Reactions of polypeptide ions with electrons in the gas phase.

Reactions of electrons in the energy range below 70 eV with polypeptide cations and anions are reviewed, as well as their applications for the structural analysis of polypeptides. At very low energies (</= 0.1 eV), the major outcome is electron-capture dissociation (ECD) of S-S and backbone N-C(alpha) bonds, leading to c' and z. fragments. ECD is useful in sequencing and characterization of post-translational modifications (PTMs), because c', z. fragmentation is abundant and the fragments usually retain labile groups. Electron capture at higher energies (3-13 eV) induces secondary fragmentation in radical z. fragments; this hot ECD (HECD) allows one to distinguish between the isomeric leucine and isoleucine residues. If a hot electron is not captured, then the induced electronic excitation converts internally into vibrational energy, resulting in fragmentation of the C(O)bond;N backbone bond (so-called EIEIO process). Above 9-10 eV, further ionization of n-charged cations occurs. If the formed (n + 1)+. cations capture electrons, then the C(alpha)bond;C backbone bond is usually broken. For anions that collide with approximately 20 eV electrons, the ejection of an electron leads to the creation of a radical positive charge (hole) that recombines internally with a negative charge. Such recombination leads to various backbone bond cleavages. This electron-detachment dissociation (EDD) is analogous to ECD for negative ions.