Molecular emission cavity analysis—a new flame analytical technique: Part V. The determination of some sulphur anions

An investigation of the MECA behaviour of sulphate, sulphite, metabisulphite, peroxodisulphate, thiocyanate, thiosulphate and sulphide ions, has shown that ng amounts of these anions may be determined in samples of a few μl. Metal ion interferences in the determination of sulphate and sulphite are removed by addition of phosphoric acid.     

Internally contracted multiconfiguration-reference configuration interaction calculations for excited states

Summary The calculation of electronically excited states with the internally contracted multiconfiguration-reference configuration interaction (CMRCI) method is discussed. A straightforward method, in which contracted functions for all states are included in the basis, is shown to be very accurate and stable even in cases of narrow avoided crossings. However, the expense strongly increases with the number of states. A new method is proposed, which employs different contracted basis sets for each state, and in which eigensolutions of the Hamiltonian are found using an approximate projection operator technique. The computational effort for this method scales only linearly with the number of states. The two methods are compared for various applications.Dedicated in honor of Prof. Klaus Ruedenberg     

Analysis of various classes of drugs by capillary supercritical fluid chromatography

Three classes of drugs were screened for analysis feasibility by capillary column supercritical fluid chromatography. These included steroids, therapeutic antibiotic drugs, and drugs of abuse, such as cannabinoids. Supercritical fluid carbon dioxide was used as the mobile phase in conjunction with a methylpolysiloxane stationary phase capillary column and a flame ionization detector. All compounds considered were analyzed either as single component solutions, simple mixtures, or in actual complex mixtures.     

On damped linear dynamical systems

Summary This paper is concerned with damped linear dynamical systems which fail to satisfy the socalled convenience hypothesis, an assumption that guarantees the existence of classical normal modes. Under suitable conditions, estimates are given for the error committed when such a system is described approximately by a differential equation which does satisfy the convenience hypothesis. One of these estimates is used to derive an upper bound on the frequency response of systems which do not violate the convenience hypothesis too severely.

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Ein Beitrag zu den gedämpften, linearen, dynamischen Systemen

Übersicht Die Arbeit befaßt sich mit gedämpften, linearen, dynamischen Systemen, welche die Bequemlichkeitshypothese nicht erfüllen. Dabei wird untersucht, welchen Fehler man macht, wenn man eine allgemeine viskose Dämpfung durch eine zugeordnete modale Dämpfung entsprechend dieser Hypothese ersetzt. Eine der Fehlerabschätzungen wird verwendet zur Herleitung einer oberen Schranke der Antwortamplitude von Systemen, welche die Bequemlichkeitshypothese nicht allzu sehr verletzen.

     

Oxidation of guanosine derivatives by a platinum(IV) complex: internal electron transfer through cyclization

Many transition-metal complexes mediate DNA oxidation in the presence of oxidizing radiation, photosensitizers, or oxidants. The DNA oxidation products depend on the nature of the metal complex and the structure of the DNA. Earlier we reported trans-d,l-1,2-diaminocyclohexanetetrachloroplatinum (trans-Pt(d,l)(1,2-(NH(2))(2)C(6)H(10))Cl(4), [Pt(IV)Cl(4)(dach)]; dach = diaminocyclohexane) oxidizes 2'-deoxyguanosine 5'-monophosphate (5'-dGMP) to 7,8-dihydro-8-oxo-2'-deoxyguanosine 5'-monophosphate (8-oxo-5'-dGMP) stoichiometrically. In this paper we report that [Pt(IV)Cl(4)(dach)] also oxidizes 2'-deoxyguanosine 3'-monophosphate (3'-dGMP) stoichiometrically. The final oxidation product is not 8-oxo-3'-dGMP, but cyclic (5'-O-C8)-3'-dGMP. The reaction was studied by high-performance liquid chromatography, (1)H and (31)P nuclear magnetic resonance, and matrix-assisted laser desorption ionization time-of-flight mass spectrometry. The proposed mechanism involves Pt(IV) binding to N7 of 3'-dGMP followed by nucleophilic attack of a 5'-hydroxyl oxygen to C8 of G and an inner-sphere, 2e(-) transfer to produce cyclic (5'-O-C8)-3'-dGMP and [Pt(II)Cl(2)(dach)]. The same mechanism applies to 5'-d[GTTTT]-3', where the 5'-dG is oxidized to cyclic (5'-O-C8)-dG. The Pt(IV) complex binds to N7 of guanine in cGMP, 9-Mxan, 5'-d[TTGTT]-3', and 5'-d[TTTTG]-3', but no subsequent transfer of electrons occurs in these. The results indicate that a good nucleophilic group at the 5' position is required for the redox reaction between guanosine and the Pt(IV) complex.     

Structural analysis of phosphatidylcholines by post-source decay matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

The utility of post-source decay (PSD) matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) was investigated for the structural analysis of phosphatidylcholine (PC). PC did not produce detectable negative molecular ion from MALDI, but positive ions were observed as both [PC+H](+) and [PC+Na](+). The PSD spectra of the protonated PC species contained only one fragment corresponding to the head group (m/z 184), while the sodiated precursors produced many fragment ions, including those derived from the loss of fatty acids. The loss of fatty acid from the C-1 position (sn-1) of the glycerol backbone was favored over the loss of fatty acid from the C-2 position (sn-2). Ions emanating from the fragmentation of the head group (phosphocholine) included [PC+Na-59](+), [PC+Na-183](+) and [PC+Na-205](+), which corresponded to the loss of trimethylamine (TMA), non-sodiated choline phosphate and sodiated choline phosphate, respectively. Other fragments reflecting the structure of the head group were observed at m/z 183, 146 and 86. The difference in the fragmentation patterns for the PSD of [PC+Na](+) compared to [PC+H](+) is attributed to difference in the binding of Na(+) and H(+). While the proton binds to a negatively charged oxygen of the phosphate group, the sodium ion can be associated with several regions of the PC molecule. Hence, in the sodiated PC, intermolecular interaction of the negatively charged oxygen of the phosphate group, along with sodium association at multiple sites, can lead to a complex and characteristic ion fragmentation pattern. The preferential loss of sn-1 fatty acid group could be explained by the formation of an energetically favorable six-member ring intermediate, as apposed to the five-member ring intermediate formed prior to the loss of sn-2 fatty acid group.