Infrared Multiple Photon Dissociation Spectroscopy of Cationized Histidine: Effects of Metal Cation Size on Gas-Phase Conformation

Abstract

The gas phase structures of cationized histidine (His), including complexes with Li(+), Na(+), K(+), Rb(+), and Cs(+), are examined by infrared multiple photon dissociation (IRMPD) action spectroscopy utilizing light generated by a free electron laser, in conjunction with quantum chemical calculations. To identify the structures present in the experimental studies, measured IRMPD spectra are compared to spectra calculated at B3LYP/6-311+G(d,p) (Li(+), Na(+), and K(+) complexes) and B3LYP/HW*/6-311+G(d,p) (Rb(+) and Cs(+) complexes) levels of theory, where HW* indicates that the Hay-Wadt effective core potential with additional polarization functions was used on the metals. Single point energy calculations were carried out at the B3LYP, B3P86, and MP2(full) levels using the 6-311+G(2d,2p) basis set. On the basis of these experiments and calculations, the only conformation that reproduces the IRMPD action spectra for the complexes of the smaller alkali metal cations, Li(+)(His) and Na(+)(His), is a charge-solvated, tridentate structure where the metal cation binds to the backbone carbonyl oxygen, backbone amino nitrogen, and nitrogen atom of the imidazole side chain, [CO,N(α),N(1)], in agreement with the predicted ground states of these complexes. Spectra of the larger alkali metal cation complexes, K(+)(His), Rb(+)(His), and Cs(+)(His), have very similar spectral features that are considerably more complex than the IRMPD spectra of Li(+)(His) and Na(+)(His). For these complexes, the bidentate [CO,N(1)] conformer in which the metal cation binds to the backbone carbonyl oxygen and nitrogen atom of the imidazole side chain is a dominant contributor, although features associated with the tridentate [CO,N(α),N(1)] conformer remain, and those for the [COOH] conformer are also clearly present. Theoretical results for Rb(+)(His) and Cs(+)(His) indicate that both [CO,N(1)] and [COOH] conformers are low-energy structures, with different levels of theory predicting different ground conformers.

Structural characterization of gas-phase cysteine and cysteine methyl ester complexes with zinc and cadmium dications by infrared multiple photon dissociation spectroscopy

“…Zero point energy (ZPE) corrections scaled by 0.989 were determined in order to provide 0 K relative energies and thermal corrections to free energies at 298 K. For comparison to IRMPD experimental spectra, calculated frequencies were scaled by 0.975, as this factor leads to good agreement between calculated and experimentally well-resolved peaks and is consistent with previous IRMPD studies of metal-amino acid complexes. 13,[30][31][32][33][34] Calculated vibrational frequencies are broadened using a 15 cm À1 full width at half maximum Gaussian line shape for comparison with experimental spectra.…”

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

“…Compared with their CID spectra (generated on a Finnigan-LTQ-FT mass spectrometry (Thermo, Bremen, Germany), which has a low-mass cutoff of 50 Th), ours show more fragment ions, including the loss of neutral Trp giving the M + cation in high abundance for all the metals (Table 1 and Supporting Information, Figure S3). + upon IR photodissociation [35]. In an earlier publication, we reported detailed fragmentation reactions of protonated aromatic AAs [43].…”

“…The precursor ion scans (Tables S1, Supporting Information) show that this ion may be formed by one of two routes, through the initial loss of either NH 3 however, no mechanism was reported [35]. Reaction pathways showing the losses of NH 3 and CO 2 in different sequential orders are outlined in Scheme 1 (pathways 2 and 3).…”

“…Several groups have examined the structures of complexes of these amino acids with alkali metal [26,34,35], alkaline earth metal [36,37], silver, and other transition metals [38] The sample solution was continuously infused into the pneumatically assisted electrospray ionization source by means of a syringe pump (Harvard Apparatus, model 22; South Natick, MA, USA). Dry air was used as the nebulizer gas and the electrospray voltage was set to 5.5 kV.…”

Structure and further fragmentation of significant [a3 + Na − H]+ions from sodium-cationized peptides

“…It has been suggested that all available carbonyl oxygens are sodium binding sites in the most stable structures of sodium‐cationized peptides . As a result, the conformation of a sodiated peptide ion will be different from its protonated counterpart in which a proton often associates with one or two basic sites.…”