Separation of 18 modern plant protectants using cyclodextrin modified micellar electrokinetic chromatography including an ion-pairing reagent

A cyclodextrin-modified micellar electrokinetic chromatography separation for 18 different pesticides (metsulfuron-methyl, rimsulfuron, thifen-sulfuron-methyl, desethylatrazine, desisopropylatrazine, atrazine, simazine, terbuthylazine, 2,4-D, MCPP, MCPB, dicamba, linuron, alachlor, metolachlor, orbencarb, propiconazole, prochloraz) from eight different substance classes with very varying chemical and physical properties is presented. In particular acid-base characteristics and water-octanol distribution coefficients diverge in wide ranges. -cyclodextrin was successfully employed as a modifier in separating the hydrophobic analytes. Peak distortion of some neutral analytes, which is devoted to the methanol content of the sample zone, was reduced by increasing the SDS concentration. Methanol in the sample is necessary for a better solubility of the hydrophobic pesticides. Few optima of SDS concentration exist allowing the separation of sulfonylureas as well as phenoxy acids and hydrophobic pesticides. An improved resolution of the sulfonylureas was achieved with tetramethylammonium chloride, which was used as an ion-pairing reagent.Final operating conditions for the separation of all these plant protectant compounds by MEKC in just one single run are a 27 mmolL^–1 phosphate buffer, pH 8.03, with 95 mmolL^–1SDS, 5 mmolL^–1 -cyclodextrin and 10 mmol L^–1 tetramethylammonium-chloride.     

Capillary zone electrophoresis of metal-binding proteins in formic acid with UV- and mass spectrometric detection using cationic transient capillary isotachophoresis for preconcentration

A capillary zone electrophoresis (CZE) method with preceding cationic transient capillary isotachophoresis (tCITP-CZE) was developed for uncoated fused-silica capillaries to analyze metal-binding proteins (MBPs) of clinical relevance. UV detection was followed by mass spectrometry (MS). Optimization was done with model proteins of properties similar to relevant human MBPs. Using 1.0 mol x L(-1) formic acid (pH 1.78) as electrolyte resulted in up to 165000 plates m(-1) in CZE and 230000 plates m(-1) in combination with tCITP and analysis time was less than 5 min in uncoupled mode. Cationic tCITP with 125 mmol x L(-1) ammonium formate, buffered to pH 4.00, as leading electrolyte improved sample loadability considerably in comparison with sample stacking without impairing resolution. Following systematic optimization of the electrospray ionization process (ESI) the coupled system ((tCITP)-CZE-UV-ESI-MS) was tested with protein model mixtures and human MBPs. Repeatability of migration times was < 0.64% in pure CZE mode and in tCITP-CZE mode and < 0.83% in CZE-ESI-MS coupled mode. Mass accuracy was < 0.015%. Limits of detection were found to be in the range 50-160 fmol.     

Bifurcations on potential energy surfaces of organic reactions

A single transition state may lead to multiple intermediates or products if there is a post-transition-state reaction pathway bifurcation. These bifurcations arise when there are sequential transition states with no intervening energy minimum. For such systems, the shape of the potential energy surface and dynamic effects, rather than transition-state energetics, control selectivity. This Minireview covers recent investigations of organic reactions exhibiting reaction pathway bifurcations. Such phenomena are surprisingly general and affect experimental observables such as kinetic isotope effects and product distributions.     

Validation of capillary zone electrophoresis and capillary isoelectric focusing separations optimized for the characterization of two recombinant products of the birch pollen allergen Bet v 1a

CZE and CIEF separation systems, both developed previously for a quality control of two recombinant products of the major birch pollen allergen Bet v 1a of Betula verrucosa, were validated including aspects of the International Conference on Harmonization. One product contained carbamylated variants as impurities. Linearity of response was confirmed by Mandel's fitting test between 0.028 and 1.90 mg/mL for CZE and between 0.016 and 0.26 mg/mL for CIEF. Repeatability and intermediate precision were evaluated for the effective mobility (mu(eff)) in CZE, for relative mobilization time in CIEF and the peak area ratio of Bet v 1a. LOQ for Bet v 1a was between 10 and 23 microg/mL for both methods. Evaluation of robustness for CZE revealed susceptibility of micro(eff) of Bet v 1a to alterations in of buffer pH and separation temperature. Selectivity was impaired by an increase in temperature, pH, and buffer concentration. In addition, pH variations influenced the separation profile of impurities. For CIEF, the ratio of narrow pH range carrier ampholytes is the critical parameter to retain robustness. Results demonstrate the suitability of both separation systems to discriminate between nonmodified Bet v 1a and carbamylated variants in the selected recombinant allergen products.     

