100-nm particle than between lysozyme and a 4-nm particle. As a result, the protein loses more of its intrinsic structure and function when adsorbed to the 100-nm particle.
SERS with nanoparticles-on-electrodes Juan Pérez and colleagues at the Universitat d’Alacant (Spain) performed surface-enhanced Raman spectroscopy (SERS) on electrodes that were prepared by spontaneously depositing pure platinum or palladium nanoparticles on the surfaces. The researchers say that this “nanoparticle-on-electrode” approach is simpler than electrochemical roughening of electrode surfaces and that it produces high-quality spectra. Carbon monoxide, hydrogen, and cyanide were adsorbed on electrodes prepared with the new method. In the case of cyanide, the researchers found a surface enhancement factor 3–4 better than those reported for experiments conducted with electrochemically roughened electrodes. Such improved enhancement factors could lead to faster experiments. The researchers also speculate that the new approach might make it straightforward to extend SERS studies to alloys of platinum-group metals or to deposit nanoparticles with well-defined shapes onto surfaces for SERS analysis. (J. Phys. Chem. B 2004, 108, 9943–9949) A U G U S T 1 , 2 0 0 4 / A N A LY T I C A L C H E M I S T R Y
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