A molecular mechanics study of the selectivity of crown ethers for metal ions on the basis of their size

A molecular mechanics (MM) analysis is carried out on complexes of crown ethers CH₂(OCH₂CH₂)_nCH₂O, 12-crown-4 (n=3), 15-crown-5 (n=4), 18-crown-6 (n=5), 24-crown-8 (n=7), and 30-crown-11 (n=9) to determine the nature of the selectivity shown by these ligands for metal ions on the basis of metal ion size. The MM program used is SYBYL, and M-O bonds are represented using a covalent model, i.e. the M-O bonds are modelled with ideal M-O bond lengths and force constants. The previously used technique of calculating strain energy as a function of M-O bond length is used for all the complexes, and also the complexes of the non-macrocyclic polyethylene glycol analogues. It is concluded that the crown ethers fall into three groups with regard to selectivity for metal ions. Group one consists of the smaller macrocycles such as 12-crown-4 and 15-crown-5, where metal ions generally are too large to enter the cavity of the macrocycle, and the metal ion is coordinated lying outside the plane of the donor atoms of the ligand. Here factors that control selectivity are the same as in non-macrocyclic ligands, chiefly the size of the chelate ring. Group 2 contains only 18-crown-6 of the ligands studied here. 18-Crown-6 complexes have three important conformers, one of which, theD _3d , shows sharp size match selectivity, preferring metal ions with M-O bond lengths of about 2.9 . The other two conformers are adopted by metal ions too small for theD _3d conformer, and are more flexible, exerting little size-match selectivity. These other two conformers are of higher energy than theD _3d conformer for metal ions with M-O bond lengths greater than 2.55 . Thus, a genuine size match selectivity is found for K⁺ with 18-crown-6. With an ideal M-O bond length of 2.88 , K⁺ fits the cavity of theD _3d conformer of 18-crown-6 very closely. The third group consists of very large macrocycles such as 24-crown-8 and 30-crown-10. These enfold the metal ion in extremely folded conformations, but may, as does 30-crown-10, exert considerable selectivity for metal ions on the basis of their size by virtue of the conformation resulting in a set of torsional angles in the ring atoms of the macrocycle which confer considerable rigidity on the ligand.     

INAA of fluorine in plastics using the medium-flux McMaster Nuclear Reactor

The fluorine contents of plastics, ranging from about 20 μg·g⁻¹ to 66%, may be measured instrumentally using a conventional research nuclear reactor, an automated sample irradiation and counting system, and a set of well-calibrated, in-house, fluorine standards. Plastics with low to medium fluorine contents may be analyzed using ²⁰F by placing the gamma-ray detector at appropriate distances from the irradiated sample. For high-F plastics, samples may be irradiated in a cadmium lined irradiation site, using ¹⁹O and ²⁰F. Counting statistics of <3% translate into reproducibility of measurements within ±3% and analytical accuracies of ±1% to ±10%.     

Free flow electrophoresis coupled with liquid chromatography-mass spectrometry for a proteomic study of the human cell line (K562/CR3)

The requirement for prefractionation in proteomic analysis is linked to the challenge of performing such an analysis on complex biological samples and identifying low level components in the presence of numerous abundant housekeeping and structural proteins. The employment of a preliminary fractionation step results in a reduction of complexity in an individual fraction and permits more complete liquid chromatography/mass spectrometry (LC/MS) analysis. Free flow electrophoresis (FFE), a solution-based preparative isoelectric focusing technique, fractionates and enriches protein fractions according to their charge differences and is orthogonal in selectivity to the popular reversed phase high performance liquid chromatography (HPLC) fractionation step. In this paper, we explored the advantages of a combination of FFE and liquid chromatography/mass spectrometry to extend the dynamic range of a proteomic analysis of a complex cell lysate. In this study, the whole cell lysate of a chronic myelogeneous leukemia cell line, K562/CR3, was prefractionated by FFE into 96 fractions spanning pH 3-12. Of these, 35 fractions were digested with trypsin and then analyzed by LC/MS. Depending on the algorithm used for peptide assignment from MS/MS data, at least 319 proteins were identified through database searches. The results also suggested that pI could serve as an additional criterion besides peptide fragmentation pattern for protein identification, although in some cases, a pI shift might indicate post-translational modification. In summary, this study demonstrated that free flow electrophoresis provided a useful prefractionation step for proteomic analysis and when combined with LC/MS allowed the identification of significant number of low level proteins in complex samples.     

Mass spectrometric analysis of chemical warfare agents and their degradation products in soil and synthetic samples

