Collision-induced ionization of neutral peptides

Pseudo-MS3 in a MALDI orthogonal quadrupole-time of flight mass spectrometer

Both the matrix selected and the laser fluence play important roles in MALDI-quadrupole/time of flight (QqTOF) fragmentation processes. "Hot" matrices, such as alpha-cyano4-hydroxycinnamic acid (HCCA), can increase fragmentation in MS spectra. Higher laser fluence also increases fragmentation. Typical peptide fragment ions observed in the QqTOF are a, b, and y ion series, which resemble low-energy CID product ions. This fragmentation may occur in the high-pressure region before the first mass-analyzing quadrupole. Fragment ions can be selected by the first quadrupole (Q1), and further sequenced by conventional MS/MS. This allows pseudo-MS3 experiments to be performed. For peptides of higher molecular weight, pseudo-MS3 can extend the mass range beyond what is usually accessible for sequencing, by allowing one to sequence a fragment ion of lower molecular weight instead of the full-length peptide. Peptides that predominantly show a single product ion after MS/MS yield improved sequence information when this technique is applied. This method was applied to the analysis of an in vitro phosphorylated peptide, where the intact enzymatically-generated peptide showed poor dissociation via MS/MS. Sequencing a fragment ion from the phosphopeptide enabled the phosphorylation site to be unambiguously determined.

Electron loss from multiply protonated lysozyme ions in high energy collisions with molecular oxygen

We report on the electron loss from multiply protonated lysozyme ions Lys-Hn(n)+ (n = 7 - 17) and the concomitant formation of Lys-Hn(n+1)+. in high-energy collisions with molecular oxygen (laboratory kinetic energy = 50 x n keV). The cross section for electron loss increases with the charge state of the precursor from n = 7 to n = 11 and then remains constant when n increases further. The absolute size of the cross section ranges from 100 to 200 A2. The electron loss is modeled as an electron transfer process between lysozyme cations and molecular oxygen.

Do amines react with protonated peptides in the gas phase via transacylation reactions to induce peptide bond cleavage?

The proposal that protonated peptides react with NH(3) in the gas phase via transacylation reactions (Tabet et al., Spectros. Int. J. 5: 253 1987) has been investigated by studying the reactions of the fixed charge derivatives [RC(O)NMe(2)CH(2)CO(2)H](+) (R=Me and Ph) with pyridine and triethylamine and the reactions of protonated glycine oligomers and leucine enkenphalin with butylamine. Under the near thermal conditions of the quadrupole ion trap, both the fixed charge derivatives as well as the protonated peptides react with the amines via either proton transfer or proton bound dimer formation. Collision induced dissociation of protonated peptides in the presence of butylamine yields b(n) and y(n) sequence ions as well as [b(n) + BuNH(2)](+) and [y(n) + BuNH(2)](+) ions. MS(3) experiments reveal that a major route to these [b(n) + BuNH(2)](+) and [y(n) + BuNH(2)](+) ions involves ion-molecule reactions between the b(n) and y(n) sequence ions and butylamine. MS(4) experiments, carried out to determine the nature of the [b(n) + BuNH(2)](+) ions, reveal that they correspond to a mixture of hydrogen bonded (i.e. proton bound dimer) and covalent amide bond structures.

On the photoionization of large molecules

There is no apparent limit to the size of a molecule for which photoionization can occur. It is argued that it is difficult to obtain useful photoionization mass spectra of peptides (above ∼ 2000 u), proteins, and oligonucleotides, because of the high internal energy of these polar molecules as a result of the desorption event and because vibrationally excited radical cations readily fragment. Evidence to support this hypothesis is presented from the 118-nm single-photon ionization (SPI) mass spectra of the cyclic decapeptide gramicidin S and of fullerenes, from null SPI results with the linear peptides substance P and gramicidin D and oligonucleotides, and from a variety of data found in the literature. The literature data include mass spectra from jet-cooled peptides, perfluorinated polyethers, collisional ionization of small neutral peptides, and the ultraviolet photoelectron spectroscopy of polymeric solids.

Interface for a four-sector mass spectrometer with a dual-purpose collision cell: High transmission at low to intermediate energies

A new interface system that consists of an ion decelerator, a floating collision cell-chemical ionization ion source, and an ion extractor was designed and installed in the third field-free region of a four-sector tandem mass spectrometer. Important features include the use of cylindrical deceleration lenses and an extraction lens assembly. This new design was found to provide enhancement of ion transmission at low to intermediate ion kinetic energies (3 eV to 1 keV) compared with the standard collision cell design. Collision-induced dissociation experiments from 3 eV to 10 keV and ion-molecule reactions of mass-selected ions can be performed conveniently. A second, grounded, collision cell is located after the extraction lenses, which allows MS(4) experiments to be carried out via the normal linked (B/E) scan function in MS2. Incorporation of chemical ionization capability into the electrically isolated collision cell makes it possible to carry out neutralization chemical-reionization mass spectrometry.