Thermodynamic, kinetic and solid-state study of divalent metal complexes of 1,4,8,11-tetraazacyclotetradecane (cyclam) bearing two trans (1,8-)methylphosphonic acid pendant arms

Divalent metal complexes of macrocyclic ligand 1,4,8,11-tetraazacyclotetradecane-1,8-bis(methylphosphonic acid)) (1,8-H4te2p, H4L) were investigated in solution and in the solid state. The majority of transition-metal ions form thermodynamically very stable complexes as a consequence of high affinity for the nitrogen atoms of the ring. On the other hand, complexes with Mn2+, Pb2+ and alkaline earth ions interacting mainly with phosphonate oxygen atoms are much weaker than those of transition-metal ions and are formed only at higher pH. The same tendency is seen in the solid state. Zinc(II) ion in the octahedral trans-O,O-[Zn(H2L)] complex is fully encapsulated within the macrocycle (N4O2 coordination mode with protonated phosphonate oxygen atoms). The polymeric {[Pb(H2L)(H2O)2].6H2O}n complex has double-protonated secondary amino groups and the central atom is bound only to the phosphonate oxygen atoms. The phosphonate moieties bridge lead atoms creating a 3D-polymeric network. The [{(H2O)5Mn}2(micro-H2L)](H2L).21H2O complex contains two pentaaquamanganese(II) moieties bridged by a ligand molecule protonated on two nitrogen atoms. In the complex cation, oxygen atoms of the phosphonate groups on the opposite sites of the ring occupy one coordination site of each metal ion. The second ligand molecule is diprotonated and balances the positive charge of the complex cation. Complexation of zinc(II) and cadmium(II) by the ligand shows large differences in reactivity of differently protonated ligand species similarly to other cyclam-like complexes. Acid-assisted dissociations of metal(II) complexes occur predominantly through triprotonated species [M(H3L)]+ and take place at pH < 5 (Zn2+) and pH < 6 (Cd2+).     

Vanadium determination in chloride matrices using ICP-MS: finding the optimum collision/reaction cell parameters for suppressing polyatomic interferences

Efficiencies of He/NH₃ and He/H₂ collision gases were compared in a conventional type of hexapole cell of an inductively coupled plasma mass spectrometer (ICP-MS). The optimum conditions [hexapole and quadrupole bias voltage (V_H and V_Q) and collision/reaction gas flow rates] were tested for vanadium determination (⁵¹V) in chloride matrices. When the He/H₂ mixture was used, the optimum values of V_H and V_Q were −10.0 and −8.0 V, respectively. This set-up corresponds to the kinetic energy discrimination effect. When the He/NH₃ mixture was used, the optimum values of V_H and V_Q were +10.0 and −7.0 V, respectively. Positive V_H values correspond to the ion kinetic energy effect, which allows the reactivity of the ions entering the collision/reaction cell with the reaction gas to be controlled. The obtained results showed that the He/H₂ mixture is not optimal for V determination in samples containing chlorides due to the insufficient suppression of the polyatomic interference of ³⁵Cl¹⁶O⁺. Data obtained from vanadium determination using the He/NH₃ mixture were consistent for all selected Cl⁻ concentrations, and the results were acceptable. The detection limit was comparable with detection limits obtained from ICP-MS equipped with a dynamic reaction cell. Analyses of elements forming interfering molecules, e.g., iron (⁵⁶Fe), arsenic (⁷⁵As) and selenium (⁸⁰Se), were in good agreement with the certified values for both studied collision/reaction gas mixtures.     

Bio-assay based on single molecule fluorescence detection in microfluidic channels

A rapid bioassay is described based on the detection of colocalized fluorescent DNA probes bound to DNA targets in a pressure-driven solution flowing through a planar microfluidic channel. By employing total internal reflection excitation of the fluorescent probes and illumination of almost the entire flow channel, single fluorescent molecules can be efficiently detected leading to the rapid analysis of nearly the entire solution flowed through the device. Cross-correlation between images obtained from two spectrally distinct probes is used to determine the target concentration and efficiently reduces the number of false positives. The rapid analysis of DNA targets in the low pM range in less than a minute is demonstrated.     

Different elution modes and field programming in gravitational field-flow fractionation: field programming using density and viscosity gradients

In previous papers, several approaches to programming of the resulting force field in GFFF were described and investigated. The experiments were dealing with flow-velocity and channel thickness, i.e. factors influencing hydrodynamic lift forces (HLF). The potential of density and viscosity of carrier liquid for field programming was predicted and demonstrated by preliminary experiments. This work is devoted to experimental verification of the influence of carrier liquid density and viscosity. Several carrier liquid density and simultaneously viscosity gradients using water-methanol mixtures are in this work implemented in the separation of a model silica mixture. Working with the water-methanol gradients, one is not able to separate the influence of density from the contribution of viscosity. However, we found experimental conditions to show the isolated effect of carrier liquid density (two water-methanol mixtures of equal viscosity differing in their densities). In order to demonstrate the isolated effect of viscosity, we implemented in this work a new system of (hydroxypropyl)methyl cellulose (HPMC) carrier liquids. Three different HPMC compositions enabled to vary the viscosity more than two times at almost constant density. With increasing carrier liquid viscosity, the focusing and elevating trend was clearly pronounced for 5 and 10 microm silica particles. By the isolated effect of increased viscosity, the centre of the 10 microm particle zone was elevated to the streamline at 16% of the channel height. These experiments have shown that the influence of carrier liquid viscosity on HLF should be taken into account even at higher levels above the channel bottom, i.e. beyond the near-wall region. Further, it is shown that higher value of carrier liquid viscosity improves the separation of the model mixture in terms of time and resolution.     

