M. Tereza Fernandez

Iron and copper chelation by flavonoids: an electrospray mass spectrometry study

Flavonoids are well known as effective free radical scavengers exhibiting therefore an antioxidant behaviour. Another antioxidant mechanism however may result from the ability they have to chelate metal ions, rendering them inactive to participate in free radical generating reactions. Electrospray mass spectrometry has been used to study metal ion interactions with a set of flavonoids from different classes. Complexes with a range of stoichiometries, of metal: flavonoid, 1:1, 1:2, 2:2, 2:3 have been observed. The stoichiometry 1:2 is in general the preferred one. It is established for flavones and for the flavanone naringenin that the binding metal sites are preferentially at the 5-hydroxyl and 4-oxo groups. Redox reactions are also observed through the change of the oxidation state of the metal, jointly with the oxidation of the flavonoid by loss of hydrogen. Structures of the oxidized species of some flavonoids are proposed.

Thermodynamics of some proton-transfer reactions. Dynamic ion structures and the measurement of entropies of ‘internal translation’

Entropies well in excess of expected values have been measured for some protonated aromatic compounds (benzene, halogenobenzenes, halogeno-toluenes and xylenes). It is proposed that this occurs only when very facile proton migration is possible and that the large excess arises out of rapid ‘internal translation’ of the proton across a broad potential well within the molecule. An attempt has been made to model these systems, using a ‘particle-in-a-box’ approach to estimate the molecular partition functions. It was possible to generate sections of the potential-energy surfaces involved by using ab initio calculations at the 4–31G basis set level. The ideas presented are consistent with independent experimental evidence not based on thermodynamic measurements. In particular, the measurements and calculations involving protonated benzene in the gas phase can be compared with a low-temperature study in the liquid phase, using n.m.r. spectroscopy. Making some assumptions with regard to the total excess entropy available in the dynamic protonated species, it is possible to estimate barrier heights for proton migration. The general implications are that such barriers are probably somewhat lower than those currently predicted by the best ab initio methods and that the relative proton affinities measured are not ground-state values and may therefore appear to be significantly lower than expected. Although the idea of ‘internal translation’ has been used in the past in the context of the transition-state theory, the concept of a stable, but dynamic ion structure, as suggested here, is new.

Experimental and theoretical proton affinity of limonene

Gas-phase basicity (GB) and proton affinity (PA) of limonene were derived from measurements of proton transfer equilibria carried out by high pressure pulsed electron beam source mass spectrometry. Experimental GB and PA are 842±5 kJ mol-1 and 875±5 kJ mol-1, respectively. The proton affinity of C10H16 is compared to abinitio (HF and MP2) and density functional predictions for the protonation energy. Theoretical calculations based on density functional theory are in very good agreement with experimental results. Our best theoretical predictions for the enthalpy of protonation range from 869.6 kJ mol-1 (B3PW91/6-31G*) to 873.9 kJ mol-1 (BLYP/6-31G*). Theoretical calculations also suggest an important reorganisation of the molecular structure and charge distribution of limonene after protonation.

Study of doubly charged alkaline earth metal and 3-azidopropionitrile complexes by electrospray ionization mass spectrometry

The present work describes a study of the complexation of calcium and magnesium by 3-azidopropionitrile by means of electrospray ionization mass spectrometry (ESI-MS). Complexes were obtained from solutions of calcium and magnesium salts of the type CaX2 and MgX2 (where X = Cl or NO3) in water and methanol/water. The complexes detected were mainly double positively charged, with various stoichiometries not depending on the solvent, since water and 3-azidopropionitrile were always the main ligands. Solvation with methanol was not observed unlike in a previous study of complexation of nickel and cobalt by 3-azidopropionitrile. The complex ions [M(II)Az4(H2O)](2+), [M(II)Az5](2+) (where M = Ca and Mg) are the most abundant for both metals, and both counter ions. Tandem mass spectrometric (MS/MS) analysis showed that, under collision-induced dissociation (CID) conditions, the most important processes occurring were loss of neutral ligands and the replacement of 3-azidopropionitrile by water. A complex species containing reduced alkaline earth metal was due to radical loss, resulting from homolytic cleavage in the azide ligand. Some terminal ions, in the fragmentation sequences, point to the nitrile group as the coordination site in the 3-azidopropionitrile. Density functional theory (DFT) calculations confirmed this coordination site and proved that 3-azidopropionitrile behaves as a monodentate ligand in the systems under study. Moreover, the theoretical study proved that the presence of water ligand introduces stability through a hydrogen bond established between the water molecule and one nitrogen atom of the azido group. In addition, the strong dipole moment of 3-azidopropionitrile (4.76 D), which is mainly related to presence of the nitrile group, favors the stabilization of the metal-ligand complexes through charge-dipole interactions and the coordination of the metal to the nitrile group.

