Characterization and differentiation of heterocyclic isomers. Part 2. Mass spectrometry and molecular orbital calculations on pyrrolo[1,2-a][1,4]benzodiazepin-4-one, -6-one, and -4,6-dione

Differentiation of heterocyclic regioisomers: a combined tandem mass spectrometry and computational study of N-acridin-4-ylbenzylamide and N-acridin-2-yl-benzylamide

Electrospray ionization (ESI) tandem mass spectrometry (MS/MS) has been used to differentiate two positional isomers of acridine derivatives, N-acridin-4-ylbenzylamide and N-acridin-2-ylbenzylamide. The study revealed that the isomeric ion structures produced by these heterocycles could be distinguished upon collision-induced dissociations (CID). In particular, the loss of a water molecule was shown to be a regiospecific reaction of the protonated N-acridin-4-ylbenzylamide, in which the location of the benzylamide substituent with respect to the acridinic nitrogen greatly assists proton migration by allowing the creation of intramolecular hydrogen bonds. To a lesser extent, the two isomers could also be distinguished by the difference in the abundance of the benzoyl cation in the MS/MS spectra of the [M+H]+ ions, as this ion is produced with a much higher rate from N-acridin-4-ylbenzylamide. Calculations based on quantum-mechanical models have been performed to evaluate the stability of the ion structures and to support mechanisms proposed for these two dissociation reactions.

Structural characterization and regiochemical differentiation of alpha-cyanoethylindole isomers in the gas phase

Mass spectrometry and tandem mass spectrometry techniques have been used to study the gas phase ion chemistry of isomeric alpha-cyanoethylindoles obtained by photochemical reactions. Both the fragmentation reactions occurring in the ion source, as well as metastable decompositions produced by the molecular and selected fragment ions, have allowed us to structurally characterize and differentiate each isomer from the others. The experiments, carried out also on deuterium labeled analogs, have shown the role exerted by the alpha-cyanoethyl substituent and by its position at the indolic ring on gas phase reactions that resulted to be distinctive and selective for each isomer. Ab initio theoretical calculations have been used to evaluate the stability and chemico-physical properties of different ion structures.

Gas phase ion chemistry of the heterocyclic isomers 3-methyl-1,2-benzisoxazole and 2-methyl-1,3-benzoxazole

Different mass spectrometric methods, stable isotope labeling, and theoretical calculations have allowed us to structurally characterize and differentiate the isomeric ion structures produced by the two heteroaromatic isomers 3-methyl-1,2-benzisoxazole and 2-methyl-1,3-benzoxazole. The low-energy collision induced dissociation spectra of their molecular ions show large differences. Although both of them produce abundant loss of CO, that involves a carbon atom of the benzene ring, the 2-methyl-1,3-benzoxazole also shows abundant [M-CHO](+) ions at m/z 104, the intensity of which is quite low in the case of its isomer 3-methyl-1,2-benzisoxazole. In addition, MS/MS measurements of fragment ions show characteristic differences that allow distinction among the isomers depending on the original arrangement of the atoms in the five-membered ring. Theoretical ab initio calculations have allowed to determine chemico-physical properties of different ions and to propose a rationalization of the decomposition pathways followed by the two benz(is)oxazole isomers.

Application of self-ionization for enhancing stereochemical and positional effects from arylglycosides under electron ionization conditions in an ion trap mass spectrometer

Ion trap mass spectrometry has been used to structurally characterize and differentiate positional and stereo isomers of arylglycosides having potential antioxidant properties. The use of the self-ionization (SI) technique has allowed to evidence a strong reactivity of fragment ions produced from dissociations of the molecular ion towards the molecules introduced into the trap. Specific structural effects due to positional isomers and anomers have been also envisaged through the occurrence of bimolecular processes inside the ion trap analyzer. Under self-ionization conditions, even-electron ions are produced. The charge is retained on the sugar moiety, in agreement with its proton affinity higher than that of the substituted phenol moiety. Most of the fragmentation pathways involve elimination of acetic acid that protects the hydroxylic groups of the glycoside. SI also produces adduct ions, likely as covalent species, having higher m/z values than the molecular ion. The reaction site is mainly the double bond present in the pyranosidic ring. Even if some fragment ions have lost the initial stereochemistry, their formation can be related to the structure of the parent neutrals introduced into the cell. Collision-induced dissociation (CID) experiments, carried out on ions formed by ion-molecule reactions, have allowed to obtain further information on gas phase ion structures. The study of mass-selected ion-molecule reactions and their kinetics have evidenced a spectacularly different reactivity of the ion at m/z 111 towards the two anomers 2alpha and 2beta, with the latter showing a much more pronounced reactivity. The approach developed in this work revealed to be an useful tool in structural characterization, as well as in stereo and regiochemical differentiation of arylglycosides.

Structural characterization and differentiation of modified isomeric tryptophans

Different mass spectrometry (MS) techniques have been applied to the study of modified tryptophan isomers obtained by photochemical reactions. The gas phase behavior of the molecular ions and the most abundant fragment ions produced under electron ionization has been selectively studied by MS/MS experiments. Both the fragmentation reactions occurring in the ion source, as well as those produced under collision-induced dissociation conditions have allowed to characterize and differentiate each isomer from the others. Investigation of a bisubstituted derivative has been useful in the rationalization of the gas phase behavior of this series of modified tryptophans. This study has allowed the evaluation of the role played by the substituents and their positions at the indolic ring on the gas phase decompositions that are distinctive and selective for each isomer. The occurrence of regiospecific reactions suggests that isomerization phenomena do not occur either in the molecular ions or in the main fragment ions in the gas phase.

Evidence for an aryl migration during the electron impact induced fragmentation of substituted aryloxymethylquinoxalines

The electron impact (EI) mass spectra of 34 differently substituted 2-phenoxymethyl-, 2-naphthyloxymethyl-, 2-pyridinyloxymethyl- and 2-chinolinyloxymethylquinoxalines were recorded. The fragmentation patterns were examined by metastable ion analysis and exact mass measurements, employing finally also selective deuterium labelling. The inclusion of the substituted aryl ring moiety appears to be important for the fragmentation of the aryloxymethylquinoxalines. A molecular ion rearrangement is proposed for the observed loss of OH* and CHO* radicals. The influence of the different substituents on the aryl ring moiety on the rearrangement in the gas phase and on the resulting fragmentation was investigated.