Determination of selected persistent organochlorine pollutants in human milk using solid phase disk extraction and narrow bore capillary GC-MS

Summary A simple and rapid procedure based on solid phase disk extraction (SPDE), adsorption chromatography on acidified silica gel and GC-MS analysis was developed for the determination of 8 organochlorine pesticides and 19 PCB congeners in human milk. By using the SPDE procedure, a high throughput and parallel sample processing could be achieved. Method variables were optimized on whole cow's milk (3.5% fat) fortified at levels close to concentrations found in human milk. Recoveries of target analytes were acceptable and ranged from 69 to 102% and 86 to 120% for whole and skimmed milk, respectively. By the use of two stage clean-up and narrow bore capillary columns, detection limits as low as 20 pg mL⁻¹ could be obtained. The method was used for the determination of organochlorine pollutants in human milk from 19 individuals from Romania. The concentrations of PCBs were low, whereas those of organochlorine pesticides were higher than the values reported from other European countries.     

Energy dependence of multiplicity in proton-nucleus collisions and models of multiparticle production

This is a continuation of our earlier investigation (Gurtuet al 1974Phys. Lett. 50 B 391) on multiparticle production in proton-nucleus collisions based on an exposure of emulsion stack to 200 GeV/c beam at the NAL. It is found that the ratioR _em = 〈n _s〉/〈n _ch〉, where 〈n _ch〉 is the charged particle multiplicity in pp-collisions, increases slowly from about 1 at 10 GeV/c to 1·6 at 68 GeV/c and attains a constant value of 1·71 ± 0·04 in the region 200 to 8000 GeV/c. Furthermore,R _em = 1·71 implies an effectiveA-dependence ofR _A =A ^0.18,i.e., a very weak dependence. Predictions ofR _em on various models are discussed and compared with the emulsion data. Data seem to favour models of hadron-nucleon collisions in which production of particles takes place through adouble step mechanism,e.g., diffractive excitation, hydrodynamical and energy flux cascade as opposed to models which envisage instantaneous production.     

Computational Investigation of the Substituent Effect on the Intramolecular Proton Transfer Reaction of 3-Hydroxytropolone

The effects of substituent type and position on the proton transfer reaction of 3-hydroxytropolone(3-OHTRN) have been investigated theoretically by using density functional theory at the level of B3LYP/ 6-31+G** method. The influence of solvent on the proton transfer reactions of substituted 3-OHTRN has been examined using the self-consistent isodensity polarized continuum model(SCI-PCM) in water. As a result, while the proton transfer reaction is kinetically the easiest by substitution on position 3 of-NH2 group in the gas phase, it is kinetically the easiest by substitution on position 5 of the same group in water. In addition, these reactions are either kinetically or thermodynamically easier in the gas phase than that in water, except the reaction of structure with-NH2 group at position 6.     

The acidic domains of the Toc159 chloroplast preprotein receptor family are intrinsically disordered protein domains

Background  

The Toc159 family of proteins serve as receptors for chloroplast-destined preproteins. They directly bind to transit peptides, and exhibit preprotein substrate selectivity conferred by an unknown mechanism. The Toc159 receptors each include three domains: C-terminal membrane, central GTPase, and N-terminal acidic (A-) domains. Although the function(s) of the A-domain remains largely unknown, the amino acid sequences are most variable within these domains, suggesting they may contribute to the functional specificity of the receptors.     

Structural transition of nanogel star polymers with pH by controlling PEGMA interactions with acid or base copolymers

We use small angle X-ray scattering (SAXS) to characterise a class of star diblock polymers with a nanogel core on which the outer block arms are comprised of random copolymers of temperature sensitive PEGMA with pH sensitive basic (PDMAEMA) and acidic (PMAA) monomers. The acquired SAXS data show that many of the nanogel star polymers undergo a sharp structural transition over a narrow range of pH, but with unexpectedly large shifts in the apparent pKa with respect to that of the acidic or basic monomer unit, the linear polymer form or even an alternate star polymer with a tightly cross-linked core chemistry. We have demonstrated a distinct and quantifiable structural response for the nanogel star copolymers by altering the core or by pairing the monomers PDMAEMA–PEGMA and PMAA–PEGMA to achieve structural transitions that have typically been observed in stars through changes in arm length and number.     

Development of an improved four-site water model for biomolecular simulations: TIP4P-Ew

A re-parameterization of the standard TIP4P water model for use with Ewald techniques is introduced, providing an overall global improvement in water properties relative to several popular nonpolarizable and polarizable water potentials. Using high precision simulations, and careful application of standard analytical corrections, we show that the new TIP4P-Ew potential has a density maximum at approximately 1 degrees C, and reproduces experimental bulk-densities and the enthalpy of vaporization, DeltaH(vap), from -37.5 to 127 degrees C at 1 atm with an absolute average error of less than 1%. Structural properties are in very good agreement with x-ray scattering intensities at temperatures between 0 and 77 degrees C and dynamical properties such as self-diffusion coefficient are in excellent agreement with experiment. The parameterization approach used can be easily generalized to rehabilitate any water force field using available experimental data over a range of thermodynamic points.     

Software for the high-throughput collection of SAXS data using an enhanced Blu-Ice/DCS control system

Biological small-angle X-ray scattering (SAXS) provides powerful complementary data for macromolecular crystallography (MX) by defining shape, conformation and assembly in solution. Although SAXS is in principle the highest throughput technique for structural biology, data collection is limited in practice by current data collection software. Here the adaption of beamline control software, historically developed for MX beamlines, for the efficient operation and high-throughput data collection at synchrotron SAXS beamlines is reported. The Blu-Ice GUI and Distributed Control System (DCS) developed in the Macromolecular Crystallography Group at the Stanford Synchrotron Radiation Laboratory has been optimized, extended and enhanced to suit the specific needs of the biological SAXS endstation at the SIBYLS beamline at the Advanced Light Source. The customizations reported here provide a potential route for other SAXS beamlines in need of robust and efficient beamline control software. As a great deal of effort and optimization has gone into crystallographic software, the adaption and extension of crystallographic software may prove to be a general strategy to provide advanced SAXS software for the synchrotron community. In this way effort can be put into optimizing features for SAXS rather than reproducing those that have already been successfully implemented for the crystallographic community.