A packed capillary liquid chromatography-electrospray ionization mass spectrometry (LC-ESI-MS) method was developed for the identification of chemical warfare agents, their degradation products and related compounds in synthetic tabun samples and in soil samples collected from a former mustard storage site. A number of organophosphorus and organosulfur compounds that had not been previously characterized were identified, based on acquired high-resolution ESI-MS data. At lower sampling cone voltages, the ESI mass spectra were dominated by protonated, sodiated and protonated acetonitrile adducts and/or their dimers that could be used to confirm the molecular mass of each compound. Structural information was obtained by inducing product ion formation in the ESI interface at higher sampling cone voltages. Representative ESI-MS mass spectra for previously uncharacterized compounds were incorporated into a database as part of an on-going effort in chemical warfare agent detection and identification. The same samples were also analyzed by capillary column gas chromatography (GC)-MS in order to compare an established method with LC-ESI-MS for chemical warfare agent identification. Analysis times and full-scanning sensitivities were similar for both methods, with differences being associated with sample matrix, ease of ionization and compound volatility. GC-MS would be preferred for organic extracts and must be used for the determination of mustard and relatively non-polar organosulfur degradation products, including 1,4- thioxane and 1,4-dithiane, as these compounds do not ionize during ESI-MS. Diols, formed following hydrolysis of mustard and longer-chain sulfur vesicants, may be analyzed using both methods with LC-ESI-MS providing improved chromatographic peak shape. Aqueous samples and extracts would, typically, be analyzed by LC-ESI-MS, since these analyses may be conducted directly without the need for additional sample handling and/or derivatization associated with GC-MS determinations. Organophosphorus compounds, including chemical warfare agents, related compounds and lower volatility hydrolysis products may all be determined during a single LC-ESI- MS analysis. Derivatization of chemical warfare agent hydrolysis products and other compounds with hydroxyl substitution would be required prior to GC-MS analysis, giving LC-ESI-MS a definite advantage over GC-MS for the analysis of samples containing chemical warfare agents and/or their hydrolysis products.     

Mass spectral rearrangements of N-(4′-arylthio-2′-butynyl) N-(2″-arylthio-2″-propenyl)-p-toluidines,N,N-bis(2′-arylthio-2′-propenyl)-p-toluidines and 1,2-bis(arylthio)acenaphthenes

Under electron impact the title compounds display, in addition to the sequential loss of the arylthio groups, the elimination of a bisaryl disulphide moiety, but they do not eliminate sulphur. The behaviour of the p-toluidine derivatives supports the rearrangement pathway proposed earlier for N,N-bis(4′-arylthio-2′-butynyl)anilines.     

N,N'-Ethylenedi-L-cysteine (EC) and Its Metal Complexes: Synthesis, Characterization, Crystal Structures, and Equilibrium Constants

N,N'-ethylenedi-L-cysteine (EC) and its indium(III) and gallium(III) complexes have been synthesized and characterized. The crystal structures of the ligand and the complexes have been determined by single-crystal X-ray diffraction. EC.2HBr.2H(2)O (C(8)H(22)Br(2)N(2)O(6)S(2)) crystallizes in the orthorhombic space group P2(1)2(1)2 with a = 12.776(3) ?, b = 13.735(2) ?, c = 5.1340 (10) ?, Z = 2, and V = 900.9(3) ?(3). The complexes Na[M(III)EC].2H(2)O (C(8)H(16)MN(2)O(6)S(2)Na) are isostructural for M = In and Ga, crystallizing in the tetragonal space group P4(2)2(1)2 with the following lattice constants for In, (Ga): a = 10.068(2) ?, (9.802(2) ?), b = 10.068(2) ?, (9.802(2) ?), c = 14.932(2) ?, (15.170(11) ?), Z = 4 (4), and V = 1513.6(5) ?(3), (1457.5(11) ?(3)). In both metal complexes, the metal atoms (In and Ga) are coordinated by six donor atoms (N(2)S(2)O(2)) in distorted octahedral coordination geometries in which two sulfur atoms and two nitrogen atoms occupy the equatorial positions, and the axial positions are occupied by two oxygen atoms of two carboxylate groups. The structures of the complexes previously predicted by molecular mechanics are compared with the crystal structures of the Ga(III) and In(III) complexes obtained experimentally. In contrast to the oxygen donors in phenolate-containing ligands, such as 1,2-ethylenebis((o-hydroxyphenyl)glycine) (EHPG) and N,N'-bis(o-hydroxybenzyl)ethylenediamine-N,N'-diacetic acid (HBED), the thiolate donors of EC enhances affinity for In(III) relative to Ga(III). The following stability sequence has been obtained: In(III) > Ga(III) > Ni(II) > Zn(II) > Cd(II) > Pb(II) > Co(II). Evidence was also obtained for several protonated and hydroxo species of the complexes of both divalent and trivalent metals, where the corresponding protonation constants (K(MHL)) decrease with increasing stability of the chelate, ML(n)(-)(4), where M(n)()(+) represent the metal ion.     

Analysis of O-ethyl S-[2-(diisopropylamino)ethyl] methylphosphonothiolate (VX) and its degradation products by packed capillary liquid chromatography-electrospray mass spectrometry

Packed capillary column liquid chromatography (LC)-electrospray mass spectrometry (ESI-MS) was used for the first time to detect and identify O-ethyl, S-[2-(diisopropylamino)ethyl] methylphosphonothiolate (VX) and its degradation products, including compounds containing a P-CH3 bond, bis(diisopropylamino)thioalkanes and ureas commonly employed as VX stabilizers. The reported ESI-MS data were generally acquired with a higher sampling cone voltage, a setting that promoted collisionally activated dissociation, and resulted in the acquisition in informative mass spectra containing both molecular and product ion information. The developed method appears to be an attractive alternative to GC-MS for the analysis of aqueous sample containing the degradation products of VX, since they may be analysed directly with little risk of thermal decomposition and without the need for additional sample handling or derivatization. Application of this method to a degraded VX sample resulted in the detection of a number of novel polar and higher-molecular-mass degradation products, not previously associated with VX during GC-MS analysis.