Analysis of carrageenans by matrix-assisted laser desorption/ionization and electrospray ionization mass spectrometry. I. kappa-Carrageenans.

Matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI) mass spectra of small kappa-carrageenans are reported and discussed. MALDI spectra can be obtained in both positive and negative ion mode. In the absence of extraneous metal ions, positive ions are formed by the attachment of one Na(+) ion to the carrageenan, whereas for negative ions one Na(+) ion is detached from the sulfate group. Multiply charged species are not observed in MALDI. Intense ESI spectra can be obtained in negative ion mode and now multiply charged species are seen. Alkali exchange experiments show that in these small carrageenan anions one, but only one, alkali metal ion is bound in a bidentate coordination with two ionic sulfate groups. G2-type ab initio calculations on model ions HO(-) [M(+)] (-)OH (M = Li, Na, K, Cs), as well as arguments based on a simple Coulombic interaction model, show that the bidentate stabilization energy drops rapidly as the size of the alkali cation increases. Exchange of Na(+) with Li(+) leads to expulsion of the Na(+) ion generating, in ESI, intense multiply charged anions. An attempt is made to rationalize this behavior in terms of hydration effects.

Characterization of ginseng saponins using electrospray mass spectrometry and collision-induced dissociation experiments of metal-attachment ions.

Electrospray mass spectrometry (ESMS) and collision-induced dissociation (CID) methodologies have been developed for the structural characterization of ginseng saponins (ginsenosides). Ginsenosides are terpene glycosides containing a triterpene core to which one to four sugars may be attached. They are neutral molecules which readily form molecular metal-attachment ions in positive ion ESMS experiments. In the presence of ammonium hydroxide intense deprotonated ions are generated. Both positive and negative ion ESMS experiments were found to be useful for molecular mass and structure determination of ten ginsenoside standards. Negative ion experiments made possible the determination of the molecular mass of each ginsenoside standard, the mass of the triterpene core and the masses and sequences of the sugar residues. Positive ion ESMS experiments with the alkali metal cations Li+ or Na+ and the transition metal cations Co2+, Ni2+ and Zn2+ were also useful in determining molecular masses. These alkali and transition metal cations form strongly bonded attachment ions with the ginsenosides. As a result, the CID mass spectra of the metal attachment ions show a variety of (structure characteristic) fragmentations. These experiments can be used to determine the identity of the triterpene core, the types and attachment points of sugars to the core and the nature of the O-glycosidic linkages in the appended disaccharides. Combining the results from the negative and positive ion experiments provides a promising approach to the structure analysis of this class of natural products.

The thiazole ylide: a frequently invoked intermediate is a stable species in the gas phase.

The 1, 2-hydrogen shift isomers of neutral (singlet and triplet) thiazole (1) and its radical cation have been investigated by a combination of mass spectro-metric experiments and hybrid density functional theory calculations. The latter were used to probe the structures and stabilities of selected C3 H3 NS and C3 H3 NS(.+) isomers and transition state structures. Although 3H-thiazole-2-ylidene (2) is less stable than 1, by 31.5 kcalmol(-1) , it is expected to be capable of independent existence, since the 1, 2-hydrogen shift from carbon to nitrogen involves a very large energy barrier of 72.4 kcalmol(-1) . The other 1, 2-hydrogen shift reaction from C(2) leads not to the expected cyclic 1H-thiazole-2-ylidene structure (3), which is apparently unstable, but rather to the ring-opened species HSCHCHNC (4), which is 34.5 kcalmol(-1) higher in energy than 1. The barrier in this case is lower but still large (54.9 kcalmol(-1) ). The triplet ground states of 1, 2 and 4 are considerably destabilised (69.5, 63.2 and 58.7 kcalmol(-1) ) relative to their singlet states. Interestingly, in addition to 2(.+) and 4(.+) , the cyclic radical cation 3(.+) is predicted to be stable although it is substantially higher in energy than ionised thiazole 1(.+) (by 53.9 kcalmol(-1) ), whereas 2(.+) and 4(.+) are much closer in energy (only 10.2 and 27.0 kcalmol(-1) higher, respectively). Dissuading 2(.+) and 3(.+) from isomerising to 1(.+) are energy barriers of 52.6 and 15.3 kcalmol(-1) , respectively. Experimentally, dissociative ionisation of 2-acetylthiazole enabled the generation of 2(.+) , which could be differentiated from 1(.+) by collisional activation mass spectrometry. Reduction of the ylide ion 2(.+) in neutralisation-reionisation mass spectrometry experiments yielded the corresponding neutral molecule 2. This direct observation of a thiazolium ylide provides support for postulates of such species as discrete intermediates in a variety of biochemical transformations.

1-Phenyl-5-vinyl-2-imidazolidinethione, a proposed causative agent of Spanish toxic oil syndrome: synthesis, and identification in one of a group of case-associated oil samples.

A method is described for the synthesis and characterization of N-(2-hydroxy-3-butenyl)-N'-phenylthiourea, and its cyclization product, 1-phenyl-5-vinyl-2-imidazolidinethione (PVIZT). Fourteen coded oil samples associated with toxic oil syndrome cases in Spain were examined by gas chromatography-electron impact mass spectrometry for the presence of PVIZT. Although these samples were obtained from households where cases of toxic oil syndrome had been recorded, they differed extremely with regard to their anilide and sulphur contents. In one sample PVIZT was detected at an estimated concentration of 1 mg/kg.

Isolation and identification of 25-hydroxydihydrotachysterol2 1 alpha,25-dihydroxydihydrotachysterol2 and 1 beta,25-dihydroxydihydrotachysterol2.

Three metabolites of orally administered dihydrotachysterol2 have been isolated in impure form from serum of rats. These metabolites have been identified as 25-hydroxydihydrotachysterol2 and two epimers of formula 1-ambo,25-dihydroxydihydrotachysterol2 by means of gas chromatography-mass spectrometry and ultraviolet absorption spectrometry. For the first time this provides evidence for 9,10-seco steroid hydroxylation at pseudo C3. The stereochemistry of the 1-hydroxyl group of the two epimers could be established tentatively by quantitative comparison of the mass spectra of their respective trimethylsilyl derivatives. Since purity requirements were not achieved, biological activities could not be determined.

Mass spectrometric investigation of copper chelates with 1-(2-pyridylazo)-2-naphthol.

The mass spectra of copper chelates of 1-(2-pyridylazo)-2-naphthol, HX, prepared by different methods, are discussed. Peaks due to Cu(2)X(2)(+), CuX(2)(+) and CuXCl(+) have been positively identified and all copper-containing species have the expected isotopic ratios. The significance of probe temperature and impurities are discussed and it is concluded that the spectra are more in accordance with the known chemical facts than those reported earlier by Betteridge and John.