Investigation of cation-pi interactions in biological systems

Noncovalent interactions, such as van der Waals interactions, hydrogen bonds, salt bridge and cation-Pi interactions play extremely important roles in biological systems and, in contrast to covalent bonds, many such noncovalent interactions are not well understood. In the present work a new protocol has been developed to measure the enhancement of binding energies due to cation-Pi interactions between aromatic amino acids and organic or metal ions. Investigation of the cation-Pi interactions will provide further insight into the structure and function of biological molecules.     

An LC-MS method for the determination of pharmaceutical compounds in wastewater treatment plant influent and effluent samples

Pharmaceuticals are continually introduced into the environment as a result of industrial and domestic use. In recent years they have emerged as environmental pollutants. An analytical method has been developed allowing for simultaneous detection and identification of 20 pharmaceutical compounds from various therapeutic classes using solid phase extraction (SPE) followed by liquid chromatography-electrospray ionisation mass spectrometry (LC-MS/MS). The limits of detection and limits of quantitation for the method were in the ng/L-microg/L range. The method was applied to influent and effluent samples from three wastewater treatment plants (WWTPs). Fifteen compounds were identified in the sample matrix with salicylic acid and ibuprofen being the most abundant at 9.17 and 3.20 microg/L respectively.     

Use of thin collodion films to prevent recoil-ion contamination of alpha-spectrometry detectors

Recoil ions from alpha-particle emission can contaminate surface-barrier detection systems. This contamination results in increased measurement uncertainty, and may require the replacement of expensive detectors. Disposable thin Collodion films are easily prepared and effectively retard the recoil ions when either directly applied to the surface of alpha-sources or as catcher foils between the source and the detector. The thin films are particularly effective for relatively low-level sources, but can sustain structural damage when exposed to high levels of recoil ions (tens of thousands per second) over extended periods of time.     

The predictably elusive form II of aspirin

The elusive form II of aspirin has been obtained during co-crystallization experiments with levetiracetam or acetamide, and it has been characterized by IR, DSC, HPLC, and single-crystal X-ray diffraction.     

Crystal engineering of pharmaceutical co-crystals from polymorphic active pharmaceutical ingredients

The carboxylic acid-primary amide supramolecular heterosynthon is exploited for the generation of pharmaceutical co-crystals that contain two active pharmaceutical ingredients that are polymorphic in their pure forms.     

Metastable and collision-induced fragmentation studies and thermochemistry of isomeric C(4)H(11)Si(+) ions and their adducts with C(4)H(12)Si silanes

Metastable Ion (MI) and collision-induced dissociation (CID) mass spectra for all isomeric even-electron [C(4)H(12)Si - H](+) ions were recorded and compared. Deuterium labeling experiments indicated that most precursors give rise to silylium ions. Silylium ions with two or more methyl groups are found to lose C(2)H(4) after isomerization via a straightforward hydrogen transfer to the appropriate ethylsilylium ion. Similarly, all isomeric propyl- and butyl-containing silylium ions are found to lose C(2)H(4) by rearrangement preceding dissociation. In the CI source of the mass spectrometer many of the silylium ions are found to cluster with the parent neutral silane present in the source to give stable [M - H](+)+M adduct ions. The MI and CID spectra of these adduct ions were also obtained. In the MI spectra of all adducts, except i-BuSiH(3), only the starting silylium ion is observed. Under CID conditions generation of silylium ions dominates. Deuterium labeling studies show that this dissociation may be accompanied by some rearrangement, in particular for the adducts from i-BuSiH(3). High-pressure mass spectrometric clustering equilibrium measurements were also carried out to determine the enthalpies and entropies of binding of the silylium ions to the neutral silanes. These measurements yield insight into the effects of various alkyl group substitutions on the association thermochemistry in these adducts. Copyright 2000 John Wiley & Sons, Ltd.     

Tren centered tris-macrocycles as polytopic ligands for Cu(II) and Ni(II)

Two novel symmetric polytopic ligands L(1) and L(2) have been synthesized. They are composed of three 1,4,8,11-tetraazacyclotetradecane macrocycles which are connected to a central tren moiety via an ethylene and a trimethylene bridge, respectively. The complexation potential and the speciation diagrams of L(1) and L(2) towards Cu(2+) and Ni(2+) were determined by spectrophotometric and potentiometric titrations. Insight into the geometry of the Cu(2+) complexes is provided by UV-VIS and EPR spectroscopy. The simplified ligands L(3) and L(4) are utilized as references for an aminoethyl- and a tren-substituted tetraaza macrocycle to help assign the EPR spectra of the polytopic ligands L(1) and L(2). At a metal-to-ligand ratio of 3 : 1, the metal cations are preferentially bound to the tetraaza macrocycles of L(1) and L(2) in a square planar geometry. At high pH values, a nitrogen atom of the tren moiety in L(1) serves as an additional ligand in an axial position leading to a square pyramidal coordination around Cu(2+), whereas in L(2) no such geometry change is observed. At a metal-to-ligand ratio of 4 : 1, the additional metal cation resides in the central tren moiety of L(1) and L(2). However, in contrast to the typical trigonal bipyramidal geometry found in the [Cutren](2+) complex, the fourth Cu(2+) has a square pyramidal coordination caused by the interaction with the Cu(2+) cations in the macrocycles (as evidenced by EPR spectra). Since the sequence of metal complexation is such that the first three metal ions always bind to the three macrocycles of L(1) and L(2) and the fourth to the tren unit, it is possible to prepare heteronuclear complexes such as [Cu(3)NiL](8+) or [Ni(3)CuL](8+), which can be unambiguously identified by their spectral properties.