Nonaqueous capillary electrophoresis with indirect electrochemical detection

Nonaqueous capillary electrophoresis (NACE) which makes use of organic solvents in place of conventional aqueous electrophoresis buffers is gaining increasing importance among modern separation techniques. Recently, it has been shown that amperometric detection in conjunction with acetonitrile-based NACE offers an extended accessible potential range and an enhanced long-term stability of the amperometric responses generated at solid electrodes. The present contribution takes advantage of the latter aspect to develop reliable systems for NACE with indirect electrochemical detection (IED). In this context, several compounds such as (ferrocenylmethyl)trimethylammonium perchlorate, tris(1,10-phenanthroline)cobalt(III) perchlorate and bis(1,4,7-triazacyclononane)nickel(II) perchlorate were studied regarding their suitability to act as electroactive buffer additives for IED in NACE. The performance characteristics for the respective buffer systems were evaluated. Tetraalkylammonium perchlorates served as model compounds for the optimization of the NACE-IED system. Target analytes choline and acetylcholine could easily be separated and determined by means of NACE-IED. In the case of a buffer system containing 10(-4) M tris(1,10-phenanthroline)cobalt(III) perchlorate the limits of detection were 2.5 x 10(-7) M and 4.6 x 10(-7) M for choline and acetylcholine, respectively. With the elaborated analytical procedure choline could be determined in pharmaceutical preparations.     

Hierarchy of hydrogen bonding in bis­(1,4,7‐trioxa‐10‐azoniacyclo­dodecane) bis­(4‐amino­benzoate) trihydrate

In the title compound, 2C₈H₁₈NO₃⁺·2C₇H₆NO₂⁻·3H₂O, proton transfer occurs from the carboxylic acid group of the 4‐amino­benzoic acid (PABA) mol­ecule to the amine group of the macrocycle, resulting in the formation of a salt‐like adduct. The anions are combined into helical chains which are further bound by the water mol­ecules into sheets. The macrocyclic cations are situated between these layers and are bound to the anions both directly and via bridging water mol­ecules. The structure exhibits a diverse system of hydrogen bonding.     

Theory of long-lived nuclear spin states in solution nuclear magnetic resonance. I. Singlet states in low magnetic field

We have recently demonstrated the existence of exceptionally long-lived nuclear spin states in solution-state nuclear magnetic resonance. The lifetime of nuclear spin singlet states in systems containing coupled pairs of spins-12 may exceed the conventional relaxation time constant T1 by more than an order of magnitude. These long lifetimes may be observed if the long-lived singlet states are prevented from mixing with rapidly relaxing triplet states. In this paper we provide the detailed theory of an experiment which uses magnetic field cycling to observe slow singlet relaxation. An approximate expression is given for the magnetic field dependence of the singlet relaxation rate constant, using a model of intramolecular dipole-dipole couplings and fluctuating external random fields.     

Novel selenium-containing non-natural diamino acids

The general synthesis of a new class of non-natural diamino acids, 2-amino-3-[(2′-aminoalkyl)seleno]propanoic acids, or Se-(aminoalkyl)selenocysteines, is reported. Under the conditions devised, enantiopure N-Boc-protected β-l-iodoamines, which are readily generated from proteinogenic α-amino acids, were treated with the selenolate anion obtained from NaBH₄ splitting of the Se-Se bond in commercial l-selenocystine. The Se-alkylation products were enantiomerically pure and the reaction is high yielding (92-98%), without any detectable traces of accompanying by-products.     

Combined use of liquid chromatography-nuclear magnetic resonance spectroscopy and liquid chromatography-mass spectrometry for the characterization of an acarbose degradation product

Directly coupled LC-MS and LC-NMR were applied to identify and structurally characterize an acarbose degradation product A in acidic media. A comparative analysis of the stop-flow LC-NMR (1H and TOCSY) and LC-MS data provided evidence that A is structurally related to acarbose, differing from the parent compound in a number of subunits present in the molecule. Spectral analysis revealed that A was the alpha-glucosidase inhibitor amylostatin XG. Complementary information obtained from the two methods led to the structural elucidation of A which was later corroborated by high-resolution NMR spectroscopy of the isolated molecule.     

