This work describes the identification of an aptamer that binds with high affinity and specificity to ochratoxin A (OTA), a mycotoxin that occurs in wheat and other foodstuffs, and a quantitative detection method for OTA based on the use of this aptamer. Aptamers are single-stranded oligonucleotides selected in vitro to bind to molecular targets. The aptamer selected in this work exhibited a dissociation constant in the nanomolar range and did not bind compounds with structures similar to OTA such as N-acetylphenylalanine or warfarin. The aptamer bound with a 100-fold less affinity to ochratoxin B. The selected aptamers could be used for the determination of ppb quantities of OTA in naturally contaminated wheat samples. Further work is ongoing to broaden the application demonstrated here with the development of sensors, affinity columns, and other analytical systems for field and laboratory determination of this toxin in food and agricultural products.

Bax promotes cell death by permeabilizing mitochondrial outer membranes by an unresolved mechanism. However, in cells lacking the gene c-myc, membrane permeabilization by Bax is blocked by changes in the mitochondria that prevent Bax oligomerization. Drug-treated c-myc null cells and cells expressing Myc were used to map the topology of Bax in membranes prior to and after mitochondrial permeabilization. Chemical labeling of single cysteine mutants of Bax using a membrane bilayer impermeant cysteine-specific modifying agent revealed that Bax inserted both the 'pore domain' (helices alpha5-alpha6), and the tail-anchor (helix alpha9) into membranes prior to oligomerization and membrane permeabilization. Additional topology changes for Bax were not required in Myc-expressing cells to promote oligomerization and cytochrome c release. Our results suggest that unlike most pore-forming proteins, Bax membrane permeabilization results from oligomerization of transmembrane monomers rather than concerted insertion of the pore domains of a preformed oligomer.

The conversion of an aptamer-target binding event into a detectable signal is an important step in the development of aptamer-based sensors. In this work, we show that the displacement of a fluorescently labeled oligo from the aptamer by the target can be detected by fluorescence polarization (FP). We used Ochratoxin A (OTA), a small organic molecule (MW = 403) as a case study. A detection limit of 5 nM OTA was achieved. The method presented here provides an advantage over fluorophore-quenching systems and other steady-state fluorescence approaches in that no modification of the aptamer or the target is required. Additionally, the signal is produced by the displacement event itself, so no further aggregation or conformational events have to be considered. This analytical method is particularly useful for small targets, as for large targets a direct measurement of the FP change of a labeled aptamer upon binding can be used to determine the concentration of the target. The results presented here demonstrate that aptamers and inexpensive labeled oligos can be used for rapid, sensitive, and specific determination of small molecules by means of FP.

Firefly luciferase (FL) was entrapped in sol-gel-derived silica containing precursors based on covalent linkage of d-gluconolactone or d-maltonolactone to (aminopropyl)triethoxysilane to form N-(3-triethoxysilylpropyl)gluconamide or N-(3-triethoxysilylpropyl)maltonamide. The enzyme was active and stable in this material and showed catalytic constants close to those in solution. As little as 20 amol ATP could be detected with the entrapped FL, and the entrapped enzyme could be used over several cycles.

Silica derived from biocompatible silane precursors and containing covalently bound sugar moieties has recently been reported to be a much more biocompatible matrix for protein entrapment than any previously synthesized materials. To better understand the nature of these new materials, the steady-state and time-resolved fluorescence of human serum albumin (HSA) was used to examine the conformation, dynamics, accessibility, thermal stability, and degree of ligand binding after entrapment of the protein into sol−gel-processed glasses derived from either tetraethyl orthosilicate (TEOS) or diglycerylsilane (DGS), which in some cases contained covalently bound gluconamidylsilane (GLS) moieties. It was observed that the initial conformation, accessibility to external analytes, thermal stability, long-term stability, and degree of ligand binding to HSA were best in DGS-derived materials that contained covalently tethered GLS relative to unmodified DGS-derived materials, TEOS, or TEOS/GLS-derived materials. Measurement of protein rotational dynamics showed that entrapment led to an immediate loss of global motion in all materials. However, the restriction of motion was most dramatic in GLS-doped materials, suggesting preferential interactions of the protein with the sugar-coated surfaces. As aging proceeded, both protein dynamics and the degree of ligand binding decreased, with a gradual loss of segmental motion and a significant increase in local motion in the vicinity of the probe, consistent with unfolding and surface adsorption of the protein, leading to loss of function. Overall, our findings suggest that the use of a biocompatible precursor (DGS) and the addition of a covalently bound sugar both contribute to improved protein performance. However, of these two the use of a biocompatible precursor is the most important factor, and in such cases addition of sugars further improves protein performance. In contrast, the use of the sugar-based additive with a nonbiocompatible precursor such as TEOS imparted essentially no benefit, demonstrating the importance of biocompatible processing conditions.