Evaluation of a novel approach for peptide sequencing: Laser-induced acoustic desorption combined with P(OCH₃)₂⁺ chemical ionization and collision-activated dissociation in a fourier transform ion cyclotron resonance mass spectrometer

Abstract

A novel mass spectrometric method has been developed for obtaining sequence information on small peptides. The peptides are desorbed as intact neutral molecules into a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR) by means of laser-induced acoustic desorption (LIAD). Reactions of the neutral peptides with the dimethoxyphosphenium ion, P(OCH 3 ) 2 ϩ , occur predominantly by addition of the peptide to P(OCH 3 ) 2 ϩ followed by the loss of two methanol molecules, thus yielding product ions with the composition (peptide ϩ P Ϫ 2H) ϩ . Upon sustained off-resonance irradiation for collision-activated dissociation (SORI-CAD), the (peptide ϩ P Ϫ 2H) ϩ ions undergo successive losses of CO and NH¢CHR or H 2 O, CO, and NH¢CHR to yield sequence-related fragment ions in addition to the regular a n -and b n -type ions. Under the same conditions, SORI-CAD of the analogous protonated peptides predominantly yields the regular a n -and b n -type ions. The mechanisms of the reactions of peptides with P(OCH 3 ) 2 ϩ and the dissociation of the (peptide ϩ P Ϫ 2H) ϩ ions were examined by using model peptides and molecular orbital calculations. (J Am Soc Mass Spectrom 2007, 18, 525-540)

Laser-Induced Acoustic Desorption Mass Spectrometry

“…The probe contains internal optics that direct the laser beam off-center to the foil attached to the probe. 23 This off-center focusing allows for rotation of the probe and a circular desorption pattern on the foil. Although the laser beam is off center at the foil, the plume of the desorbed molecules overlaps the center axis of the trap.…”

“…A maximum power of 25 mJ can be used for each pulse. 23 A major strength of LIAD compared to other laser desorption methods 14 is that the laser light never interacts with the analyte molecules, which makes this method much gentler than direct laser desorption. However, the need to generate strong acoustic waves makes LIAD a more complicated experiment.…”

“…10,11 This has been shown in our own laboratories for peptides 21 and oligonucleotides. 9,22,23 The mechanism that has generally been accepted for LIAD is the "shake-off" mechanism, which was used to explain LIAD in some of the first LIAD mass spectrometry experiments. 7 Based on this mechanism, the analyte molecules are pushed off by the sudden displacement of the foil surface caused by acoustic waves.…”

“…22 In many cases, the analytes have been ionized with minimal fragmentation via a careful selection of the ionization technique. Further, various large or thermally labile compounds, including peptides, 11,21,23,31,32 oligonucleotides 33,34 and steroids, 1,11,13,35 have been successfully desorbed using this technique. Very large particles, even entire cells, [36][37][38] have been desorbed by LIAD.…”

Gas Chromatographic-Ion Trap Mass Spectrometric Analysis of Volatile Organic Compounds by Ion–Molecule Reactions Using the Electron-Deficient Reagent Ion CCl3+

“…Furthermore, ion-molecule reactions are also regarded as a useful tool for solving challenging analytical problems because these reactions are often highly selective and extremely fast, as well as combined with chromatographic separation methods for qualitative analysis of target compounds in complicated samples. Recently, more studies have focused on the development of mass spectrometric methods based on structurally diagnostic ion-molecule reactions and MS/MS techniques for identification and structural characterization of isomeric compounds by using selective reagents [7][8][9][10][11][12][13][14][15][16][17][18][19]. The demand for fast, highly selective and sensitive ionmolecule reactions in analytical applications stimulated researchers' interest in searching for new alternative reagents used for such purposes [20,21].…”