REMGa3Ge and RE3Ni3Ga8Ge3 (M = Ni, Co; RE = rare-earth element): new intermetallics synthesized in liquid gallium. X-ray, electron, and neutron structure determination and magnetism

New quaternary intermetallic phases REMGa(3)Ge (1) (RE = Y, Sm, Tb, Gd, Er, Tm; M = Ni, Co) and RE(3)Ni(3)Ga(8)Ge(3) (2) (RE = Sm, Gd) were obtained from exploratory reactions involving rare-earth elements (RE), transition metal (M), Ge, and excess liquid Ga the reactive solvent. The crystal structures were solved with single-crystal X-ray and electron diffraction. The crystals of 1 and 2 are tetragonal. Single-crystal X-ray data: YNiGa(3)Ge, a = 4.1748(10) A, c = 23.710(8) A, V = 413.24(2) A(3), I4/mmm, Z = 4; Gd(3)Ni(3)Ga(8)Ge(3), a = 4.1809(18) A, c = 17.035(11) A, V = 297.8(3) A(3), P4/mmm, Z = 1. Both compounds feature square nets of Ga atoms. The distribution of Ga and Ge atoms in the REMGa(3)Ge was determined with neutron diffraction. The neutron experiments revealed that in 1 the Ge atoms are specifically located at the 4e crystallographic site, while Ga atoms are at 4d and 8g. The crystal structures of these compounds are related and could be derived from the consecutive stacking of disordered [MGa](2) puckered layers, monatomic RE-Ge planes and [MGa(4)Ge(2)] slabs. Complex superstructures with modulations occurring in the ab-plane and believed to be associated with the square nets of Ga atoms were found by electron diffraction. The magnetic measurements show antiferromagnetic ordering of the moments located on the RE atoms at low temperature, and Curie-Weiss behavior at higher temperatures with the values of mu(eff) close to those expected for RE(3+) free ions.     

Kinetics and mechanism of the wet oxidation of propene to acetone in the presence of Pd2+ ions and Mo-V-phosphoric heteropoly acids ?

Summary Oxidation of propene to acetone in water solutions in the presence of homogeneous catalysts (Pd²⁺ + HPA-x, where HPA-x = H_3+xPV_xMo_12-xO₄₀, x = 1-4) is studied. This reaction is shown to be of the 1st order with respect to C₃H₆ and of the 0.5th order with respect to Pd. The reaction rate does not depend on the concentration of HPA-x and acidity of the catalyst solution. The apparent activation energy of the reaction is 21 kJ/mol. A reaction mechanism is proposed.     

Solubility Behavior in Ternary Water-Salt Systems under Sub- and Supercritical Conditions

Summary.  Two main types of binary systems with distinctive solubility behavior under sub- and supercritical conditions were used to subdivide ternary water-salt systems into three classes. Characteristic features of solubility behavior and phase equilibria in ternary water-salt systems of each class at temperatures above 200°C are discussed on the basis of available experimental data and some conclusions obtained as a result of theoretical derivation of fluid and complete phase diagrams. Corresponding author. E-mail: Valyashko@IGIC.RAS.RU Received September 25, 2002; accepted (revised) November 28, 2002 Published online April 24, 2003 RID="a" ID="a" Dedicated to Prof. Dr. H. Gamsjaeger on the occasion of his 70^th birthday anniversary     

Parton distributions for high energy collisions

Recent data from deep inelastic scattering experiments atx10^–2 are used to fix the parton distributions down tox10^–4 andQ ²0.3 GeV². The predicted extrapolations are uniquely determined by the requirement of avalence-like structure ofall parton distributions at some low resolution scale and are furthermore shown to be insensitive in the small-x region, 10^–4x10^–2, to the detailed experimental input at the presently accessiblex>10^–2. Simple parameterizations of the resulting parton distributions are presented in the range 10^–5x<1 and=">Q ²10⁸ GeV² as obtained from the leading- and higher-order evolution equations.