ELECTROSPRAY MASS SPECTROMETRY OF ANGIOTENSIN-CONVERTING ENZYME INHIBITORS

Electrospray ionisation (ESI) mass spectrometry combined with collision-induced decomposition of ions given by captopril, lisinopril and enalaprilat, angiotensin-converting enzyme (ACE) inhibitors, has been used to characterise these compounds. As expected, the ESI mass spectra are characterised by the presence of abundant [M + H]+ ions with very little fragmentation at low cone voltages. Fragment ions are produced by collision-induced decomposition of these ions at higher cone voltages, and in MS/MS experiments. Fragmentation pathways and structures of fragment ions observed have been proposed. Copyright

Thermodynamic studies of gas-phase proton transfer equilibria involving benzene: a reassessment of earlier data

Temperature-variable equilibrium constant measurements have been performed for a number of proton-transfer equilibria in which benzene was a partner, using a newly built high-pressure pulsed source mass spectrometer. Entropy values obtained showed that the protons in protonated benzene are not as mobile as previously thought. Systems involving ethanol are found to give anomalous, though self-consistent results owing to the onset of thermal decomposition. In the light of this, previous data involving halotoluenes and xylenes which appear to show unusually large increases in entropy on protonation, have been reassessed. There is evidence of proton-induced isomerisation. In the reaction [4-CIC6H4CH3]H+→[3-CIC6H4CH3]H+ the free energy of activation is derived to be ca. 90 kJ mol–1 from a computer model fit to the results, consistent with the energy calculated to be needed for a proton shift from the 3 to the 4 position in the precursor. The equivalent reaction for protonated 4-fluorotoluene has a barrier which is ca. 10 kJ mol–1 higher. A kinetic scheme is presented which shows how this could account for the observed ‘thermodynamic’ behaviour, and also give rise to the ‘isokinetic effects’ previously noted. There has therefore been some readjustment of the recommended proton affinity values for some of these compounds.

Observation and lifetime of autoionising states of Ar produced in a glow discharge ion source

Abstract In experiments to measure the bombardment energies of cathodic Ar ions in a glow discharge mass spectrometer ion source, ions were observed, dependent on the experimental conditions, which had been created by autoionisation of neutral metastable Ar in the accelerating region of the mass spectrometer. These excited Ar atoms are carried into the vacuum by the discharge gas escaping through the ion exit aperture. An expression is derived to describe the expansion of the gas into the vacuum and hence the relative decrease in gas density as the gas envelope moves away from the ion exit, giving the relative pressure, at a distance x along the direction of the ion beam, Px/P0 = S2/(S2 + x2), where S is the radius of the ion exit orifice. This is very similar to the form of the measured autoionisation profile. Two classes of metastable atom can be distinguished with lifetimes of about 1 μs and > 10 μs respectively. This is consistent with previous electron spectroscopy measurements [P. Marchand and J. Cardinal, Can. J. Phys., 57 (1979) 1624] and it may explain anomalous results reported for the energy distribution of cathodic Ar+ in earlier glow discharge studies [W.D. Davies and T.A. Vanderslice, Phys. Rev., 131 (1963) 219].

