The excessive use of antibiotics in food-producing animals causes a steady rise of multiple antibiotic resistance in foodborne bacteria. Next to sulfonamides, the most common antibiotics groups are fluoroquinolones, aminoglycosides, and ß-lactams. Therefore, there is a need for a quick, efficient, and low-cost detection procedure for antibiotics. In this study, we propose an inkjet-printed aptamer-based biosensor developed for the detection of the fluoroquinolone ciprofloxacin. Due to their extraordinary high affinity and specificity, aptamers are already widely used in various applications. Here we present a ciprofloxacin-binding RNA aptamer developed by systematic evolution of ligands by exponential enrichment (SELEX). We characterized the secondary structure of the aptamer and determined the KD to 36 nM that allow detection of antibiotic contamination in a relevant range. We demonstrate that RNA aptamers can be inkjet-printed, dried, and resolved while keeping their functionality consistently intact. With this proof of concept, we are paving the way for a potential range of additional aptamer-based, printable biosensors.

RNAs play major roles in the regulation of gene expression. Hence, designer RNA molecules are increasingly explored as regulatory switches in synthetic biology. Among these, the TetR-binding RNA aptamer was selected by its ability to compete with operator DNA for binding to the bacterial repressor TetR. A fortuitous finding was that induction of TetR by tetracycline abolishes both RNA aptamer and operator DNA binding in TetR. This enabled numerous applications exploiting both the specificity of the RNA aptamer and the efficient gene repressor properties of TetR. Here, we present the crystal structure of the TetR-RNA aptamer complex at 2.7 Å resolution together with a comprehensive characterization of the TetR–RNA aptamer versus TetR–operator DNA interaction using site-directed mutagenesis, size exclusion chromatography, electrophoretic mobility shift assays and isothermal titration calorimetry. The fold of the RNA aptamer bears no resemblance to regular B-DNA, and neither does the thermodynamic characterization of the complex formation reaction. Nevertheless, the functional aptamer-binding epitope of TetR is fully contained within its DNA-binding epitope. In the RNA aptamer complex, TetR adopts the well-characterized DNA-binding-competent conformation of TetR, thus revealing how the synthetic TetR-binding aptamer strikes the chords of the bimodal allosteric behaviour of TetR to function as a synthetic regulator.

structure structure (solids and liquids) D 2000 -002Crystal Structures of Two Polymorphs of Ca 3 [Al 2 N 4 ].-Green crystals of α-Ca 3 [Al 2 N 4 ] are obtained by reacting mixtures of the metals with N 2 at temperatures above 1000 • C. β-Ca 3 [Al 2 N 4 ] is formed as a byproduct in a reaction of Ca with Al under N 2 at 930 • C. As revealed by single crystal XRD, α-Ca 3 [Al 2 N 4 ] crystallizes in the monoclinic space group P2 1 /c with Z = 4 and β-Ca 3 [Al 2 N 4 ] in the monoclinic space group C2/c with Z = 4. The crystal structures consist of layers of edge-and vertex-sharing AlN 4 tetrahedra, leading to different layered anionic partial structures of the two polymorphs. -(LUDWIG, M.; JAEGER, J.; NIEWA, R.; KNIEP, R.; Inorg. Chem. 39 (2000) 26, 5909-5911; Eduard-Zintl-Inst., TH Darmstadt, D-64289 Darmstadt, Germany; EN)

Nitridocuprates(I): Quaternary Phases in the Systems Li-Ca/Sr-Cu-N. -Attempts to synthesize nitridocuprates(I) in the quaternary title systems result in the phases (I)-(III). (I) and (II) are obtained by heating mixtures of Li, Sr and Cu3N under N2 at 800 • C (7:3:1, Ta crucible, 48 h) whereas (III) is prepared by reaction of the metals with N2 at 920 • C (5:1:1, Ta crucible, 16 h). Single crystal data show the quaternary nitridocuprates(I) to be characterized by structural units, in which the (Li1-xCux) substitutional positions are two-gold (linear) coordinated to nitrogen. (I) crystallizes in the tetragonal system (I41/amd, Z = 4) and is isotypic to Li4SrN2. (II) belongs to the orthorhombic space group Pnma with Z = 12 (isotype of Ba(NiN)) and (III) crystallizes in the Sr2(Li(CoN2)) structure (P42/ mnm, Z = 2). The bond length Cu(I)-N in 2-fold (linear) coordinated Cu( I) in nitrido compounds is estimated to be 184.5 pm. -(JAEGER, J.; KNIEP, R.; Z. Naturforsch., B: Chem. Sci. 47 (1992) 9, 1290-1296; Eduard-Zintl-Inst., TH Darmstadt, W-6100 Darmstadt, Germany; DE)