A spectral chart method for estimating the mean turbulent kinetic energy dissipation rate

We present an empirical but simple and practical spectral chart method for determining the mean turbulent kinetic energy dissipation rate $ \left\langle \varepsilon \right\rangle $ in a variety of turbulent flows. The method relies on the validity of the first similarity hypothesis of Kolmogorov (C R (Doklady) Acad Sci R R SS, NS 30:301–305, 1941) (or K41) which implies that spectra of velocity fluctuations scale on the kinematic viscosity ν and $ \left\langle \varepsilon \right\rangle $ at large Reynolds numbers. However, the evidence, based on the DNS spectra, points to this scaling being also valid at small Reynolds numbers, provided effects due to inhomogeneities in the flow are negligible. The methods avoid the difficulty associated with estimating time or spatial derivatives of the velocity fluctuations. It also avoids using the second hypothesis of K41, which implies the existence of a ?5/3 inertial subrange only when the Taylor microscale Reynods number R _λ is sufficiently large. The method is in fact applied to the lower wavenumber end of the dissipative range thus avoiding most of the problems due to inadequate spatial resolution of the velocity sensors and noise associated with the higher wavenumber end of this range.The use of spectral data (30?≤?R _λ?≤?400) in both passive and active grid turbulence, a turbulent mixing layer and the turbulent wake of a circular cylinder indicates that the method is robust and should lead to reliable estimates of $ \left\langle \varepsilon \right\rangle $ in flows or flow regions where the first similarity hypothesis should hold; this would exclude, for example, the region near a wall.     

Probing the potential of N-heterocyclic carbenes in molecular electronics: redox-active metal centers interlinked by a rigid ditopic carbene ligand

Bimetallic homonuclear iron(II) and ruthenium(II) N-heterocyclic carbene complexes have been synthesized and crystallographically analyzed. As a spacer ligand for interconnecting the two redox-active metal centers, a ditopic carbene ligand has been used that comprises two carbene sites annelated to benzene. Detailed electrochemical and spectroelectrochemical analyses of the bimetallic systems revealed that despite the potentially pi-delocalized nature of the ditopic ligand, the iron centers are only moderately coupled. In the ruthenium complexes, the intermetallic interactions are very weak and the centers are electrochemically nearly independent. A model is proposed for rationalizing these observations which is based on (I) relatively weak charge delocalization in the spacer ligand and (II) on electrostatic factors governing the metal-carbene bond.     

Analytical method for authentication of Traditional Balsamic Vinegar of Modena

(13)C NMR spectroscopy was employed for evaluating glucose and fructose isoforms content in Traditional Balsamic Vinegar of Modena. With the use of reference spectra recorded in water for samples obtained with respect to set rules, a shift determination method for fructose carbon isoforms was introduced to determine the frauds present in unknown Traditional Balsamic Vinegar of Modena samples. No sample preparation yields this approach highly reliable and time saving. Following this approach, for the first time, an objective analytical technique can be used alternatively to the actual procedures for Traditional Balsamic Vinegar certification.     

Water Activity Regulates the Q(A)(-) to Q(B) electron transfer in photosynthetic reaction centers from Rhodobacter sphaeroides

We report on the effects of water activity and surrounding viscosity on electron transfer reactions taking place within a membrane protein: the reaction center (RC) from the photosynthetic bacterium Rhodobacter sphaeroides. We measured the kinetics of charge recombination between the primary photoxidized donor (P(+)) and the reduced quinone acceptors. Water activity (aW) and viscosity (eta) have been tuned by changing the concentration of cosolutes (trehalose, sucrose, glucose, and glycerol) and the temperature. The temperature dependence of the rate of charge recombination between the reduced primary quinone, Q(A)(-), and P(+) was found to be unaffected by the presence of cosolutes. At variance, the kinetics of charge recombination between the reduced secondary quinone (Q(B)(-)) and P(+) was found to be severely influenced by the presence of cosolutes and by the temperature. Results collected over a wide eta-range (2 orders of magnitude) demonstrate that the rate of P(+)Q(B)(-) recombination is uncorrelated to the solution viscosity. The kinetics of P(+)Q(B)(-) recombination depends on the P(+)Q(A)(-)Q(B) <--> P(+)Q(A)Q(B)(-) equilibrium constant. Accordingly, the dependence of the interquinone electron transfer equilibrium constant on T and aW has been explained by assuming that the transfer of one electron from Q(A)(-) to Q(B) is associated with the release of about three water molecules by the RC. This implies that the interquinone electron transfer involves at least two RC substates differing in the stoichiometry of interacting water molecules.     

