3 research outputs found
Ultraviolet Laser Induced Hydrogen Transfer Reaction: Study of the First Step of MALDI In-Source Decay Mass Spectrometry
The early mechanisms of matrix-assisted laser desorption/ionization
in-source decay (MALDI-ISD) are described herein. MALDI-ISD is initiated
by the hydrogen transfer from excited matrix molecules to the carbonyl
oxygen of the peptide backbone, which is followed by a radical-induced
cleavage, producing the <i>c</i>ā²/<i>z</i>ā¢ fragment pair. As expected, the use of 2,5-DHB or 1,5-DAN
was efficient to induce MALDI-ISD, and the strongest intensity of
MALDI-ISD fragments was observed when laser shots were performed on
matrix crystals. In contrast, the hydrogen radical transfer reaction
was suppressed by using ionic liquid and amorphous structure of 2,5-DHB
and 1,5-DAN mixture as a matrix. Our results suggest that the hydrogen
transfer occurs on the matrix crystal during the dissipation of the
laser energy and before desorption, following ISD fragments formed
in the MALDI plume
In-Source Decay during Matrix-Assisted Laser Desorption/Ionization Combined with the Collisional Process in an FTICR Mass Spectrometer
The type of ions detected after in-source
decay (ISD) in a MALDI
source differs according to the ion source pressure and on the mass
analyzer used. We present the mechanism leading to the final ISD ions
for a Fourier transform-ion cyclotron resonance mass spectrometer
(FTICR MS). The MALDI ion source was operated at intermediate pressure
to cool the resulting ions and increase their lifetime during the
long residence times in the FTICR ion optics. This condition produces
not only <i>c</i>ā², <i>z</i>ā², and <i>w</i> fragments, but also <i>a</i>, <i>y</i>ā², and <i>d</i> fragments. In particular, <i>d</i> ions help to identify isobaric amino acid residues present
near the N-terminal amino acid. Desorbed ions collide with background
gas during desorption, leading to proton mobilization from Arg residues
to a less favored protonation site. As a result, in the case of ISD
with MALDI FTICR, the influence of the Arg residue in ISD fragmentation
is less straightforward than for TOF MS and the sequence coverage
is thus improved. MALDI-ISD combined with FTICR MS appears to be a
useful method for sequencing of peptides and proteins including discrimination
of isobaric amino acid residues and site determination of phosphorylation.
Additionally we also used new software for in silico elimination of
MALDI matrix peaks from MALDI-ISD FTICR mass spectra. The combination
of high resolving power of an FTICR analyzer and matrix subtraction
software helps to interpret the low <i>m</i>/<i>z</i> region of MALDI-ISD spectra. Finally, several of these developed
methods are applied in unison toward a MALDI ISD FTICR imaging experiment
on mouse brain to achieve better results
Selected Protein Monitoring in Histological Sections by Targeted MALDI-FTICR In-Source Decay Imaging
Matrix-assisted laser desorption/ionization mass spectrometry
imaging
(MALDI MSI) is a rapidly growing method in biomedical research allowing
molecular mapping of proteins on histological sections. The images
can be analyzed in terms of spectral pattern to define regions of
interest. However, the identification and the differential quantitative
analysis of proteins require off line or in situ proteomic methods
using enzymatic digestion. The rapid identification of biomarkers
holds great promise for diagnostic research, but the major obstacle
is the absence of a rapid and direct method to detect and identify
with a sufficient dynamic range a set of specific biomarkers. In the
current work, we present a proof of concept for a method allowing
one to identify simultaneously a set of selected biomarkers on histological
slices with minimal sample treatment using in-source decay (ISD) MSI
and MALDI-Fourier transform ion cyclotron resonance (FTICR). In the
proposed method, known biomarkers are spotted next to the tissue of
interest, the whole MALDI plate being coated with 1,5-diaminonaphthalene
(1,5-DAN) matrix. The latter enhances MALDI radical-induced ISD, providing
large tags of the amino acid sequences. Comparative analysis of ISD
fragments between the reference spots and the specimen in imaging
mode allows for unambiguous identification of the selected biomarker
while preserving full spatial resolution. Moreover, the high resolution/high
mass accuracy provided by FTICR mass spectrometry allows the identification
of proteins. Well-resolved peaks and precise measurements of masses
and mass differences allow the construction of reliable sequence tags
for protein identification. The method will allow the use of MALDI-FTICR
MSI as a method for rapid targeted biomarker detection in complement
to classical histology