Gas-phase fragmentation of the Ag⁺—phenylalanine complex: Cation—π interactions and radical cation formation

Abstract

Collision-induced dissociation experiments on the Ag ϩ -phenylalanine complex using several collision energies were shown to yield ten different fragment ions. Unambiguous assignment of these fragment ions were made by careful analysis of deuterium labeling experiments. The losses of H 2 O, CO, CO 2 , and AgH were commonly observed; also encountered were the losses of H 2 , Ag, and H. Deuterium labeling experiments and density functional calculations have been employed to probe fragmentation mechanisms that account for all experimental results. (J Am Soc Mass Spectrom 2002, 13, 408 -416) © 2002 American Society for Mass Spectrometry T he interactions of metal ions with amino acids and peptides in the gas phase is a topic of much current interest [1][2][3]. Attempts have been made to further our understanding of complicated biological processes by modeling them using carefully planned gas-phase studies. In the absence of solvent, one may study the intrinsic modes of binding governing metalamino acid complexes. Mass spectrometric experiments thus offer great potential for exploring the formation and reactivity of isolated cationic species.Amino acid complexes of transition metal ions were first observed and studied using fast atom bombardment (FAB) [4,5]. However, the limited solubility of metal salts and poor sensitivity prevented widespread systematic study of these complexes. Sensitivity is not typically an issue in matrix-assisted laser desorption/ ionization (MALDI) [6,7], but the fragmentation yields of its metastably decomposing ions are often poor. As the fragment ions potentially give information on atom connectivity and hence structure, MALDI is therefore non-ideal for examining fragmentation pathways. One technique that has proven effective for the study of metal-containing amino acids and peptides, and the one employed here, is electrospray tandem mass spectrometry. First, electrospray ionization [8] is a soft ionization method and one that is efficient at producing gas-phase metal-containing ions [9 -15], whose solubility in water or water/methanol is typically high. Second, fragmentation is efficient and is easily controlled via collision-induced dissociation, the energy of which is crucial in directing fragmentation products and their yield.Molecular orbital calculations are frequently combined with experiments in studies to obtain a better understanding of the structure and thermochemistry of the organometallic species of interest [1,2,16,17]. Recent density functional molecular orbital studies have shown that Ag ϩ can be mono-, di-, or tricoordinate in complexes with ␣-amino acids [18]; tetracoordinate Ag ϩ has been postulated for relatively small peptides [19,20]. The binding of silver(I) to glycine, diglycine, and triglycine, and to a number of other polypeptides, has been investigated [19,20]. In particular, the structures of argentinated glycine and its oligomers have been examined in detail by means of density functional theory [21].The focus of this paper is the Ag ϩ -phenylalanine complex. ...

Dissociations of Complexes Between Monovalent Metal Ions and Aromatic Amino Acid or Histidine

“…Coordination of the metal ion to NH 2 is first broken, followed by a proton transfer to the NH 2 group from the carboxyl group to produce the most stable zwitterionic form, which also is a tridentate complex, but with an O/O/ring binding motif [40]. NH 3 can then be displaced via neighboring group participation by the aromatic ring to give I, a phenonium-type ion [15,44,45]. The indole ring of tryptophan provides the most charge-delocalization and, perhaps for this reason, Trp gives the highest abundance of [(AA -NH 3 ) + M)] + ions.…”

“…The elimination of neutral silver hydride, AgH, from the [Phe + Ag] + complex has been previously reported [15,17] [56], so it is surprising that only this aromatic amino acid produces a radical cation. Furthermore, the ionization energy of Ag is 7.57 eV [57], suggesting that dissociation of [His + Ag] + should yield predominantly Ag + , the second most abundant signal as shown in Table 1.…”

“…The binding of Ag + to amino acids and peptides is of interest to the gas-phase chemistry community. A frequently employed technique to examine the structures and chemistries of these complexes is collision-induced dissociation (CID) under tandem mass spectrometry (MS/MS) conditions [12][13][14][15]. The binding energies of Ag + to all twenty essential amino acids have been theoretically determined [16] and the dissociations of the Ag + -Phe and Ag + -Pro complexes have been examined in detail [14,15,17].…”

“…A frequently employed technique to examine the structures and chemistries of these complexes is collision-induced dissociation (CID) under tandem mass spectrometry (MS/MS) conditions [12][13][14][15]. The binding energies of Ag + to all twenty essential amino acids have been theoretically determined [16] and the dissociations of the Ag + -Phe and Ag + -Pro complexes have been examined in detail [14,15,17]. These studies revealed the presence of noncovalent binding between Ag + and the π-systems of amino acids (e.g., Phe, despite the fact that the interaction with Ag + may not be viewed strictly as cation-π interaction as a result of significant d-orbital participation in the binding.…”

Gas-Phase Intramolecular Cyclization of Argentinated N-Allylbenzamides

“…In the gas-phase chemistry of numerous argentinated organic ligands, product ions retaining the silver ion are generally observed [23][24][25][26][27] except when elimination of AgR (R = CH 3 , ph) [27,28] or AgH [24][25][26][27][28][29][30] takes place. In the case of argentinated amines, aminocarboxylic acids, ethers, or compounds possessing at least one α-hydrogen to the amino nitrogen or ether oxygen, the α-hydrogen tends to cleave with the silver in a 1,2-elimination [28][29][30].…”