In-lens cryo-high resolution scanning electron microscopy: methodologies for molecular imaging of self-assembled organic hydrogels.

The micro- and nanoarchitectures of water-swollen hydrogels were routinely analyzed in three dimensions at very high resolution by two cryopreparation methods that provide stable low-temperature specimens for in-lens high magnification recordings. Gemini surfactants (gS), poly-N-isopropylacrylamides (p-NIP Am), and elastin-mimetic di- (db-E) and triblock (tb-E) copolymer proteins that form hydrogels have been routinely analyzed to the sub-10-nm level in a single day. After they were quench or high pressure frozen, samples in bulk planchets were subsequently chromium coated and observed at low temperature in an in-lens field emission SEM. Pre-equilibrated planchets (4-40 degrees C) that hold 5-10 microl of hydrogel facilitate dynamic morphological studies above and below their transition temperatures. Rapidly frozen samples were fractured under liquid nitrogen, low-temperature metal coated, and observed in-lens to assess the dispersion characteristics of micelles and fragile colloidal assemblies within bulk frozen water. Utilizing the same planchet freezing system, the cryoetch-HRSEM technique removed bulk frozen water from the hydrogel matrix by low-temperature, high-vacuum sublimation. The remaining frozen solid-state sample faithfully represented the hydrogel matrix. Cryo- and cryoetch-HRSEM provided vast vistas of hydrogels at low and intermediate magnifications whereas high magnification recordings and anaglyphs (stereo images) provided a three-dimensional prospective and measurements on a molecular level.     

New stereoselective syntheses of cis- and trans-2-methyl-4-arylpiperidines

A stereoselective approach has been developed for the synthesis of cis- and trans-2-methyl-4-arylpiperidines from a common intermediate. The Ni-catalyzed hydrogenolysis of N-Boc-2-methyl-4-aryl-4-piperidinols, obtained by addition of organometallic reagents on N-Boc-2-methyl-4-piperidone, afforded the trans derivatives with up to 95% selectivity whereas the corresponding cis isomers were obtained in the presence of palladium catalysts.     

Zirconium-mediated conversion of homoallylic ethers into cyclopropane derivatives

Homoallylic ethers react with Cp₂ZrCl₂/2 n-BuLi reagent to afford cyclopropane derivatives. Cyclopropylcarbinyl-homoallyl rearrangements involving zirconium species are observed depending on the substrate structure.     

Short metal-metal separations in a highly luminescent trimetallic Ag(I) complex stabilized by bridging NHC ligands

Reaction of 1,3-bis(2-pyridinylmethyl)-1H-imidazolium salt, [H(pyCH(2))(2)im]X (X = BF(4)(-) or Cl(-)), with silver oxide in acetonitrile readily yields yellow-brown [((pyCH(2))(2)im)(2)Ag]X, 1.BF(4) or 1.Cl. The chloride salt crystallizes with 3.650 A intermolecular Ag...Ag interactions while 1.BF(4) shows no short intermolecular interaction. Addition of excess Ag(BF(4)) produces the homoleptic carbene bridged trimetallic species, [(mu-NHC)(3)Ag(3)](BF(4))(3), 2. This species contains very short Ag-Ag separations between 2.7249(10) and 2.7718(9) A. In solution, these complexes are photoluminescent.     

Redox modulation of benzene triimides and diimides via noncovalent interactions

[reaction: see text] Mellitic triimides undergo three sequential one-electron reduction processes whose potentials are significantly lowered in the presence of alkyl thioureas. The two sequential reductions of benzene diimides are similarly stabilized. Calculation of the relative free energy change between the different electronic states of the imide acceptors and their corresponding alkyl thiourea complexes indicates dramatic increases in hydrogen bond strength with increasing acceptor charge density.     

Synthesis and characterization of mannosyl mimetic derivatives based on a beta-cyclodextrin core

The synthesis of branched beta-cyclodextrins substituted with mannosyl mimetic derivatives at one primary hydroxy group is described. It was shown that the self-inclusion phenomenon observed for the target compounds in water did not preclude the inclusion properties of the cyclodextrin moiety.     

Electron ionization mass spectra and their reproducibility for trialkylsilylated derivatives of organic acids, sugars and alcohols

Mass spectra of trialkylsilyl derivatives of fatty acids, dicarboxylic acids, hydroxyacids, oxoacids, sugars, amino acids and alcohols were obtained. Amino acids were analyzed as tert-butyldimethylsilyl derivatives; all other model compounds were analyzed as trimethylsilyl derivatives. Reproducibility of the electron ionization (EI) mass spectra for the derivatives obtained was discussed. It was shown that, for many investigated derivatives, composition of the respective mass spectra depended greatly on ion source contamination. The trimethylsilylated alpha-tocopherol mass spectrum composition was most significantly influenced by ion source contamination. This compound can be used to test ion source contamination.     

Validation of a method for arsenic speciation in food by ion chromatography-inductively coupled plasma/mass spectrometry after ultrasonic-assisted enzymatic extraction

A fully validated and rapid quantitative method is presented for determination of inorganic arsenic [arsenite, As(III) and arsenate, As(V)] and organic arsenic species (methylarsonic acid, dimethylarsinic acid, and arsenobetaine) by ion chromatography paired with inductively coupled plasma/MS after ultrasonic-assisted enzymatic extraction (UAEE) in rice- and seafood-based raw materials and finished products. This method gives toxicological meaning to arsenic analysis, since the sum of the toxic chemical forms As(III) and As(V) can be determined. In contrast to classical water-methanol extraction, UAEE enables drastic acceleration of sample extraction (5 min instead of several hours), while total arsenic extraction efficiency is improved without species conversion. Validation was performed to evaluate the method for selectivity, linearity, LOD/LOQ (0.007-0.020 mg/kg), trueness, precision (HorRat values, 0.2-0.6), recovery (93-122%), and uncertainty. The method was also satisfactorily tested using two proficiency tests. Performance characteristics are reported for four certified reference materials, standard reference material (SRM) 1568a (rice flour), Institute for Reference Materials and Measurements 804 (rice flour), SRM 2976 (mussel tissue), certified reference material-627 (tuna fish), and several commercial food samples populating five AOAC triangle food sectors. The results indicated that this speciation method is cost-efficient, time-saving, and accurate, as well as fit-for-purpose, according to International Organization for Standardization/International Electrotechnical Commission 17025:2005 standard, and could be used for routine analysis.     

A short access to the macrocyclic core of cycloviracin and glucolipsin

The macrocyclic core of cycloviracin and glucolipsin has been synthesised in ten steps from levoglucosan and (S)-(−)-dimethyl malate. The limited number of steps to obtain this macrolide makes it a valuable procedure for the synthesis of analogues of cycloviracin and glucolipsin.