Gas analyzer for continuous monitoring of trace level methanethiol by microchannel collection and fluorescence detection

The highly odorous compound methanethiol, CH₃SH, is commonly produced in biodegradation of biomass and industrial processes, and is classed as 2000 times more odorous than NH₃. However, there is no simple analytical method for detecting low parts-per-billion in volume ratio (ppbv) levels of CH₃SH. In this study, a micro gas analysis system (μGAS) was developed for continuous or near real time measurement of CH₃SH at ppbv levels. In addition to a commercial fluorescence detector, a miniature high sensitivity fluorescence detector was developed using a novel micro-photomultiplier tube device. CH₃SH was collected by absorption into an alkaline solution in a honeycomb-patterned microchannel scrubber and then mixed with the fluorescent reagent, 4-(N,N-dimethylaminosulfonyl)-7-fluoro-2,1,3-benzoxadiazole (DBD-F). Gaseous CH₃SH was measured without serious interference from other sulfur compounds or amines. The limits of detection were 0.2 ppbv with the commercial detector and 0.3 ppbv with the miniature detector. CH₃SH produced from a pulping process was monitored with the μGAS system and the data agreed well with those obtained by collection with a silica gel tube followed by thermal desorption–gas chromatography–mass spectrometry. The portable system with the miniature fluorescence detector was used to monitor CH₃SH levels in near-real time in a stockyard and it was shown that the major odor component, CH₃SH, presented and its concentration varied dynamically with time.     

Enantioselective hydrogenation of α-phenylcinnamic acids over cinchonidine-modified Pd/C commercial catalysts

Enantioselective hydrogenation of α-phenylcinnamic acid (PCA) and p,p′-dimethoxyphenylcinnamic acid (DMPCA) was studied over a variety of commercial 5 % Pd/C catalysts to reveal catalyst properties suitable for obtaining high enantioselectivity. The catalysts were characterized by CO adsorption, X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). It is confirmed that pretreatment at 353 K under atmospheric pressure of H₂ before modification with cinchonidine is very effective for all the Pd/C catalysts used here to improve the selectivity and reaction rate. It is suggested that the distribution of Pd metal particles is crucial to attain high selectivity (ee% = 79 ± 1 for PCA, 89 ± 2 for DMPCA): a uniform or eggshell-type distribution of Pd is more suitable than an egg-white or egg-yolk-type distribution. It is also suggested that the dispersion of Pd metal particles controls the enantioselectivity over cinchonidine (CD)-modified Pd/C catalysts. XPS techniques are proposed to provide a convenient method to find desirable catalysts. The choice of such Pd/C catalysts could facilitate high-throughput guided study on highly enantioselective hydrogenation of α,β-unsaturated carboxylic acids.     

Interfacing collective atomic excitations and single photons

We study the performance and limitations of a coherent interface between collective atomic states and single photons. A quantized spin-wave excitation of an atomic sample inside an optical resonator is prepared probabilistically, stored, and adiabatically converted on demand into a sub-Poissonian photonic excitation of the resonator mode. The measured peak single-quantum conversion efficiency of chi=0.84(11) and its dependence on various parameters are well described by a simple model of the mode geometry and multilevel atomic structure, pointing the way towards implementing high-performance stationary single-photon sources.     

Highly Fluorescent Flavins: Rational Molecular Design for Quenching Protection Based on Repulsive and Attractive Control of Molecular Alignment

Unprecedented intense fluorescent emission was observed for a variety of flavin compounds bearing a perpendicular cyclic imide moiety at the C7 position of an isoalloxazine platform. A series of alloxan‐substituted flavins was prepared selectively by reduction of the corresponding N‐aryl‐2‐nitro‐5‐alkoxyanilines with zinc dust and subsequent reaction with alloxan monohydrate in the presence of boric acid. Analogues bearing oxazolidine‐2,4‐dione functionality were obtained on methylation of the alloxan‐substituted flavins with methyl iodide and subsequent rearrangement in the presence of an inorganic base. The flavin compounds exhibit intense white‐green fluorescent emission in the solution state under UV excitation at 298 K, with emission efficiencies Φ_{298 K} greater than 0.55 in CH₃CN, which are higher than the values for all reported flavin compounds under similar conditions. The highest Φ_{298 K} value of 0.70 was obtained in CH₃CN for isoalloxazine bearing C7‐alloxan and N10‐2,6‐diisopropylphenyl groups. The temperature dependence of the emission intensities indicates that the pronounced emission properties at 298 K are attributable to the highly heat resistant properties towards emission decay with increasing temperature. Mechanistic studies, including X‐ray diffraction analysis, revealed that the good emission properties and high heat resistance of the alloxan‐substituted flavins are due to a synergetic effect of the associative nature of the C7‐alloxan unit and the repulsive nature of the perpendicular bulky substituents at the C7 and N10 positions.     

