Solid-state (14)N MAS NMR of ammonium ions as a spy to structural insights for ammonium salts

The high resolution offered by magic-angle spinning (MAS), when compared to the static condition in solid-state NMR of powders, has been used to full advantage in a (14)N MAS NMR study of some ammonium salts: CH(3)NH(3)Cl, (NH(4))(2)(COO)(2) x H(2)O, (CH(3))(3)(C(6)H(5)CH(2))NCl, (CH(3))(3)(C(6)H(5))NI, [(n-C(4)H(9))(4)N](2)Mo(2)O(7), (NH(4))(2)HPO(4), and NH(4)H(2)PO(4). It is shown that the high-quality (14)N MAS NMR spectra, which can be obtained for these salts, allow determination of the (14)N quadrupole coupling parameters, i.e. C(Q) (the quadrupole coupling constant) and eta(Q) (the asymmetry parameter), with very high precision. In particular, it is shown that precise C(Q), eta(Q) parameters can be determined for at least two different (14)N sites in case the individual spinning-sideband (ssb) intensities arise from a single manifold of ssbs, i.e. the ssbs for the two sites cannot be resolved. This feature of (14)N MAS NMR, which is the first demonstration for manifolds of ssb in MAS NMR without the potential information from a central transition, becomes especially useful at the slow spinning frequencies (nu(r) = 1000-1500 Hz) applied to some of the ammonium salts studied here. The detection of the number of sites has been confirmed by the corresponding crystal structures determined from single-crystal X-ray diffraction (XRD), either in this work for the unknown structure of benzyl trimethylammonium chloride or from reports in the literature. The magnitudes of the (14)N quadrupole coupling constants for the ammonium salts studied here are in the range from C(Q) approximately 20 kHz to 1 MHz while the asymmetry parameters span the full range 0 < or = eta(Q) < or = 1. Clearly, the (14)N quadrupole coupling parameters (C(Q), eta(Q)) for ammonium ions appear highly sensitive toward crystal structure and therefore appreciably more informative for the characterization of ammonium salts in comparison to the isotropic (14)N (or (15)N) chemical shifts.     

Protolytic properties of polyamine wasp toxin analogues studied by 13C NMR spectroscopy

Acid-base properties of the natural polyamine wasp toxin PhTX-433 (1) and seven synthetic analogues [PhTX-343 (2), PhTX-334 (3), PhTX-443 (4), PhTX-434 (5), PhTX-344 (6), PhTX-444 (7), and PhTX-333 (8)], each having four protolytic sites, were characterized by 13C NMR spectroscopy. Nonlinear, multiparameter, simultaneous fit of all chemical shift data obtained from the NMR titration curves yielded macroscopic pKa values as well as intrinsic chemical shift data of all differently protonated macrospecies. Analyses of the chemical shift data demonstrated strong interactions between all four sites and provided information about complex relationships between chemical shift values and protonation state. Deprotonation of fully protonated forms starts at the central amino group of the polyamine moiety, and the extent of this trend depends on the distance to the flanking, protonated amino groups. The pKa1 values of 1-8 are in the range 8.2-9.4. Hence, some of the toxins are incompletely protonated at the pH and ionic strength conditions used for assessment of their interactions with ionotropic glutamate and nicotinic acetylcholine receptors, and the degree of protonation is expected to have pharmacological importance in the ion-channel binding event.     

Silylenes,Silenes, and Disilenes: Novel Silicon‐Based Reagents for Organic Synthesis?

Silylenes, silenes, and disilenes are silicon analogues of carbenes and alkenes. Since the first detection and isolation of these species a few decades ago, focus has been given to their fundamental structure and reactivity properties. Recent developments show that the time is set to exploit their unique chemistry in applied areas. Emerging applications in catalysis and stereoselective synthesis point to a new field within synthetic organosilicon chemistry.     

AC electroosmotic pump with bubble-free palladium electrodes and rectifying polymer membrane valves

We present the design, test and theoretical analysis of a novel micropump. The purpose is to make a pump with large flow rate (approximately 10 microL min-1) and high pressure capacity (approximately 1 bar) powered by a low voltage DeltaV<30 V. The pump is operated in AC mode with an electroosmotic actuator in connection with a full wave rectifying valve system. Individual valves are based on a flexible membrane with a slit. Bubble-free palladium electrodes are implemented in order to increase the range of applications and reduce maintenance.     

