Hilde Van den Heuvel

Internal glucose residue loss in protonated O-diglycosyl flavonoids upon low-energy collision-induced dissociation

The low-energy collision-induced dissociation of protonated flavonoid O-diglycosides, i.e., flavonoid O-rutinosides and O-neohesperidosides, containing different aglycone types has been studied. The results indicate that the unusual [M + H − 162]+ ion formed by internal glucose residue loss, which in a previous study was shown to be a rearrangement ion, is strongly dependent upon the aglycone type. For 7-O-diglycosides, the internal glucose loss is very pronounced for aglycones of the flavanone type, but is completely absent for aglycones of the flavone and flavonol types. Internal glucose residue loss was found to correspond to a minor fragmentation pathway for flavonol 3-O-diglycosides. A plausible mechanism is proposed based on proton mobilization from the aglycone to the disaccharidic part of the flavonoid O-diglycosides which is supported by theoretical calculations and model building.

Mechanistic Aspects of Charge-remote Fragmentation in Saturated and Mono-unsaturated Fatty Acid Derivatives. Evidence for Homolytic Cleavage

The high-energy collision-induced dissociation (CID) of [M+Li]+ ions of n-butyl ester derivatives of palmitic acid and oleic acid as well as 9,9-2H2-palmitic acid and 11,11-2H2-oleic acid has been studied in order to obtain information on the charge-remote fragmentation mechanism of saturated and mono-unsaturated fatty acid ions containing a stable charge centre. The results obtained in the present study indicate that homolytic cleavage reactions, involving C—H cleavage as an initial rate-determining step, operate during the charge-remote fragmentation observed for high-energy CID of [M+Li]+ ions of n-butyl palmitate and correspond to a major fragmentation route. With respect to the charge-remote fragmentation of n-butyl oleate, our 2H-labelling results point to the same mechanism, involving an initial C—H cleavage at allylic positions, for the formations of ions corresponding to a formal homo-allylic cleavage, and are also consistent with a direct allylic C—C cleavage for the formation of ions due to a formal allylic C—C cleavage. These results, however, do not exclude the possibility of other minor homolytic fragmentation pathways for the formation of ions involving formal allylic and homo-allylic cleavages.

Triterpenoid saponins from Maesa lanceolata

Six new homologous triterpenoid saponins were isolated from the methanol extract of the leaves of Maesa lanceolata and characterized as 3 beta-O-[alpha-L-rhamnopyranosyl(1 --> 2)-beta-D-galactopyranosyl (1 --> 3)]-[beta-D-galactopyranosyl(1 --> 2)]-beta-D-glucuronopyranosides alpha-diol, 22 alpha-angeloyloxy-16 alpha-butanoyloxy-13 beta,28-oxydoolean-21 beta,28 alpha-diol, 16 alpha,22 alpha-diangeloyloxy-13 beta,28-oxydoolean-21 beta,-28 alpha-diol, 22 alpha-angeloyloxy-13 beta,28-oxydo-16 alpha-(2-methyl-butanoyloxy)-olean-21 beta,28 alpha-diol, 21 beta-acetoxy-22 alpha-angeloyloxy-13 beta,28-oxydo-16 alpha-propanoyloxyolean-28 alpha-ol, 21 beta-acetoxy-22 alpha-angeloyloxy-16 alpha-butanoyloxy-13 beta,28-oxydoolean-28 alpha-01. The structures were established on the basis of chemical and spectral evidence.

Charge‐remote and charge‐proximate fragmentation processes in alkali‐cationized fatty acid esters upon high‐energy collisional activation. A new mechanistic proposal

The effect of the metal ion on the high-energy collision-induced dissociation (CID) of alkali metal-cationized n-butyl and methyl ester derivatives of palmitic and oleic acid was examined. The results show that the alkali metal ion has a pronounced effect and does not act as a mere ‘spectator’ ion with respect to the fragmentation process. While C–H cleavage is a dominant process for [M+Li]+ as well as [M+Na]+ precursor ions, C–C cleavage is also significant for the [M+Na]+ ions. Homolytic mechanisms involving the formation of a transient biradical cation are proposed which enable us to rationalize in a straightforward manner all product ions formed by both charge-remote and charge-proximate fragmentations. The mechanistic proposal is discussed in view of available knowledge on electron impact, CID and related processes. In order to predict how the alkali metal ion could affect the reactivity of the postulated biradical state formed following electronic excitation of the alkali metal-cationized molecules, quantum chemical calculations were performed on methyl and n-butyl acetate as model substances. The decreased spin density at the carbonyl oxygen atom in the biradical state may provide an explanation for the greater tendency towards C–C cleavage reactions of the sodium-cationized fatty acid esters relative to the corresponding lithium complexes.

