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Keywords:

  • peptides;
  • ionization energy;
  • odd-electron ions;
  • electron impact;
  • fourier transform ion cyclotron resonance

Abstract

Ionization energies (IE) of [M + zH]z+ (z+) electrospray-produced polypeptides were determined by electron ionization in a Penning cell of 4.7 and 9.4 T Fourier transform mass spectrometers. For z = 1+ and substance P, the found IE value of 11.0 ± 0.4 eV is in agreement with that obtained earlier for ions generated with matrix-assisted laser desorption/ionization. For higher z, the following values were found: 11.7 ± 0.3 eV for 2+ of [Arg-8]-vasopressin, 11.1 ± 0.6 eV for 2+ of substance P, 12.2 ± 0.7 eV for 2+ of renin substrate, 13.3 ± 0.4 eV for 3+ of B-chain of insulin and 14.6 ± 0.6 eV for 4+ and 15.1 ± 0.4 eV for 5+ of melittin. It was found that 90% of existing IE data on polypeptides in the 1.0–3.5 kDa mass range are described with ≤0.5 eV uncertainty by the empirical equation IE(z) = 9.8 + 1.1z. The average IE increase of 1.1 eV/charge is attributed to Coulombic repulsion. The deduced ionization energy of a neutral polypeptide molecule, 9.8 ± 0.3 eV, is consistent with literature expectations. Copyright © 2002 John Wiley & Sons, Ltd.