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Probes with tunable colors and lifetimes Place two carefully chosen dyes—one a donor molethe emission color to be largely determined by the cule and the other an acceptor—close to each other on acceptor. The lifetime is determined by the amount a single molecule, and it is possible to measure differof energy transfer between the donor and acceptor, ent fluorescence emissions or “colors” as a function of which is affected by the distance between the two. distance. This multicolor fluorescence with compound With these ground rules, the authors report obtaindyes is a clever approach that is being used to investiing tunable wavelengths from 520 to 680 nm and gate biological molecules. However, a fluorescence sig- tunable lifetimes ranging from 50 to 500 µs. The nal is also characterized by its lifetime, which also lifetimes are particularly notable, because they fall could be varied. Paul R. Selvin and Jiyan Chen of the into a range that allows discrimination against shortUniversity of Illinois at Champaign–Urbana put the lived background fluorescence and scattering. These two together to ranges will probably create compound grow with future work. dyes that have The system was simultaneously demonstrated with tunable emission oligonucleotide duplexwavelengths and es. A lanthanide chelate tunable excitedwas attached to the 5´state lifetimes. end of a DNA strand, The basic sysand the acceptor was tem consists of a connected to the middle lanthanide donor of the same strand via molecule and an C-5 deoxyuridine tetherorganic acceptor ing or to the 5´-end of a fluorophore, complementary DNA such as fluoresstrand. Differences in cein. The energy lifetimes and colors were transfer between observed with changes the lanthanide in donor–acceptor moleand fluorophore cules and separations. (J. is chosen to be Different lifetime decays for a Tb–fluorescein compound dye separated Am. Chem. Soc. 2000, by 6–10 base pairs. large, allowing 122, 657–660)
Better lead sensor To better understand the toxicology of lead, it’s important to measure ratiometric concentrations and to selectively detect free Pb2+ in vivo. Currently, only the total amount of Pb2+ in cells can be measured, but researchers really need to know the amount of bioavailable lead as a function of time, total lead in the blood, cell type, and subcellular location. To that end, Hilary Arnold Godwin and Sandhya Deo of Northwestern University describe the first ratiometric fluorescent probe that is selective for Pb2+ over other common metal ions. The probe consists of the fluorescent dye dimethylamino-naphthalene-1-sulfonamide (dansyl) conjugated to the amino terminus of a tetrapeptide. The design is based on the idea that the binding of Pb2+ would result in a conformational change in the peptide, causing the dansyl fluorophore to move to a new environment with a different polarity, which would lead to a shift in the emission spectrum. The researchers note that the generality of the design and the modular nature of the probe provide a framework for developing a second generation of probes that have a broad range of affinities for Pb2+. Future work will address the need for probes that bind lead significantly tighter and have better bioavailability, stability, and fluorescence properties. Other fluorescent dyes will also be tested. (J. Am. Chem. Soc. 2000, 122, 174–175)
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