Gas-phase proton transfer in halogenotoluene mixtures. Evidence for a ‘dynamic’ ion structure

Gas-phase proton transfer equilibria between fluoro- and chloro-toluene mixtures have been studied as a function of temperature to determine relative proton affinities and entropy changes. Some of the entropy changes are much greater than any values previously reported for proton-transfer reactions. It is suggested that this can only be explained if the ortho- and para-halogenotoluene isomers have a ‘dynamic’ protonated ion structure in which a mobile hydrogen atom is shared by two or more unsubstituted ring carbon atoms. There is also an apparent linear correlation between the proton affinities measured and the corresponding entropy of proton bonding of these species. This ‘isokinetic’ relationship has a slope of 589 ± 50 K and is consistent with the suggestion in these compounds that the internal translational movement of the proton within the plane of the aromatic ring decreases the more tightly bound it becomes. This appears to be the first time such a relationship has been observed for gas-phase proton transfer. The following proton affinities relative to Ap(p-chlorotoluene) were measured: p-chlorotoluene, 0; p-fluorotoluene, 1.2 ± 0.2; o-chlorotoluene, 16 ± 0.8; o-fluorotoluene, 28 ± 1.5; m-chlorotoluene, 33.9 ± 1.4; m-fluorotoluene, 36.9 ± 1.4 kJ mol–1.

Azidoacetone as a complexing agent of transition metals Ni2+/Co2+ promoted dissociation of the CC bond in azidoacetone

The relevance of metal interactions with azides has led us to the study of the complexation of some transition metals, nickel and cobalt, by azidoacetone by means of electrospray ionization mass spectrometry (ESI-MS). Complexes were obtained from solutions of NiCl(2) and CoCl(2) , in methanol/water. Nickel was electrosprayed with other counter ion, bromide (Br), as well as other solvent (ethanol/water). For nickel and cobalt, the complexes detected were single positively charged, with various stoichiometries, some resulted from the fragmentation of the ligand, the loss of N(2) being quite common. The most abundant species were [Ni(II)Az(2)X](+) where X = Cl, Br and Az = azidoacetone. Some of the complexes showed solvation with the solvent components. Metal reduction was observed in complexes where a radical was lost, resulting from the homolytic cleavage of a metal coordination bond. Collision-induced dissociation (CID) experiments followed by tandem mass spectrometry (MS-MS) analysis were not absolutely conclusive about the coordination site. However, terminal ions of the fragmentation routes were explained by a gas-phase mechanism proposed where a C-C bond was activated and the metal inserted subsequently. Density functional theory calculations provided structures for some complexes. In [Ni(II)Az(2)X](+) species, one azidoacetone ligand is monodentate and the dominant binding location is the alkylated nitrogen and not the carbonyl group. The other azidoacetone ligand is bidentate showing coordination through alkylated nitrogen and the carbonyl group. These are also the preferential binding sites for the most stable isomer of [Ni(II)AzX](+) species.

On the way to understand antioxidants: chromanol and dimethoxyphenols gas-phase acidities

Some antioxidant mechanisms displayed by several phenolic compounds relate with OH bond dissociation energy. One way for its determination, in the gas-phase, relies on acidity measurements. Gas-phase acidities were determined experimentally, applying the kinetic method, for chromanol and four dimethoxyphenols, and theoretically through quantum chemical DFT calculations for chromanol, six dimethoxyphenols and 3,4,5-trimethoxyphenol. The experimental acidity order, 2,3-dimethoxyphenol > 3,5-dimethoxyphenol > 2,6-dimethoxyphenol > 3,4-dimethoxyphenol ≈ phenol > chromanol shows good agreement with the theoretical acidity order, 2,5-dimethoxyphenol > 2,3-dimethoxyphenol > 3,4,5-trimethoxyphenol > 3,5-dimethoxyphenol ≈ 2,4-dimethoxyphenol > 2,6-dimethoxyphenol > 3,4-dimethoxyphenol > phenol > chromanol. These acidity trends were rationalized in terms of the substituent effects on the thermodynamic stabilities both of the parent phenols and the corresponding phenoxide ions. DFT calculations also evidenced the occurrence of intramolecular C-H···O hydrogen bonds whenever there are vicinal substituents (either OH, O(-) or OCH(3)) which induce further stabilization of the geometries.