Ochratoxin A degrading enzymes of Stenotrophomonassp. 043-1a.

Ochratoxin A is a secondary metabolite that acts as a mycotoxin and is produced by Aspergillus, Penicillium, and other fungal species. It is a threat to animal and human health due to nephrotoxic, carcinogenic, and genotoxic properties and its widespread incidence in agricultural products. To reduce this threat, biological remediation processes are of growing interest. The aerobic gram-negative bacterium Stenotrophomonassp. 043-1a, isolated from soil, was previously shown to degrade ochratoxin A into the non-toxic ochratoxin α and l-phenylalanine (Schatzmayr et al. 2002). However, the enzyme or enzymes catalyzing this reaction in this strain remained elusive. Here, we report the targeted purification of Stenotrophomonassp. 043-1a lysate via ammonium sulfate precipitation, size-exclusion chromatography, and hydrophobic interaction chromatography to identify the ochratoxin A degrading enzymes by subsequent peptide fragment fingerprinting. The metallo-dependent hydrolase Chr1_3858681_3267 and a member of the peptidase S9 family, Chr1_3858681_771, were shown to degrade ochratoxin A. This was, to our knowledge, the first report of an ochratoxin A degrading enzyme from the peptidase S9 family.

Diameter-dependent single- and double-file stacking of squaraine dye molecules inside chirality-sorted single-wall carbon nanotubes.

The filling of single-wall carbon nanotubes (SWCNTs) with dye molecules has become a novel path to add new functionalities through the mutual interaction of confined dyes and host SWCNTs. In particular cases, the encapsulated dye molecules form strongly interacting molecular arrays and these result in severely altered optical properties of the dye molecules. Here, we present the encapsulation of a squaraine dye inside semiconducting chirality-sorted SWCNTs with diameters ranging from ∼1.15 nm, in which the dye molecules can only be encapsulated in a single-file molecular arrangement, up to ∼1.5 nm, in which two or three molecular files can fit side-by-side. Through the chirality-selective observation of energy transfer from the dye molecules to the surrounding SWCNTs, we find that the absorption wavelength of the dye follows a peculiar SWCNT diameter dependence, originating from the specific stacking of the dye inside the host SWCNTs. Corroborated by a theoretical model, we find that for each SWCNT diameter, the dye molecules adopt a close packing geometry, resulting in tunable optical properties of the hybrid when selecting a specific SWCNT chirality.

Ag-catalyzed diastereo- and enantioselective vinylogous Mannich reactions of alpha-ketoimine esters. Development of a method and investigation of its mechanism.

An efficient diastereo- and enantioselective Ag-catalyzed method for additions of a commercially available siloxyfuran to alpha-ketoimine esters is disclosed. Catalytic transformations require an inexpensive metal salt (AgOAc) and an air stable chiral ligand that is prepared in three steps from commercially available materials in 42% overall yield. Aryl- as well as heterocyclic substituted ketoimines can be used effectively in the Ag-catalyzed process. Additionally, two examples regarding reactions of alkyl-substituted ketoimines are presented. An electronically modified N-aryl group is introduced that is responsible for high reaction efficiency (>98% conversion, 72-95% yields after purification) as well as diastereo- (up to >98:2 dr) and enantioselectivity (up to 97:3 er or 94% ee). The new N-aryl unit is crucial for conversion of the asymmetric vinylogous Mannich (AVM) products to the unprotected amines in high yields. Spectroscopic and X-ray data are among the physical evidence provided that shed light on the identity of the Ag-based chiral catalysts and some of the mechanistic subtleties of this class of enantioselective C-C bond forming processes.

Al-catalyzed enantioselective alkylation of alpha-ketoesters by dialkylzinc reagents. enhancement of enantioselectivity and reactivity by an achiral Lewis base additive.

An Al-catalyzed enantioselective method for additions of Me(2)Zn and Et(2)Zn to alpha-ketoesters bearing aromatic, alkenyl, and alkyl substituents is disclosed. Transformations are promoted in the presence of a readily available amino acid-based ligand and afford the desired products in excellent yields and in up to 95% ee. Investigations described illustrate that the presence of a Lewis basic additive can lead to significant enhancements in efficiency and enantioselectivity. A mechanistic model that provides a rationale for such effects is provided.

A simple and efficient chemoselective method for the catalytic deprotection of acetals and ketals using bismuth triflate.

Bismuth triflate is a highly efficient catalyst (0.1-1 mol %) for the deprotection of acetals and ketals. The procedure is very facile and selective for acetals derived from ketones and conjugated aldehydes. tert-Butyldimethylsilyl ethers are stable to the reaction conditions. The highly catalytic nature of bismuth triflate and the use of a relatively nontoxic solvent system (THF/H(2)O) make this procedure particularly attractive for large-scale synthesis.