Establishing low-energy sequential decomposition pathways of leucine enkephalin and its N- and C-terminus fragments using multiple-resonance CID in quadrupolar ion guide

Abstract

The simultaneous resonant low-energy excitation of leucine enkephalin and its fragment ions was demonstrated in a collision cell of the multipole-quadrupole time-of-flight instrument. Using low-amplitude multiple-resonance excitation CID, we were able to show the exclusive sequential fragmentation of N-and C-terminus fragments all the way to the final fragmentsimmonium ions of phenylalanine or tyrosine. In this CID mode the single-channel dissociation of each new generation of fragments followed the lowest energy decomposition pathways observable on the time scale of our experiment. Up to six generations of sequential dissociation were carried out in multiple-resonance CID experiments. The direct qualitative comparison of fragmentation of axial-acceleration versus resonant (radial) CID was performed in the same instrument. In both activation methods, fragmentation patterns suggested complex decomposition mechanisms attributable to dynamic competition between sequential and parallel dissociation channels. R esonant harmonic excitation of ions is the primary method for MS detection and activation of induced dissociation in most ion traps. Simultaneous excitation of ions of different m/z ratios is routinely employed to illuminate dissociation pathways using ion traps. It can also be used in harmonic ion guides in order to selectively eject or dissociate ions on their way to an MS analyzer. This paper reports results of validation of a nontrapping multiple frequency component low-energy resonant collision-induced dissociation (CID) in a linear quadrupole ion guide.Several of the ion activation methods used in modern MS instruments are selective, by nature, and target exclusively the parent ion, supplying no additional energy to its primary fragments. As examples of such methods, the sustained off-resonance irradiation (SORI) [1], or on-resonance CID in ion cyclotron resonance (ICR) cells [2] can be mentioned. Other activation methods, such as infrared multi-photon dissociation (IRMPD), and its variations [3,4], or black body infrared dissociation (BIRD) [5] are nonselective, by nature, and always excite primary fragments along with the parent ion. A vast majority of ion activation methods may excite primary fragments depending on the condition of the experiment. In such methods, modeling of the dissociation process can be quite complex. For example, in surface induced dissociation (SID) experiments with moderate impact energies, the dissociation event most likely comprises activation of only the parent ion at the surface, followed by its recoil and subsequent unimolecular decomposition. In contrast, at high impact energies, more extensive fragmentation can be observed due to on-surface shattering of the precursor and possibly some of its primary fragments [6].Axial CID in ion guides (or in triple-quadrupoles) achieved by accelerating the primary ions into the collision cell operated at an elevated pressure is a potentially nonselective activation technique. If initial collisions of the projectile ion with the backgr...

Fragmentation of leucine enkephalin as a function of laser fluence in a MALDI TOF-TOF

“…The C-terminal ions observed, in order of increasing energy requirements, are: y 2 Ͻ GG ϩ / GGF ϩ Ͻ y 1 . A similar progression of energy requirements was reported by Harrison [6] over ESI source cone voltages in the range of 44 -62 V. The presence of the GF ϩ internal fragment was reported by only two other groups, Alexander and Boyd [14] in a CID spectrum with E LAB ϭ 70 eV with 90% attenuation of MH ϩ using a sector instrument and Wysocki and coworkers [44] under thermal degradation conditions for which the ions were at temperatures between 658 and 679 K. In general other studies appear to have been undertaken under CID [48,51] or thermal conditions [47], which were insufficiently energetic to allow decompositions to the extent of producing either these internal fragments or immonium ions.…”

Photodissociation of protonated leucine-enkephalin in the VUV range of 8–40 eV

“…Leu-enk has the additional advantage that it has already been investigated by a whole arsenal of mass-spectrometric tools. The obtained fragmentation pattern and yields will be interpreted in the context of existing data on leu-enk surface-induced dissociation (SID), 24 blackbody infrared radiative dissociation (BIRD), 25 laser-induced dissociation (LID), 26 multiphotoninduced 27 and collision-induced (CID) 28 mass spectra. The influence of the electronic structure of the peptide and its constituent amino acids on the dissociation and ionization will be discussed.…”

Revising the proton affinity scale of the naturally occurring α-amino acids

“…Recently, the PAs of amino acids, small peptides, and their derivatives have been successfully used [2][3][4][5][6][7] to explain some fragment ion abundance relationships in the low-energy tandem mass spectra of protonated peptides. Considering appropriate PAs, one can often predict whether the N-or the C-terminal fragment remains charged upon dissociation of the respective parent ions [2].…”