A. J. Ferrer-Correia

Keggin-type polyoxotungstates as catalysts in the oxidation of cyclohexane by dilute aqueous hydrogen peroxide

Abstract Oxidation of cyclohexane by hydrogen peroxide in the presence of catalytic amounts of the Keggin-type heteropolytungstates [PW 11 O 39 ] 7− and [PW 11 M(L)O 39 ] (7− m )− , M m + =first row transition metal cation, L=H 2 O or CH 3 CN, was found to produce cyclohexanol, cyclohexanone and, in certain cases, cyclohexyl hydroperoxide. The presence of the latter was demonstrated by negative chemical ionization GC-MS. The reactions were carried out in acetonitrile, using tetra n -butylammonium salts of the catalysts and aqueous 30% hydrogen peroxide as oxidant. The polyanions [PW 11 O 39 ] 7− and [PW 11 Fe(H 2 O)O 39 ] 4− showed higher catalytic activity and different selectivity for the oxidation of cyclohexane than did the corresponding Cu-, Co-, Mn- and Ni-substituted complexes.

Identification of oxidation products and free radicals of tryptophan by mass spectrometry

New oxidation products and free radicals derived from tryptophan (Trp) oxidation under Fenton reaction conditions were identified using mass spectrometry. After the oxidation of tryptophan using hydrogen peroxide and iron (II) system (Fenton reaction), mono- and dihydoxy tryptophans and N-formylkynurenine were identified using electrospray mass spectrometry (ES-MS) and ES-MS/MS. Besides these products, new products resulting from the reaction of tryptophan and oxidized tryptophan and 3-methyl indole derivatives were also identified. The 3-methyl indole derivatives resulted, most probably, from the oxidation process and not from in-source processes. A dimer formed by cross-linking between two Trp radicals (Trp-Trp), similar to the previously described tyrosine dimer was observed, as well as the corresponding monohydroxy-dimer (Trp-Trp-OH). Tandem mass spectrometry was used to identify the structures of these new oxidation products. Free radicals derived from tryptophan oxidation under Fenton reaction were detected using as spin trap the DMPO. The free radical species originated during the oxidation reaction formed stable adducts with the spin trap, and these adducts were identified by ES-MS. New adducts of oxidized tryptophan radicals, namely monohydroxy-tryptophan and dihydroxy-Trp dimer radicals, with one and two DMPO spin trap molecules where identified. Tandem mass spectrometry was used to confirm the proposed structure of the observed adducts.

Positive and negative electrospray ionisation tandem mass spectrometry as a tool for structural characterisation of acid released oligosaccharides from olive pulp glucuronoxylans.

Xylo-oligosaccharides with degrees of polymerisation 5-13, formed by partial acid hydrolysis from an extract representative of olive pulp glucuronoxylans (GX), were analysed by electrospray ionisation mass spectrometry (ESI-MS), both in positive and negative modes. The positive spectrum showed the presence of xylo-oligosaccharides in the mass range between m/z 500 and 1500 corresponding to singly [M+Na](+) charged ions of neutral (Xyl(7-9)) and acidic xylo-oligosaccharides (Xyl(5-9)MeGlcA), and doubly [M+2Na](2+) charged ions of Xyl(9-13) and Xyl(7-11)MeGlcA. Ammonium adducts [M+NH(4)](+) were also observed for Xyl(5-9)MeGlcA. The negative spectra showed the contribution of ions in the mass range between m/z 600 and 1400, ascribed to the deprotonated molecules [M-H](-) of Xyl(3-9)MeGlcA. Tandem mass spectrometry (MS/MS) of the major ions observed in the MS spectra was performed. The MS/MS spectra of the [M+Na](+) adducts showed the loss of MeGlcA residues as the major fragmentation pathway and glycosidic fragment ions of Xyl(n) and Xyl(n)MeGlcA structures. The MS/MS spectra of the [M+NH(4)](+) adducts suggests the occurrence of isomers of Xyl(5-9)MeGlcA oligosaccharides with the MeGlcA residue at the reducing end and at the non-reducing end of the molecules, although other structural isomers can also occur. Both glycosidic bond and cross-ring cleavages in the MS/MS spectra of the [M-H](-) ion suggest the occurrence of Xyl(3-9)MeGlcA with the substituting group at the reducing end position of the xylose backbone, as the main fragmentation ions. The results obtained by ESI-MS/MS, both in positive and negative modes, of Xyl(7-13)- and Xyl(5-11)MeGlcA, allow to identify fragmentation patterns of the structural isomers with MeGlcA linked to the terminal xylosyl residues of the oligosaccharides. The occurrence of these higher molecular weight oligosaccharides with a low substitution pattern allows to infer a scatter and random distribution of MeGlcA along the xylan backbone of olive pulp.