Quantification of urinary etheno-DNA adducts by column-switching LC/APCI-MS/MS

Lipid peroxidation induced etheno-DNA adducts are promutagenic and have been suggested to play a causal role in the development of human cancers. Therefore, human biomonitoring of etheno-DNA adducts in urine has been suggested as a potential marker for oxidative stress-related DNA damage. For quantitative determination, a column-switching LC/APCI-MS/MS method was developed for simultaneous analysis of epsilonAde, epsilondC, and epsilondA in human urine. Quantitative validation parameters (precision, within-day repeatability, and between-day reproducibility) yielded satisfactory results below 10%. Limit of quantification for epsilonAde, epsilondC, and epsilondA was 5.3 fmol, 7.5 fmol, and 1.3 fmol on column, respectively. Mean urinary excretion rates of a six healthy volunteers were 45.8 pmol epsilonAde/24 h, 96.8 pmol epsilondC/24 h, and 18.1 pmol epsilondA/24 h. The demonstrated levels of performance suggest a future applicability of this method to studies of cancer and other diseases related to oxidative stress in humans. To our knowledge, this is the first method described that allows simultaneous determination of epsilonAde, epsilondC, and epsilondA in human urine samples.     

Symmetry in de Branges almost Pontryagin spaces

In many examples of de Branges spaces symmetry appears naturally. Presence of symmetry gives rise to a decomposition of the space into two parts, the ‘even’ and the ‘odd’ part, which themselves can be regarded as de Branges spaces. The converse question is to decide whether a given space is the ‘even’ part or the ‘odd’ part of some symmetric space, and, if yes, to describe the totality of all such symmetric spaces. We consider this question in an indefinite (almost Pontryagin space) setting, and give a complete answer. Interestingly, it turns out that the answers for the ‘even’ and ‘odd’ cases read quite differently; the latter is significantly more complex.     

Coupled cluster response theory in parameter subspaces

We introduce a response function formalism that enables smaller number of parameters than that defined in standard coupled cluster response theory. This is essential in the development of reduced scaling methods. The formalism is general and it applies to all parameterizations at all levels of the coupled cluster hierarchy. We show that to achieve physically reasonable results the parameterization must fulfill certain criteria. The linear response functions are derived and discussed in the context of optimized virtual orbitals and Cholesky decomposition of the cluster amplitudes. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Methodology for determination of 89Sr and 90Sr in radiological emergency: I. Scenario dependent evaluation of potentially interfering radionuclides

Early determination of ⁸⁹Sr and ⁹⁰Sr in radiological emergency is hampered by the presence of interfering short-lived fission products. In this study, three commonly used radioanalytical strategies for ⁸⁹Sr and ⁹⁰Sr were evaluated theoretically considering their suitability in a nuclear explosion scenario. The methods were evaluated with respect to the need for decay time of interfering short-lived strontium and yttrium isotopes, and reduction of other known interfering nuclides prior to measurement. The strategy shown to be most successful included initial separation of strontium and determination of ⁸⁹Sr, followed by an yttrium separation and counting of ⁹⁰Y. ⁸⁹Sr and ⁹⁰Sr could be determined about 5 and 9 days after a nuclear explosion, respectively.     

Convergence of finite volume schemes for triangular systems of conservation laws

We consider non-strictly hyperbolic systems of conservation laws in triangular form, which arise in applications like three-phase flows in porous media. We device simple and efficient finite volume schemes of Godunov type for these systems that exploit the triangular structure. We prove that the finite volume schemes converge to weak solutions as the discretization parameters tend to zero. Some numerical examples are presented, one of which is related to flows in porous media. The research of K. H. Karlsen was supported by an Outstanding Young Investigators Award from the Research Council of Norway.     

Use of 18O water and ESI-MS detection in subsite characterisation and investigation of the hydrolytic action of an endoglucanase

We present a novel method for investigating subsite-substrate interactions of glycoside hydrolases and the determination of the oligosaccharide cleavage point based on the analysis of the hydrolysis products produced in the presence of ¹⁸O-labelled water. Conventional techniques for such determination of the hydrolysis pattern call for the chemical modification of the substrate, whereas the method presented makes it possible to use natural substrates, utilising the selectivity and sensitivity of mass spectrometry. This method is very useful for the detection and analysis of enzyme-catalysed hydrolysis, provided that the conditions are chosen where ¹⁸O incorporation without the presence of the enzyme is absent or undetectable. Such conditions were found and used in incubations of cellopentaose with the well-characterised endoglucanase Cel5A from Bacillus agaradhaerens. We were able to confirm that the preferred glycoside bond to be hydrolysed is the third one counting from the non-reducing end of the cellopentaose. Thus, cellopentaose prefers to bind from the –3 to the +2 subsites, which is in accordance with published crystallographic data. The main advantage of the method presented is that there is no need for a priori chemical modification/labelling of oligosaccharide substrates, which are processes that can disturb the enzyme–substrate interaction. From ¹⁸O incorporation we could demonstrate that the enzyme also has an oxygen-exchange activity on cellotriose and cellobiose. This is in agreement with the mechanism for transglycosylation and indicates that it is possible for the enzyme to perform such reactions.