MH2+⋅ ion production from protonated polypeptides by electron impact: observation and determination of ionization energies and a cross-section

Irradiation of gas-phase MH+ ions of polypeptides up to MW 3493 by 11 to 70 eV electrons produced further ionization: MH++e−→MH2+·+2 e−, with ionization thresholds of 11.4±0.5 eV for [Arg-8]-vasopressin (MW 1084), 10.7±0.5 eV for substance P (MW 1347), 11.4±0.6 eV for renin substrate (MW 1759) and 10.6±0.4 eV for melittin (MW 2846). The ionization cross-section of vasopressin MH+ ions was found to be (1.3±0.4)·10−15 cm2 for 20 eV electrons. The gas-phase MH2+⋅ ions are rather stable at room temperature, with lifetimes of hundreds of seconds at 10−10 Torr.