Structural characterisation by MALDI-MS of olive xylo-oligosaccharides obtained by partial acid hydrolysis

Abstract Alkali extractable olive pulp and olive seed hull glucuronoxylans were submitted to mild acid hydrolysis originating low molecular weight xylo-oligosaccharides. The xylo-oligosaccharides thus formed were partially separated by size exclusion chromatography (Biogel P6) and several elution fractions were identified by MALDI-MS. The MALDI mass spectra obtained revealed the presence of several xylo-oligosaccharides ions in each fraction between the m / z ranges 500–2500. Based on the sugar and methylation analyses of the glucuronoxylans, the proposed structures are the neutral xylo-oligosaccharides (Xyl n , n =4–18) and acidic xylo-oligosaccharides substituted by glucuronic acid residues (Xyl n GlcA, n =3–16 and Xyl n GlcA 2 , n =2–14), by 4- O -methyl-glucuronic acid residues (Xyl n MeGlcA, n =3–14 and Xyl 2 MeGlcA 2 ) and by both acid residues (Xyl n GlcAMeGlcA, n =4–7). The identification of ions corresponding to long blocks of neutral and acidic oligosaccharides suggested that the acidic residues were irregularly distributed along the xylan backbone in both olive pulp and olive seed hull. In addition, MALDI mass spectra showed that the acidic xylo-oligosaccharides from the pulp were predominantly constituted by Xyl n MeGlcA oligosaccharides, while the acidic xylo-oligosaccharides from the seed hull were predominantly constituted by Xyl n GlcA and Xyl n GlcA 2 oligosaccharides.

Detection and Characterization by Mass Spectrometry of Radical Adducts Produced by Linoleic Acid Oxidation

The formation of linoleic acid radical species under the oxidative conditions of the Fenton reaction (using hydrogen peroxide and Fe (II)) was monitored by FAB-MS and ES-MS using the spin trap 5,5-dimethyl-1-pyrrolidine-N-oxide, DMPO. Both the FAB and ES mass spectra were very similar and showed the presence of ions corresponding to carbon- and oxygen centered spin adducts (DMPO/L·, DMPO/LO·, and DMPO/LOO·). Cyclic structures, formed between the DMPO oxygen and the neighboring carbon of the fatty acid, were also observed. Electrospray tandem mass spectrometry of these ions was performed to confirm the proposed structure of these adducts. All MS/MS spectra showed an ion at m/z 114, correspondent to the [DMPO+H]+, and a fragment ion due to loss of DMPO (loss of 113 Da), confirming that they are DMPO adducts. ES-MS/MS spectra of alkoxyl radical adducts (DMPO/LO·) showed an additional ion at m/z 130 [DMPO−O+H]+, while ES MS/MS of peroxyl radical adducts (DMPO/LOO·) showed a fragment ion at m/z 146 [DMPO−OO+H]+, confirming both structures. Other fragment ions were observed, such as alkyl acylium radical ions, formed by cleavage of the alkyl chain after loss of water and the DMPO molecule. The identification of fragment ions observed in the MS/MS spectra of the different DMPO adducts suggests the occurrence of structural isomers containing the DMPO moiety both at C9 and C13. The use of ES tandem mass spectrometry, associated with spin trapping experiments, has beee shown to be a valuable tool for the structural characterization of carbon and oxygen-centered spin adducts of lipid radicals.

