Reactions of polypeptide ions with electrons in the gas phase

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Abstract
I. Introduction 58 A. Challenges in the Gas‐Phase Fragmentation of Polypeptides 58 B. Ion–Electron Interactions 58 C. Nomenclature 59 II. Instrumental Realization and Applications 60 III. Electron‐Capture Dissociation 61 A. Electron Capture 61 B. NCα Bond Cleavage 62 C. SS Bond Reduction 64 D. Dissociation of Strong Bonds in the Presence of Weak Bonding 65 E. Frequencies of NCα Bond Cleavage 67 IV. Electronic Excitation 68 A. Hot‐Electron‐Capture Dissociation (HECD) 68 B. Electronic Excitation Dissociation (EED) 69 V. Vibrational Excitation 70 A. Vibrational Excitation of Even‐Electron Species 70 B. Vibrational Excitation of Radical Species 71 VI. Electron Detachment 72 A. Ionization 72 B. Electron‐Detachment Dissociation (EDD) 73 VII. Conclusions 74 References 74 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Cα 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)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 CαC backbone bond is usually broken. For anions that collide with ≈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. © 2003 Wiley Periodicals, Inc., Mass Spec Rev 22:57–77, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com)

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