Biosynthesis of the N–N-Bond-Containing Compound l-Alanosine

The formation of a N−N bond is a unique biochemical transformation, and nature employs diverse biosynthetic strategies to activate nitrogen for bond formation. Among molecules that contain a N−N bond, biosynthetic routes to diazeniumdiolates remain enigmatic. We here report the biosynthetic pathway for the diazeniumdiolate-containing amino acid l -alanosine. Our work reveals that the two nitrogen atoms in the diazeniumdiolate of l -alanosine arise from glutamic acid and aspartic acid, and we clarify the early steps of the biosynthetic pathway by using both in vitro and in vivo approaches. Our work demonstrates a peptidyl-carrier-protein-based mechanism for activation of the precursor l -diaminopropionate, and we also show that nitric oxide can participate in non-enzymatic diazeniumdiolate formation. Furthermore, we demonstrate that the gene alnA, which encodes a fusion protein with an N-terminal cupin domain and a C-terminal AraC-like DNA-binding domain, is required for alanosine biosynthesis.     

Calorimetric study on coordination of tridentate imidazolyl calix[6]arene ligands to zinc ion in organic solvents

Complexation of three kinds of tris(imidazolyl)calix[6]arenes containing alternate p-substituents (Calix-tBu, R(1) = R(2) = tert-butyl; Calix-NH(2), R(1) = tert-butyl, R(2) = NH(2); Calix-NO(2), R(1) = tert-butyl, R(2) = NO(2)) with Zn(ClO(4))(2)(H(2)O)(6) in acetonitrile, methanol, and THF was investigated via isothermal titration calorimetry (ITC). For the coordination of these calixarene ligands to Zn(II) in acetonitrile, typical one-phase exothermic titration curves were obtained, indicating the formation of 1:1 ligand-Zn(II) complexes accompanied by large conformational changes of the ligands. In contrast, the complexation in methanol was endothermic and dominated by favorable entropy changes. The entropy gains were achieved by extensive desolvation from both Zn(II) and the ligands. ITC measurements suggest a 2:1 ligand-Zn(II) complex formation in THF in the presence of excess ligands (Calix-NH(2) and Calix-NO(2)). The 2:1 complexes were converted to 1:1 complexes upon further addition of Zn(ClO(4))(2)(H(2)O)(6). The results indicate the important role of a coordinating solvent (acetonitrile) for direct formation of the 1:1 complexes under the conditions of excess ligand. Complexation of a ditopic ligand (Calix-Tri) with three triazole moieties on the wider rim was also studied via ITC. The first coordination of the imidazole moieties to Zn(II) was an exothermic process. This was followed by the entropically favorable coordination of the triazole moieties to the divalent cation. We have also investigated exchange of the fourth ligand (H(2)O) of the Zn(II) complex of Calix-NH(2) with butylamine, heptylamine, acetonitrile, and acetamide in a noncompetitive solvent, THF. The ΔH(0) tended to decrease upon increasing the electron-pair-donating ability of the guest ligand, whereas it was also affected by an entropic term due to restricted rotation of the guest ligand inside the calixarene cavity.     

Micelle formation of nonionic surfactants in a room temperature ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate: surfactant chain length dependence of the critical micelle concentration

Micellization behavior was investigated for polyoxyethylene-type nonionic surfactants with varying chain length (C(n)E(m)) in a room temperature ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF(4)). Critical micelle concentration (cmc) was determined from the variation of (1)H NMR chemical shift with the surfactant concentration. The logarithmic value of cmc decreased linearly with the number of carbon atoms in the surfactant hydrocarbon chain, similarly to the case observed in aqueous surfactant solutions. However, the slope of the straight line is much smaller in bmimBF(4) than in aqueous solution. Thermodynamic parameters for micelle formation estimated from the temperature dependence of cmc showed that the micellization in bmimBF(4) is an entropy-driven process around room temperature. This behavior is also similar to the case in aqueous solution. However, the magnitude of the entropic contribution to the overall micellization free energy in bmimBF(4) is much smaller compared with that in aqueous solution. These results suggest that the micellization in bmimBF(4) proceeds through a mechanism similar to the hydrophobic interaction in aqueous surfactant solutions, although the solvophobic effect in bmimBF(4) is much weaker than the hydrophobic effect.     

Benzylic-acetoxylation of alkylbenzenes with PhI(OAc)2 in the presence of catalytic amounts of TsNH2 and I2

Treatment of alkylbenzenes with (diacetoxyiodo)benzene in the presence of catalytic amounts of p-toluenesulfonamide or p-nitrobenzenesulfonamide, and molecular iodine in 1,2-dichloroethane at 60 °C gave the corresponding (α-acetoxy)alkylbenzenes in good to moderate yields. The present reaction is a simple method for the introduction of an acetoxy group to the benzylic position of alkylbenzenes.     

Palladium-catalyzed asymmetric synthesis of 2-pyrrolidinones with a quaternary carbon stereocenter

A palladium-catalyzed asymmetric synthesis of 2-pyrrolidinones with a quaternary stereocenter at the 3-position has been achieved by the reaction of γ-methylidene-δ-valerolactones with alkyl isocyanates. High enantioselectivity has been realized by employing a newly synthesized chiral phosphoramidite ligand.