title: Assigning structures to gas-phase peptide cations and cation-radicals. An infrared multiphoton dissociation, ion mobility, electron transfer, and computational study of a histidine peptide ion.
abstract: Infrared multiphoton dissociation (IRMPD) spectroscopy, using a free-electron laser, and ion mobility measurements, using both drift-cell and traveling-wave instruments, were used to investigate the structure of gas-phase peptide (AAHAL + 2H)(2+) ions produced by electrospray ionization. The experimental data from the IRMPD spectra and collisional cross section (Ω) measurements were consistent with the respective infrared spectra and Ω calculated for the lowest-energy peptide ion conformer obtained by extensive molecular dynamics searches and combined density functional theory and ab initio geometry optimizations and energy calculations. Traveling-wave ion mobility measurements were employed to obtain the Ω of charge-reduced peptide cation-radicals, (AAHAL + 2H)(+●), and the c(3), c(4), z(3), and z(4) fragments from electron-transfer dissociation (ETD) of (AAHAL + 2H)(2+). The experimental Ω for the ETD charge-reduced and fragment ions were consistent with the values calculated for fully optimized ion structures and indicated that the ions retained specific hydrogen bonding motifs from the precursor ion. In particular, the Ω for the doubly protonated ions and charge-reduced cation-radicals were nearly identical, indicating negligible unfolding and small secondary structure changes upon electron transfer. The experimental Ω for the (AAHAL + 2H)(+●) cation-radicals were compatible with both zwitterionic and histidine radical structures formed by electron attachment to different sites in the precursor ion, but did not allow their distinction. The best agreement with the experimental Ω was found for ion structures fully optimized with M06-2X/6-31+G(d,p) and using both projection approximation and trajectory methods to calculate the theoretical Ω values.
authors.affiliation: Department of Chemistry, University of Washington, Bagley Hall, Box 351700, Seattle, Washington 981915-1700, USA.
authors.forename: Christopher L
authors.initials: CL
authors.lastname: Moss
authors.forename: Julia
authors.initials: J
authors.lastname: Chamot-Rooke
authors.forename: Edith
authors.initials: E
authors.lastname: Nicol
authors.forename: Jeffery
authors.initials: J
authors.lastname: Brown
authors.forename: Iain
authors.initials: I
authors.lastname: Campuzano
authors.forename: Keith
authors.initials: K
authors.lastname: Richardson
authors.forename: Jonathan P
authors.initials: JP
authors.lastname: Williams
authors.forename: Matthew F
authors.initials: MF
authors.lastname: Bush
authors.forename: Benjamin
authors.initials: B
authors.lastname: Bythell
authors.forename: Bela
authors.initials: B
authors.lastname: Paizs
authors.forename: Frantisek
authors.initials: F
authors.lastname: Turecek
mesh_terms: D004579:Electron Transport; D005740:Gases / Q000737:chemistry*; D006639:Histidine / Q000737:chemistry*; D007477:Ions / Q000737:chemistry; D056004:Molecular Dynamics Simulation; D010455:Peptides / Q000737:chemistry* / Q000378:metabolism; D013055:Spectrophotometry, Infrared; D013816:Thermodynamics
chemical_list: D005740:Gases; D007477:Ions; D010455:Peptides; D006639:Histidine