Infrared multiphoton dissociation spectroscopy and ion mobility measurements were used to investigate the structure of gas-phase peptide (AAHAL + 2H)2+ ions produced by electrospray ionization. The experimental data were consistent with properties calculated for the lowest-energy peptide ion conformer obtained by extensive molecular dynamics searches and electronic structure calculations. Traveling-wave ion mobility measurements were employed to obtain the collision cross sections (Ω) of the charge-reduced peptide cation-radicals, (AAHAL + 2H)+●, and c and z fragment ions from electron-transfer dissociation (ETD) of (AAHAL + 2H)2+. The experimental Ω for the ETD charge-reduced and fragment ions were consistent with values calculated for ions that retained specific hydrogen bonding motifs from the precursor ion. These results show that the combination of multilevel theoretical calculations and ion mobility experiments is a powerful tool for assigning the structures of precursor ions and electron transfer intermediates and fragments.
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 Christopher L. Moss, Julia Chamot-Rooke, Edith Nicol, Jeffery Brown, Iain Campuzano, Keith Richardson, Jonathan P. Williams, Matthew F. Bush, Benjamin Bythell, Bela Paizs, Frantisek Turecek. J. Phys. Chem B 2012, 116, 3445–3456.