Pathways of Peptide Ion Fragmentation Induced by Vacuum Ultraviolet Light

Abstract

One Hundred Fifty-Seven nm photodissociation of singly protonated peptides generates unusual distributions of fragment ions. When the charge is localized at the C-terminus of the peptide, spectra are dominated by x-, v-, and w-type fragments. When it is sequestered at the N-terminus, a-and d-type ions are overwhelmingly abundant. Evidence is presented suggesting that the fragmentation occurs via photolytic radical cleavage of the peptide backbone at the bond between the ␣-and carbonyl-carbons followed by radical elimination to form the observed daughter ions. Low-energy fragmentation appears to be well described by the mobile proton model according to which vibrational excitation of the analyte leads to charge mobility [12]. Transfer of the charge proton to either the backbone carbonyl oxygen or amide nitrogen enables charge-induced cleavage of the peptide backbone. This process yields primarily b-and y-type fragment ions according to the standard nomenclature shown below [13,14].Higher energy activation methods involving collisions with gas molecules or surfaces can enable chargeremote fragmentation [15][16][17]. However, even in these cases, apparently, protonated peptides still generate some fragments through charge-directed processes [17]. Immobilization of the charge, either by using metal adducts [18] or charged chemical modifications [19 -22], has been used to inhibit charge-directed fragmentation. In these cases, primarily a-, d-, and w-type fragments are observed. Several mechanisms have been proposed for charge-remote peptide fragmentation, some involving homolytic radical cleavage [14,23].Electron capture dissociation (ECD) [24] and the recently reported electron-transfer dissociation (ETD)[25] appear to be nonthermal processes. These phenomena generate c-and z-type fragment ions upon the addition of an electron to a multiply charged protein or peptide ion. Two mechanisms have been proposed, one involving reactions of a free hydrogen atom generated when an electron is captured by a charged site on the analyte ion [26], and the other involving localization of the ϳ6 eV of energy that is generated upon charge neutralization. This energy then induces fragmentation through an excited electronic state [27]. Implicit in this second mechanism is the suggestion that techniques which excite appropriate electronic states can lead to unique, nonergodic fragmentation even for molecules as large as peptides and proteins [24,27]. This can occur if ions reach a dissociative electronic state and fragment before the energy is redistributed throughout the molecule. Electronic to vibrational relaxation, resulting in nonspecific excitation is an important competing process that often inhibits nonergodic processes, and rates of relaxation are predicted to increase with the size of the molecule [28].Lasers are powerful tools that can also be used to excite molecules to specific energy levels with either multiphoton or single photon processes. Light is not affected by the electric or magnetic fields of mass spectrometers, s...

Structural analysis of leukotriene C4isomers using collisional activation and 157 nm photodissociation

“…In this report, we specifically focus on photofragmentation results that are different from those obtained with CID experiments. As we reported in our earlier 157 nm photofragmentation experiments, with the current experimental configuration precursor depletion rates of ϳ25% have been obtained [22,23]. In some cases, dozens of different fragment ions form from a single precursor, and this significantly limits the intensity of each one.…”

Radical-driven dissociation of odd-electron peptide radical ions produced in 157 nm photodissociation

“…Previously we have demonstrated that 157 nm photodissociation of singly-charged peptide ions yields abundant a ϩ 1 radical ions [38,39]. In the present work, these odd-electron radical ions were isolated in an ion trap mass spectrometer to study their structure and fragmentation behavior.…”

Time-resolved photodissociation study of singly protonated peptides with a histidine residue generated by matrix-assisted laser desorption ionization: Dissociation rate constant and internal temperature

“…For small model peptide ions, investigations beyond simple mechanistic interpretation have been reported, such as the quantum chemical search for reaction paths [8 -10]. However, there has not been much study on kinetics of peptide ion dissociation [11].Tandem mass spectral patterns for protonated peptides are affected by factors, such as the charge state, the number of arginine residue, and the energy regime [2,3,12,13]. For singly protonated peptides-these will be called peptide ions from now on-without an arginine residue, b and y types (see reference [2] for product ion symbols) are the major product ions regardless of the energy regime.…”

“…For peptide ions with an arginine residue, b/y channels (rearrangement) are dominant in the lowenergy regime [3], such as in low-energy collisionally activated dissociation (CAD) and post-source decay (PSD). In the high-energy regime, such as in highenergy CAD [2] and ultraviolet photodissociation (UV-PD) [12][13][14], however, a new set of homolytic C ␣ -CO cleavage channels become dominant, even though the rearrangement channels still operate.Statistical theory of mass spectra (Rice-RamspergerKassel-Marcus theory, RRKM) [15] is known to provide an excellent description for the dissociation of small polyatomic ions. There have been some debates on whether RRKM would be valid for dissociation of large biomolecules [16].…”

“…Tandem mass spectral patterns for protonated peptides are affected by factors, such as the charge state, the number of arginine residue, and the energy regime [2,3,12,13]. For singly protonated peptides-these will be called peptide ions from now on-without an arginine residue, b and y types (see reference [2] for product ion symbols) are the major product ions regardless of the energy regime.…”