Automated on-line column-switching HPLC-MS/MS method with peak focusing for measuring parabens, triclosan, and other environmental phenols in human milk

Parabens (esters of p-hydroxybenzoic acid) and triclosan are widely used as preservatives and antimicrobial agents, respectively, in personal care products, pharmaceuticals, and food processing. Because of their widespread use and potential risk to human health, assessing human exposure to these compounds in breastfed infants is of interest. We developed a sensitive method, using a unique on-line solid-phase extraction-high performance liquid chromatography-tandem mass spectrometry system with peak focusing feature, to measure in human milk the concentrations of five parabens (methyl-, ethyl-, propyl-, butyl-, and benzyl parabens), triclosan, and six other environmental phenols: bisphenol A (BPA); ortho-phenylphenol (OPP); 2,4-dichlorophenol; 2,5-dichlorophenol; 2,4,5-trichlorophenol; and 2-hydroxy-4-methoxybenzophenone (BP-3). The method, validated by use of breast milk pooled samples, shows good reproducibility (inter-day coefficient of variations ranging from 3.5% to 16.3%) and accuracy (spiked recoveries ranging from 84% to 119% at four spiking levels). The detection limits for most of the analytes are below 1 ng mL(-1) in 100 microL of milk. We tested the usefulness of the method by measuring the concentrations of these twelve compounds in four human milk samples. We detected methyl paraben, propyl paraben, triclosan, BPA, OPP, and BP-3 in some of the samples tested. The free species of these compounds appear to be the most prevalent in milk. Nevertheless, to demonstrate the utility of these measures for exposure and risk assessment purposes, additional data about sampling and storage of the milk, and on the stability of the analytes in milk, are needed.     

Ab initio Structure Determination of [(Dimethylamino)methylene]bis[phosphonic Acid] Dihydrate from X-Ray Powder Diffraction Data: Comparison with the Corresponding Monohydrate and Unhydrated Form

The structure of [(dimethylamino)methylene]bis[phosphonic acid] dihydrate (C₃H₁₁NO₆P₂⋅2 H₂O; 1 ) was solved ab initio from synchrotron powder X-ray diffraction data. The structure determination was based on direct methods combined with difference Fourier techniques, and the refinement was carried out using the Rietveld method. Using this high-quality diffraction pattern, it was possible to index a second phase which corresponds to the structure of the known [(dimethylamino)methylene]bis[phosphonic acid] monohydrate ( 2 ). [(Dimethylamino)methylene]bis[phosphonic acid] dihydrate ( 1 ) is monoclinic, space group P2₁/c, Z=4, with a=10.6644(1), b=9.1599(1), c=10.5213(1) Å, and β=98.353(1)°. The structure analysis indicates two non-equivalent P-atoms in the molecule of 1 which are also observed in the corresponding monohydrate 2 and unhydrated form 3 . All three compounds exhibit extended H-bonding networks which result in remarkably different ³¹P-NMR spectra. The [(dimethylamino)methylene]bis[phosphonic acids] 1 – 3 crystallize in the betaine-type structure which, therefore, contains two nonequivalent P-atoms. The −P(=O)(OH)₂ and −P(=O)(OH)O⁻ groups of 1 – 3 are involved in a number of strong H-bonds which can be characterized by the different ³¹P-NMR chemical shifts of the two P-atoms of 1 – 3 .     

Development and validation of an HPLC method for the determination of seven penicillin antibiotics in veterinary drugs and bovine blood plasma

Herein a quantitative method for the determination of seven penicillins in bovine plasma and veterinary drugs has been developed. Amoxicillin (AMO), ampicillin (AMP), penicillin G (PENG), penicillin V (PENV), oxacillin (OXA), cloxacillin (CLO) and dicloxacillin (DICLO) were separated on a Perfectsil ODS‐2 (250×4 mm, 5 μm) column, using gradient elution, with a mobile phase of 0.1% v/v TFA and ACN–methanol (90:10 v/v). PDA detection was used at 240 nm. Penicillins were isolated from bovine plasma by SPE on Lichrolut RP‐18 cartridges with mean recoveries from 85.7 to 113.5%. Colchicine (3 ng/μL) was used as an internal standard. The developed method was validated in terms of selectivity, linearity, accuracy, precision, stability and sensitivity. Repeatability (n = 5) and between‐day precision (n = 5) revealed RSD < 12%. The detection limits in the bovine plasma were estimated as 18 ng for AMO and AMP, 25 for PENG, PENV and OXA, 3 ng for CLO and 12 ng for DICLO. Spiked plasma samples were stable for 1 wk, except for AMP and CLO, which were stable for 3 wk and OXA for 4 wk. AMO, PENG and PENV were stable for two freeze–thaw cycles, OXA, CLO and DICLO for four, while AMP only for one.     

Photooxidations of 2-(γ,ε-dihydroxyalkyl) furans in water: synthesis of DE-bicycles of the pectenotoxins

Photooxygenations of 2-(γ,ε-dihydroxyalkyl) furans in H(2)O followed by in situ reduction and ketalization affords, in one synthetic operation, DE-bicyclic ketals of the pectenotoxins.