Detection and characterization of hydroxyl radical adducts by mass spectrometry

The study of the influence of free radicals in the biological process depends primarily on the capacity to detect these reactive species. In this work we have studied the application of mass spectrometry to the identification of hydroxyl radical species. The detection and identification by collisional activation mass-analyzed ion kinetic energy spectrometry (CA-MIKES) of a spin adduct of DMPO with the hydroxyl radical [(DMPO + O) + H]+ (m/z 130) has demonstrated that mass spectrometry can be a powerful tool in the detection and identification of spin adducts of DMPO with hydroxyl radical species. We were also able to detect the capture of secondary free radicals using ethanol by detecting and identifying the corresponding adduct [(DMPO + ethanol) + H]+. Other spin adducts have also been detected and identified. We consider that the use of mass spectrometry is a relevant technique for the detection of free hydroxyl radicals, especially in complex mixtures, since mass spectrometry is able to discriminate these adducts in such situations. Moreover, using this approach, it was possible to identify new spin adducts.

Do charge-remote fragmentations occur under matrix-assisted laser desorption ionization post-source decompositions and matrix-assisted laser desorption ionization collisionally activated decompositions?

The precursor ions of tetraphenylporphyrins that are substituted with fatty acids can be introduced into the gas phase by matrix-assisted laser desorption ionization (MALDI) and undergo post-source and collisionally activated decompositions (CAD) in a time-of-flight mass spectrometer. The goal of the research is to obtain a better understanding of post-source decompositions (PSD); specifically, we asked the question of whether ions undergoing PSD have sufficient energy to give charge-remote fragmentations along an alkyl chain. We chose the porphyrin macrocycle because we expected it to act as an inert “support,” allowing the molecule to be desorbed by MALDI and to be amenable to charge-remote fragmentation. MALDI-PSD and MALDI-CAD spectra are similar to high-energy CAD spectra and considerably more informative than low-energy CAD spectra, showing that charge-remote fragmentations of the fatty acid moieties do occur upon MALDI-PSD and MALDI-CAD.

Constant neutral loss scanning for the characterization of glycerol phosphatidylcholine phospholipids

The product-ion spectra of the sodiated molecules of glycerol phosphatidylcholine phospholipids (GPC) were obtained, using fast-atom bombardment (FAB) as the ionization method, in a tandem mass spectrometer. The product-ion spectra of these sodiated molecules of GPCs were found to differ significantly from those of the protonated GPC molecules. This difference is due to the absence of the ion of m/z 184 (protonated-phosphocholine moiety) and to the presence of an ion resulting from the loss of trimethylamine (m=59 Da) from the polar head group, which is the dominant fragmentation. This characteristic neutral loss provides a means of identification of this class of phospholipids and of differentiation from other phospholipid classes in complex mixtures by performing a constant-neutral-loss scan of 59.

Differentiation of positional isomers of nitro meso-tetraphenylporphyrins by tandem mass spectrometry.

We studied by tandem mass spectrometry two isomers of nitro meso-tetraphenylporphyrin, one with a nitro group in the para position of a phenyl ring and the other with the same group in a β-pyrrolic position, and their copper complexes. Collisional activation of the molecular ions of both free-base porphyrins and of their copper complexes produces an array of product ions that permit ready differentiation of the two positional isomers. The diagnostic ions, when the nitro group is in a β-pyrrolic position, may be produced through intramolecular and double cyclization processes, triggered by the interaction of the nitro substituent with the neighboring meso-phenyl ring. These diagnostic ions do not form when the nitro group is in the para position. The gas-phase processes have precedents in solution chemistry.

High- and Low-Energy Collisionally Activated Decompositions of Octaethylporphyrin and Its Metal Complexes

High-energy (HE) and low-energy (LE) collisionally activated decompositions of octaethylporphyrin (OEP) and its metal complexes (ZnOEP and CuOEP) depend on whether the precursor is produced by electrospray ionization as protonated molecules or by fast atom bombardment as radical cations or protonated molecules. LE activation leads to such simple product-ion spectra that a complete picture of fragmentation emerges only after nine stages of tandem mass spectrometry (MS9). HE activation, on the other hand, gives product-ion spectra that afford an integrated view of all the decomposition channels in a single MS/MS experiment. These results are the basis of a recommendation that OEP is an appropriate model compound for investigating energy effects in the collisional activation of organic and bioorganic molecule ions.