A high performance liquid chromatographic method for the determination of ethylenethiourea in urine and on filters

Urine samples are evaporated and pretreated with silica gel and alumina. Filters undergo ultrasonic treatment in water. Ethylenethiourea (ETU) is then determined by reversed-phase high-performance liquid chromatography with detection at 230 nm. The detection limit for ETU is 0.1 ng per injection and linear response is found for the range 0.3–110 ng; 0.2 μg 1^?1 ETU can be detected in urine. Recoveries from spiked filters (7 μg ETU/filter) varied from 79 to 94%. The methods are sensitive enough for application in occupational hygiene work.     

A density functional theory (DFT) and time-dependent density functional theory (TDDFT) study on optical transitions in oligo(p-phenylenevinylene)-fullerene dyads and the applicability to resonant energy transfer

The optical transitions of three different size oligo(p-phenylenevinylene)-fullerene dyads (OPV(n)-MPC(60); n = 2-4) and of the corresponding separate molecules are studied using density functional theory (DFT) and time-dependent density functional theory. The DFT is used to determine the geometries and the electronic structures of the ground states. Transition energies and excited-state structures are obtained from the TDDFT calculations. Resonant energy transfer from OPV(n) to MPC(60) is also studied and the Fermi golden rule is used, along with two simple models to describe the electronic coupling to calculate the energy transfer rates. The hybrid-type PBE0 functional is used with a split-valence basis set augmented with a polarization function (SV(P)) in calculations and the calculated results are compared to the corresponding experimental results. The calculated PBE0 spectra of the OPV(n)-MPC(60) dyads correspond to the experimental spectra very well and are approximately sums of the absorption spectra of the separate OPV(n) and MPC(60) molecules. Also, the absorption energies of OPV(n) and MPC(60) and the emission energies of OPV(n) are predicted well with the PBE0 functional. The PBE0 calculated resonant energy transfer rates are in a good agreement with the experimental rates and show the existence of many possible pathways for energy transfer from the first excited singlet states of the OPV(n) molecules to the MPC(60) molecule.     

van der Waals interactions are critical in Car–Parrinello molecular dynamics simulations of porphyrin–fullerene dyads

The interplay between electrostatic and van der Waals (vdW) interactions in porphyrin‐C₆₀ dyads is still under debate despite its importance in influencing the structural characteristics of such complexes considered for various applications in molecular photovoltaics. In this article, we sample the conformational space of a porphyrin‐C₆₀ dyad using Car–Parrinello molecular dynamics simulations with and without empirical vdW corrections. Long‐range vdW interactions, which are poorly described by the commonly used density functional theory functionals, prove to be essential for a proper dynamics of the dyad moieties. Inclusion of vdW corrections brings porphyrin and C₆₀ close together in an orientation that is in agreement with experimental observations. The structural differences arising from the vdW corrections are shown to be significant for several properties and potentially less important for others. Additionally, our Mulliken population analysis reveals that contrary to the common belief, porphyrin is not the primary electron donating moiety for C₆₀. In the considered dyad, fullerene's affinity for electrons is primarily satisfied by charge transfer from the amide group of the linker. However, we show that in the absence of another suitable bound donor, C₆₀ can withdraw electrons from porphyrin if it is sufficiently close. © 2015 Wiley Periodicals, Inc.     

Suppressor current switching: a simple and effective means to reduce background noise in ion chromatography

Background noise in ion chromatography with suppressed conductivity detection was significantly reduced for the period of time when the electric current to an anion self regenerating suppressor (ASRS) running in the recycle mode was turned-off. With high capacity AS11-HC columns, it was possible to maintain current free conditions from the beginning of the run past the chloride peak, which enables routine high sensitivity analysis of early to mid eluting peaks. This suppressor current switching was utilized for the analysis of bromate in drinking water with large volume injection using on-line removal of chloride by an On-Guard Ag+-cartridge. The method detection limit (MDL) was 0.21 microg/l in fortified reagent water. Coelution of bromate with an unknown compound was observed, but it was solved by the optimization of gradient program.     

Modeling the interaction between two- and four-ring progestin models and a silicone-based polymer model: a density functional theory study

In this paper, we introduce a relatively fast and reliable method for determining the feasibility of drug delivery from transdermal and implant materials. We are using density functional theory for modeling the interaction of progestins, that is, progesterone and six of its hydroxyl derivatives, with a silicone-based polymer. The silicone-based polymer model is a linear molecule, which consists of four dimethylsiloxane units. The progestin models are (1) complete progestin structures, which are called four-ring models, and (2) their two-ring models, which are comprised of the C and D rings of the basic steroid skeletons. We are investigating the interaction between the four- and two-ring models and the polymer model in three different interaction configurations. Altogether, 42 different equilibrium geometries of progestin-polymer model complexes and the corresponding interaction energies have been calculated. Our computational results are in very good agreement with the experimental findings reported previously in the literature, which state that the release rates and permeabilities of progestin pharmaceuticals in silicone-based drug delivery systems decrease when the number of hydroxyl groups is increased in the steroid skeleton. The four-ring models take the total interaction of the steroid into account slightly better than the two-ring models. However, the two-ring models are very good for predicting the local interactions between the steroid and the polymer model.     