Balancing an accurate representation of the molecular surface in generalized born formalisms with integrator stability in molecular dynamics simulations

Different integrator time steps in NVT and NVE simulations of protein and nucleic acid systems are tested with the GBMV (Generalized Born using Molecular Volume) and GBSW (Generalized Born with simple SWitching) methods. The simulation stability and energy conservation is investigated in relation to the agreement with the Poisson theory. It is found that very close agreement between generalized Born methods and the Poisson theory based on the commonly used sharp molecular surface definition results in energy drift and simulation artifacts in molecular dynamics simulation protocols with standard 2-fs time steps. New parameters are proposed for the GBMV method, which maintains very good agreement with the Poisson theory while providing energy conservation and stable simulations at time steps of 1 to 1.5 fs.     

Structural aspects of long-lived C7H8(2+) dications generated by the electron ionization of toluene

The structure of the C7H8(2+) dication generated upon electron ionization of toluene is investigated by experimental and theoretical means. For the long-lived C7H8(2+) dication, the experimental findings obtained with a novel SIFT/GIB instrument suggest complete loss of structural integrity corresponding to the toluene structure. Instead, the manifold of C7H8(2+) dications most likely to be formed is assigned to a mixture of the cycloheptatriene dication and ring-protonated benzylium ions.     

Magnetism of UCoGe and U3Co4Ge7

A series of UCoGe and U₃Co₄Ge₇ polycrystalline samples has been prepared by arc melting and studied with respect to the phase composition and crystal structure, magnetization, a.c. susceptibility, electrical-resistivity and specific-heat behavior (down to 350 mK). U₃Co₄Ge₇ has been found to exhibit a spontaneous magnetization below T_C=21 K. Clear anomalies at T_C typical for a ferromagnetic transition have been observed in a.c. susceptibility, electrical-resistivity and specific-heat data. No additional anomaly, which would indicate the second magnetic phase transition below T_C reported in the literature, has been indicated. In all our UCoGe samples a transition to superconductivity has been revealed. On the other hand, no clear evidence of any transition to ferromagnetism with zero-field cooling down to 1.8 K has been obtained. The zero-field state is most probably governed by strong ferromagnetic spin fluctuations and seems to transform to a ferromagnetic state only when applying a magnetic field ≥10 mT. Simultaneously, an increase of the superconducting transition temperature is increasing with a magnetic field up to 10 mT and starts to decrease when increasing the field above this value. Measurements on bulk samples, which are by rule textured, indicate strong magnetocrystalline anisotropy in both investigated compounds.     

YC‐1, an activation inductor of soluble guanylyl cyclase

The crystal structure of 1‐benzyl‐3‐(5‐hydroxy­methyl‐2‐furyl)­indazole, C₁₉H₁₆N₂O₂, showed that the furan O and indazole N atoms lie on the same face of the mol­ecule. The crystal packing consists of intermolecular hydrogen bonding, and indazole–indazole and indazole–phenyl interactions.     

Uniqueness and decay estimates for a class of parabolic partial differential equations with hysteresis and convection

We deal in detail with the question of existence, uniqueness and asymptotic behaviour of solutions to a parabolic equation with hysteresis and convection. This equation is part of a model system which describes the magnetohydrodynamic (MHD) flow of a conducting fluid between two ferromagnetic plates. The result of this paper complements the content of a previous paper of the first author, where existence of the solution has been proved under fairly general assumptions on the hysteresis operator and the uniqueness was only obtained for a restricted class of hysteresis operators.     

Micro-hydration of the MgNO3+ cation in the gas phase.

Coordination complexes of the magnesium nitrate cation with water [MgNO(3)(H(2)O)(n)](+) up to n=7 are investigated by experiment and theory. The fragmentation patterns of [MgNO(3)(H(2)O)(n)](+) clusters generated via electrospray ionization indicate a considerable change in stability between n=3 and 4. Further, ion-molecule reactions of mass-selected [MgNO(3)(H(2)O)(n)](+) cations with D(2)O reveal the occurrence of consecutive replacement of water ligands by heavy water, and in this respect the complexes with n=4 and 5 are somewhat more reactive than their smaller homologs with n=1-3 as well as the larger clusters with n=6 and 7. For the latter two ions, the theory suggests the existence of isomers, such as complexes with monodentate nitrato ligands as well as solvent-separated ion pairs with a common solvation shell. The reactions observed and the ion thermochemistry are discussed in the context of ab initio calculations, which also reveal the structures of the various hydrated cation complexes.