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ANALYTICAL CURRENTS
Single-molecule probe Single-molecule detection, the ultimate goal of any analytical chemist, requires a special set of circumstances and conditions. David Adams and his co-workers from Columbia University introduce a new strategy to this unique art with the synthesis of a promising compound for single-molecule studies of structures, reactions, and interfacial processes. The molecule is a modified perylene with an amine group. By itself, it doesn’t fluoresce, but when the molecule binds protons, metals, TiO2 nanoparticles, or aldehydes, it becomes highly fluorescent. For example, the molecule was found to identify metal impurities in glass, protonated sites in quartz, and butylaldehyde acetal in a polyvinylbutyral matrix. The key to this molecule emerges from a molecular orbital diagram. In the unbound state, intramolecular electron transfer from a nonbonding electron pair quenches the molecule’s excited state and prevents fluorescence, but when the molecule is bound, the electron transfer is turned off and the fluorescence is turned on. This means that the molecule can be modified to change the selectivity and sensi-
Chips and carbohydrates: “tasty” combination Although carbohydrates and chips have long gone hand-inhand in the world of food, the two are just beginning to be paired in the world of microarrays. In one example of this early work, Sungjin Park and Injae Shin of Yonsei University (Korea) describe carbohydrate chips that might lead to high-throughput microarrays similar to those used for DNA or protein studies. The researchers fabricated short, maleimide-conjugated carbohydrate probes with lactose, cellobiose, maltose, or GlcNAc as model carbohydrates and with linkers of various lengths. A pin-type microarray spotter deposited the probes on glass slides coated with thiol groups. Reaction of the maleimides with the thiols formed stable thioether linkages to hold the carbohydrates in place. The largest array contained 12,000 spots, and the researchers note that the chips can be reused without detaching the probes. Probing the slides with fluorescein-labeled lectins verified that they bound specifically to their target carbohydrates, as they do in solution. Selectivity was even achieved for carbohydrates that differed in anomeric configuration, such as maltose and cellobiose. As expected, the lengths of the linkers affected the binding affinities. Lectin binding was weak for probes bearing the shortest linker, except at the highest concentration (5 mM); for longer linkers, strong binding affinity was achieved at 0.5 mM. (Angew. Chem., Int. Ed. 2002, 41, 3180–3182)
tivity for other applications. (J. Am. Chem. Soc. 2002, 124, 10,640–10,641)
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A molecular orbital diagram for the free single-molecule probe (left) illustrates how intramolecular electron transfer from a nonbonding electron pair (N–MO) to the highest occupied molecular orbital (HOMO) prevents fluorescence. Once the molecule binds to something (right), the N–MO drops in energy, and fluorescence occurs with excited electrons going from the lowest unoccupied molecular orbital (LUMO) to the HOMO.
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Lectin binding to carbohydrate microarrays is specific. As expected, (a) the lectin from Triticum vulgaris binds to GlcNAc but not lactose, (b) the lectin from Erythrina cristagalli binds to lactose but not GlcNAc, and (c) the lectin Concanavalin A binds more strongly to maltose than cellobiose. (Adapted with permission. Copyright 2002 Wiley-VCH Verlag GmbH.)