Transition states for the dimerization of 1,3-cyclohexadiene: a DFT, CASPT2, and CBS-QB3 quantum mechanical investigation

Quantum mechanical calculations using restricted and unrestricted B3LYP density functional theory, CASPT2, and CBS-QB3 methods for the dimerization of 1,3-cyclohexadiene (1) reveal several highly competitive concerted and stepwise reaction pathways leading to [4 + 2] and [2 + 2] cycloadducts, as well as a novel [6 + 4] ene product. The transition state for endo-[4 + 2] cycloaddition (endo-2TS, DeltaH(double dagger)(B3LYP(0K)) = 28.7 kcal/mol and DeltaH(double dagger)(CBS-QB3(0K)) = 19.0 kcal/mol) is not bis-pericyclic, leading to nondegenerate primary and secondary orbital interactions. However, the C(s) symmetric second-order saddle point on the B3LYP energy surface is only 0.3 kcal/mol above endo-2TS. The activation enthalpy for the concerted exo-[4 + 2] cycloaddition (exo-2TS, DeltaH(double dagger)(B3LYP(0K)) = 30.1 kcal/mol and DeltaH(double dagger)(CBS-QB3(0K)) = 21.1 kcal/mol) is 1.4 kcal/mol higher than that of the endo transition state. Stepwise pathways involving diallyl radicals are formed via two different C-C forming transition states (rac-5TS and meso-5TS) and are predicted to be competitive with the concerted cycloaddition. Transition states were located for cyclization from intermediate rac-5 leading to the endo-[4 + 2] (endo-2) and exo-[2 + 2] (anti-3) cycloadducts. Only the endo-[2 + 2] (syn-3) transition state was located for cyclization of intermediate meso-5. The novel [6 + 4] "concerted" ene transition state (threo-4TS, DeltaH(double dagger)(UB3LYP(0K)) = 28.3 kcal/mol) is found to be unstable with respect to an unrestricted calculation. This diradicaloid transition state closely resembles the cyclohexadiallyl radical rather than the linked cyclohexadienyl radical. Several [3,3] sigmatropic rearrangement transition states were also located and have activation enthalpies between 27 and 31 kcal/mol.     

Effect of Lewis acid catalysts on Diels-Alder and hetero-Diels-Alder cycloadditions sharing a common transition state

The thermal and Lewis acid catalyzed cycloadditions of beta,gamma-unsaturated alpha-ketophosphonates and nitroalkenes with cyclopentadiene have been explored by using density functional theory (DFT) methods. In both cases, only a single highly asynchronous bis-pericyclic transition state yielding both Diels-Alder and hetero-Diels-Alder cycloadducts could be located. Stepwise pathways were found to be higher in energy. On the potential energy surface, the bis-pericyclic cycloaddition transition state is followed by the Claisen rearrangement transition state. No intermediates were located between these transition states. Claisen rearrangement transition states are also highly asynchronous, but bond lengths are skewed in the opposite direction compared to the bis-pericyclic transition states. The relative positions of the bis-pericyclic and Claisen rearrangement transition states may control periselectivity due to the shape of the potential energy surface and corresponding dynamical influences. Inspection of the thermal potential energy surface (PES) indicates that a majority of downhill paths after the bis-pericyclic transition state lead to the Diels-Alder cycloadducts, whereas a smaller number of downhill paths reach the hetero-Diels-Alder products with no intervening energy barrier. Lewis acid catalysts alter the shape of the surface by shifting the cycloaddition and the Claisen rearrangement transition states in opposite directions. This topographical change qualitatively affects the branching ratio after the bis-pericyclic transition state and ultimately reverses the periselectivity of the cycloaddition giving a preference for hetero-Diels-Alder cycloadducts.     

On Sparse Parity Check Matrices

We consider the extremal problem to determine the maximal number of columns of a 0-1 matrix with rows and at most ones in each column such that each columns are linearly independent modulo . For fixed integers and , we shall prove the probabilistic lower bound = ; for a power of , we prove the upper bound which matches the lower bound for infinitely many values of . We give some explicit constructions.     

Synthesis and Reactivity of a Neutral,Three‐Coordinate Platinum(II) Complex Featuring Terminal Amido Ligation

A crystallographically characterized three‐coordinate, formally 14 electron Pt^II complex 1 featuring terminal amido ligation is reported. Computational analysis revealed relatively weak π donation from the amide lone pair to platinum and supports a 14‐electron assignment for 1 . Stoichiometric reactivity studies confirmed the viability of net O H and C H addition across, as well as isonitrile insertion into, the terminal platinum–amido linkage of 1 .