A simple and fast liquid–liquid extraction method for the determination of 2,2′,4,4′,5,5′-hexachlorobiphenyl (CB-153) and 1,1-dichloro-2,2-bis(p-chlorophenyl)-ethylene (p,p′-DDE) from human serum for epidemiological studies on type 2 diabetes

A simple and fast method is presented to be used for example in studies on the relationship between serum levels of persistent organic pollutants and type 2 diabetes mellitus. Method is based on liquid–liquid extraction and gas chromatography coupled with high-resolution mass spectrometry. In the sample pre-treatment special attention was paid to minimize the number of sample manipulation steps and the amounts of organic solvents needed. Compounds analyzed were 2,2′,4,4′,5,5′-hexachlorobiphenyl (PCB-153) and 1,1-dichloro-2,2-bis (p-chlorophenyl)-ethylene (p,p′-DDE), the major metabolite of DDT. The method included extraction and cleanup of 0.2 ml of serum in a single test tube and subsequent analysis of the extract from 0.2 ml final volume. Validation was conducted to explore the performance of the method. The limits of detection for p,p′-DDE and PCB-153 based on the standard deviation of the blank samples were 4.3 and 3.1 pg/ml, respectively. Repeatability was less than 2.5% at three concentration levels tested and recovery from Certified Reference Material SRM 1589a was 84% for p,p′-DDE and 87% for PCB-153 of the certified values, respectively. Serum samples from the AMAP intercalibration round 2008-2 were also analyzed, and results were 101–116% of the assigned values. The presented method was used for an epidemiological study with more than 700 serum samples from a type 2 diabetes cohort from Sweden.     

Effect of substituents on the absorption properties of three pyridylindolizine derivatives: A DFT and TDDFT study

Ground state properties and the UV–Vis absorption spectra of three recently synthesized pyridylindolizine derivatives have been studied by using density functional theory (DFT) and its time-dependent counterpart TDDFT. Performances of the two widely used hybrid functionals, B3LYP and PBE0, and of four different basis sets have been compared. The two functionals yield absorption spectra which have very similar shapes and characteristics but the excitations calculated with PBE0 are obtained at slightly shorter wavelengths. Basis sets affect the appearances of the calculated absorption spectra only little. Independent of the solvent polarity, simulation of the solvent effects by COSMO induces only slight changes into the ground state properties compared to those calculated in the gas-phase and into the absorption spectra.     

Molecular simulations for the conformational assessment of a porphyrin-fullerene dyad in different environments

Conformational space of a porphyrin-fullerene dyad with the donor and acceptor connected by a relatively flexible linker is studied by molecular dynamics simulations in both non-polar and polar solvents, as well as in vacuum. The most probable conformations obtained from the vacuum MD simulations were optimized with semi-empirical (SE) and density functional theory (DFT) methods and the extent of the structural changes is assessed. The computational results indicate the co-existence of different conformers in both polar and nonpolar solvents showing agreement with experimental results. The most probable vacuum conformations at 300 K are similar to the ones at 0 K, while the structures most often observed in the solvents show less compact conformations. Optimization with SE and DFT calculations leads to structures, which represent relatively well the folded conformations in solvent, which validates the electronic structure calculations relevant to describing photoinduced electron-transfer in H2P-O34-C60.     

Ab initio description of photoabsorption and electron transfer in a doubly-linked porphyrin-fullerene dyad

Structure, photoabsorption and excited states of two representative conformations obtained from molecular dynamics (MD) simulations of a doubly-linked porphyrin-fullerene dyad DHD6ee are studied by using both DFT and wavefunction based methods. Charge transfer from the donor (porphyrin) to the acceptor (fullerene) and the relaxation of the excited state are of special interest. The results obtained with LDA, GGA, and hybrid functionals (SVWN, PBE, and B3LYP, respectively) are analyzed with emphasis on the performance of used functionals as well as from the point of view of their comparison with wavefunction based methods (CCS, CIS(D), and CC2). Characteristics of the MD structures are retained in DFT optimization. The relative orientation of porphyrin and fullerene is significantly influencing the MO energies, the charge transfer (CT) in the ground state of the dyad and the excitation of ground state CT complex (g-CTC). At the same time, the excitation to the locally excited state of porphyrin is only little influenced by the orientation or cc distance. TD-DFT underestimates the excitation energy of the CT state, however for some cases (with relatively short donor-acceptor separations), the use of a hybrid functional like B3LYP alleviates the problem. Wavefunction based methods and CC2 in particular appear to overestimate the CT excitation energies but the inclusion of proper solvation models can significantly improve the results.