Comparison of high- and low-energy collision-induced dissociation tandem mass spectrometry in the analysis of glycoalkaloids and their aglycons

Four aglycons (tomatidine, demissidine, solanidine, and solasodine) and three glycoalkaloids (α-tomatine, α-chaconine, and α-solanine) have been analyzed by positive ion liquid secondary ion high-energy and low-energy collision-induced dissociation (CID) tandem mass Spectrometry, performed on a four-sector (EBEB) and a hybrid (EBQQ) instrument, respectively. Both high- and low-energy collision-induced dissociation mass spectra of [M+H]+ ions of these compounds provided structural information that aided the characterization of the different aglycons and of the carbohydrate sequence and linkage sites in the glycoalkaloids. Low-energy CID favors charge-driven fragmentation of the aglycon rings, whilst high-energy CID spectra are more complex and contain additional ions that appear to result from charge-remote fragmentations, multiple cleavages, or complex charge-driven rearrangements. With respect to the structural characterization of the carbohydrate part, low-energy CID fragmentations of sugar residues in the glycoalkaloids generate Yn+ ions and some low intensity Zn+ ions; the high-energy spectra also exhibit strong 1,5Xn+ ions, formed by multiple cleavage of the sugar ring, and significant Zn+ ions.

The Application of Liquid Chromatography-Electrospray Ionization Mass Spectrometry and Collision-Induced Dissociation in the Structural Characterization of Acylated Flavonol O-Glycosides from the Seeds of Carrichtera Annua:

The flavonoid fraction from the seeds of Carrichtera annua was studied using high-performance liquid chromatography simultaneously coupled to a photodiode array detector (LC/UV-DAD) and a mass spectrometer equipped with an electrospray source (LC/ESI-MS). Collision-induced dissociation (CID) mass spectral data obtained off-line by nanospray (nano-ESI) analysis provided a wealth of complementary structural information, which was consistent with structures established by NMR or led to the proposal of base structures of the flavonol O-glycosides present in the Carrichtera annua seed extract. The flavonoid fraction was found to contain 12 structurally related flavonol O-glycosides. Eleven flavonoids, of which several were new compounds, were acylated with one or more benzoyl, feruloyl or sinapoyl groups. These acyl groups gave rise to characteristic product ions in the [M + H]+ and [M + Na]+ CID spectra as well as to radicalar acid-related product ions at high-energy collisional activation. In addition to the characterization of the acyl substituents, the mass spectral data allowed the identification of the aglycone, the determination of the base structure and the differentiation of several positional isomers.

Charge-remote fragmentation characteristics of functionalized alkanes in high-energy collision- induced dissociation

In this study we report on high-energy, collision-induced dissociation processes leading to charge-remote fragmentations, using three alkyl cations, namely n-hexadecylpyridinium, n-hexadecyltriphenylphosphonium and n-hexadecyltriethylammonium, each with and without (2)H(2)-labelling at the C(9) position of the hexadecyl chain. The characteristic patterns corresponding to the formal elimination of alkane elements were observed, and the (2)H(2)-labelling at C(9) clearly affected only one charge-remote fragment ion of the homologous series. However, in addition to the expected fragment ion containing only one deuterium atom, a significant ion retaining two deuterium atoms was observed. MS/MS/MS experiments demonstrated clearly that the latter ion showed partial deuteration around the charge site, the level of deuteration depending on the structure of the original precursor cation. These results can be interpreted in terms of two novel, distinct mechanisms, one of which involves an excited state in an aromatic ring. Mixed-site fragmentation (MSF) ions were also observed from the phosphonium and ammonium ion precursors. We believe that the observation of the MSF process occurring at an sp(2)-hybridized center in the phosphonium series has not been reported previously. It thus becomes apparent that high-energy collisions leading to charge-remote reactions in fact lead to a broad range of pathways. Copyright 2000 John Wiley & Sons, Ltd.

Comparison of Low- and High-energy Collision-induced Dissociation Tandem Mass Spectrometry in the Analysis of Ricinoleic and Ricinelaidic Acid

Ricinoleic acid and itstransisomer ricinelaidic acid, and their methyl esters, were analyzed by low- and high-energy collision-induced dissociation (CID) tandem mass spectrometry. It is shown that a stable charge centre is required to observe the rearrangement, which occurs in the β-hydroxyalkene part and results in the loss of heptaldehyde, and that this rearrangement corresponds to a low-energy CID reaction. Differences between the CID behaviour of ricinoleic acid and that of itstransisomer are related to the loss of water, which can also be regarded as a low-energy rearrangement reaction. Comparison of low- and high-energy CID spectra further revealed that high-energy CID gives rise to loss of a H•radical and H2 together with low-energy fragmentation. Examination of different molecule ions, including [M-H]-, [M+Li]+ and [M-H+2Li]+ ions of free fatty acids and [M+Li]+ ions of the methyl esters shows that charge-remote homolytic fragmentation is most pronounced for the [M+Li]+ ions of the methyl esters.

CHARACTERIZATION OF OZONATED C60 AND C70 BY HIGH PERFORMANCE LIQUID CHROMATOGRAPHY AND LOW- AND HIGH-ENERGY COLLISION-INDUCED DISSOCIATION TANDEM MASS SPECTROMETRY

The formation of C60 and C70 oxides by reaction with ozone has been examined using reversed-phase high-performance liquid chromatography and mass spectrometry including liquid secondary ion mass spectrometry, collision-induced dissociation (CID) and tandem mass spectrometry. C60 and C70 oxides containing up to four oxygen atoms could be detected and were found to be chemically unstable. The mass spectral results show that low-energy CID of molecular anions of the higher C60 and C70 oxides results in the loss of one CO molecule, whereas molecular cations eliminate two molecules of CO. In both ion modes the formation of odd-numbered fullerenes was noticed. In contrast, high-energy CID results in the loss of O and O2, followed by sequential C2 losses. For some product ions the formation of fullerene–helium adduct ions was observed.