Influence of a ring substituent on the tendency to form H₂O adducts to Ag⁺ complexes with phenylalanine analogues in an ion trap mass spectrometer

Abstract

In a previous report we showed that certain binary Ag(+)-amino acid complexes formed adduct ions by the attachment of a single water and methanol molecule when stored in an ion trap mass spectrometer: complexes with aliphatic amino acids and with 4-fluorophenylalanine formed the adduct ions whereas complexes with phenylalanine and tryptophan did not. In this study we compared the tendency of the Ag(+) complexes derived from phenylalanine, 4-fluorophenylalanine, 4-hydroxyphenylalanine (tyrosine), 4-bromophenylalanine, 4-nitrophenylalanine and aminocyclohexanepropionic acid to form water adducts when stored, without further activation, in the ion trap for times ranging from 1 to 500 ms. Because the donation of pi electron density to the Ag(+) ion is a likely determining factor in complex reactivity, our aim in the present study was to determine qualitatively the influence of para-position substituents on the aromatic ring on the formation of the water adducts. Our results show that the reactivity of the complexes is influenced significantly by the presence of the various substituents. Decreases in [M + Ag](+) ion abundance, and increases in adduct ion abundance, both measured as a function of storage time, follow the trend -NO(2) > -Br > -F > -OH > -H. The complex of Ag(+) with 4-nitrophenylalanine was nearly as reactive towards water as the Ag(+) complex with aminocyclohexanepropionic acid, the last being an amino acid devoid of pi character in the ring system. Collision induced dissociation of the [M + Ag](+) species derived from the amino acids produces, among other products, Ag(+) complexes with a para-substituted phenylacetaldehyde: complexes that also form adduct species when stored in the ion trap. The trends in adduct ion formation exhibited by the aldehyde-Ag(+) complex ions were similar to those observed for the precursor complexes of Ag(+) and the amino acids, confirming the influence of the ring substituent.

Gas-phase reactions of divalent Ni complex ions with acetonitrile: Chelate ring size, inductive, and steric effects

“…The trend indicates that as the charge on the metal decreases the reactivity of the complex decreases. This result is not surprising, and it agrees with our previous report [18] and the work of others [48,49] in which the ability of the ligand donor groups to donate electron density to the metal center was observed to directly affect complex reactivity. Essentially, greater metal-donor overlap, which leads to stronger metal-ligand bonds, decreases the charge on the metal and weakens the metal-CH 3 CN interaction, which leads to lower equilibrium constants.…”

Elucidation of the collision induced dissociation pathways of water and alcohol coordinated complexes containing the uranyl cation

“…During the isolation step, all ionized species except the one chosen for storage are resonantly ejected from the ion trap. The appearance of peaks 18 u higher than the isolated ion is indicative of the formation of H 2 O adducts [23][24][25]; the abundance of the adducts increased as the isolation and storage time was extended. The formation of abundant H 2 O adducts indicates that several of the uranyl complex ions show a significant tendency to accept ligands via gas-phase association reactions in the ion trap.…”

“…To gain a better understanding of the intrinsic interactions between different uranyl species and solvent, we have begun an investigation of uranyl-anion complexes in the gas phase using ion-trap mass spectrometry (IT-MS). Several recent reports have demonstrated that intrinsic metal and metal complex chemistry can be investigated by the (controlled) addition of reagent gas to an ion trap [12][13][14][15][16][17][18][19][20][21][22], or by way of the presence of H 2 O and other small molecule contaminants within the He bath gas used to collisionally cool ions and improve trapping efficiency [23][24][25]. The reactions of uranium ions in the gas phase have been the subject of several earlier investigations.…”

Intrinsic hydration of monopositive uranyl hydroxide, nitrate, and acetate cations

“…ESI-MS, multiple-stage CID and ion-molecule reactions were carried out using established procedures described elsewhere [22,24,26]. Briefly, ESI mass spectra were collected using a Finnigan LCQ-Deca ion-trap mass spectrometer (ThermoFinnigan Corporation, San Jose, CA).…”

“…To gain a better understanding of the intrinsic interactions between different uranium species and solvent, we have turned to investigations of uranyl-anion complexes in the gas phase. Recent studies have shown that ion-trap mass spectrometry can be used to probe intrinsic metal and metal-complex chemistry by exposing metal species to reagent gases deliberately added to the ion trap [9 -20], or to adventitious H 2 O present in the He bath gas used to collisionally cool ions and improve trapping efficiency [21][22][23][24]. Further motivating the present study is the fact that ion traps will shortly be used to characterize actinide speciation in samples from radioactive waste disposal sites, and a concise understanding of actinide behavior will be critical for correctly interpreting environmental data [25].…”