Fragmentation study of peptides using Fourier transform ion cyclotron resonance with infrared multiphoton dissociation: experiment and simulation

Infrared multiple photon dissociation spectroscopy and computational studies of O-glycosylated peptides

The infrared multiple photon dissociation (IRMPD) spectra of O-glycosylated peptides in the gas phase were studied in the IR scanning range of 5.7-9.5 μm. Fragmentation of protonated and sodiated O-glycopeptides was investigated using electrospray ionization (ESI) Fourier-transform ion cyclotron resonance (FTICR) mass spectrometry (MS) with a free electron laser (FEL). FEL is used in the IRMPD technique as a tunable IR light source. In the IRMPD spectroscopic analysis of the protonated O-glycopeptide, fragment ions of the b/y and B/Y types were observed in the range of 5.7-9.5 μm, corresponding to the cleavage of the backbone in the parent amino acid sequence and glycosyl bonds, whereas the spectra of the sodiated glycopeptide showed major peaks of photoproducts of the B/Y type in the range of 8.4-9.5 μm. The IRMPD spectra of the O-glycopeptides were compared with simulated IR spectra for the structures obtained from the molecular dynamics.

Comparison of collision-induced dissociation and electron-induced dissociation of singly protonated aromatic amino acids, cystine and related simple peptides using a hybrid linear ion trap-FT-ICR mass spectrometer

The gas-phase fragmentation reactions of singly protonated aromatic amino acids, their simple peptides as well as simple models for intermolecular disulfide bonds have been examined using a commercially available hybrid linear ion trap-Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Low-energy collision-induced dissociation (CID) reactions within the linear ion trap are compared with electron-induced dissociation (EID) reactions within the FT-ICR cell. Dramatic differences are observed between low-energy CID (which occurs via vibrational excitation) and EID. For example, the aromatic amino acids mainly fragment via competitive losses of NH(3) and (H(2)O+CO) under CID conditions, while side-chain benzyl cations are major fragment ions under EID conditions. EID also appears to be superior in cleaving the S-S and S-C bonds of models of peptides containing an intermolecular disulfide bond. Systematic studies involving fragmentation as a function of electron energy reveal that the fragmentation efficiency for EID occurs at high electron energy (more than 10 eV) compared with the low-electron energy (less than 0.2 eV) typically observed for electron capture dissociation fragmentation. Finally, owing to similarities between the types of fragment ions observed under EID conditions and those previously reported in ultraviolet photodissociation experiments and the electron-ionization mass spectra, we propose that EID results in fragmentation via electronic excitation and vibrational excitation. EID may find applications in analyzing singly charged molecular ions formed by matrix-assisted laser desorption ionization.

Infrared multiphoton dissociation spectroscopic analysis of peptides and oligosaccharides by using fourier transform ion cyclotron resonance mass spectrometry with a midinfrared free-electron laser

The fragmentation of peptides and oligosaccharides in the gas phase was investigated by means of electrospray ionization Fourier transform ion cyclotron resonance (FTICR) mass spectrometry coupled with dissociation by a laser-cleavage infrared multiphoton dissociation (IRMPD) technique. In this technique, an IR free-electron laser is used as a tunable source of IR radiation to cause cleavage of the ionized samples introduced into the FTICR cell. The gas-phase IRMPD spectra of protonated peptides (substance P and angiotensin II) and two sodiated oligosaccharides (sialyl Lewis X and lacto-N-fucopentaose III) were obtained over the IR scan range of 5.7-9.5 microm. In the IRMPD spectra for the peptide, fragment ions are observed as y/b-type fragment ions in the range 5.7-7.5 microm, corresponding to cleavage of the backbone of the parent amino acid sequence, whereas the spectra of the oligosaccharides have major peaks in the range 8.4-9.5 microm, corresponding to photoproducts of the B/Y type.

Improving fragmentation of poorly fragmenting peptides and phosphopeptides during collision-induced dissociation by malondialdehyde modification of arginine residues

Despite significant technological and methodological advancements in peptide sequencing by mass spectrometry, analyzing peptides that exhibit only poor fragmentation upon collision-induced dissociation (CID) remains a challenge. A major cause for unfavorable fragmentation is insufficient proton 'mobility' due to charge localization at strongly basic sites, in particular, the guanidine group of arginine. We have recently demonstrated that the conversion of the guanidine group of the arginine side chain by malondialdehyde (MDA) is a convenient tool to reduce the basicity of arginine residues and can have beneficial effects for peptide fragmentation. In the present work, we have focused on peptides that typically yield incomplete sequence information in CID-MS/MS experiments. Energy-resolved tandem MS experiments were carried out on angiotensins and arginine-containing phosphopeptides to study in detail the influence of the modification step on the fragmentation process. MDA modification dramatically improved the fragmentation behavior of peptides that exhibited only one or two dominant cleavages in their unmodified form. Neutral loss of phosphoric acid from phosphopeptides carrying phosphoserine and threonine residues was significantly reduced in favor of a higher abundance of fragment ions. Complementary experiments were carried out on three different instrumental platforms (triple-quadrupole, 3D ion trap, quadrupole-linear ion trap hybrid) to ascertain that the observation is a general effect.

Fragmentation of Singly Protonated Peptides via a Combination of Infrared and Collisional Activation

The coupling of matrix-assisted laser desorption/ionization (MALDI) to Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) provides an exceptionally capable platform for peptide analysis, but an important limitation of this approach is the difficulty in obtaining informative tandem mass spectra (MS/MS) of singly protonated peptides. This difficulty is especially pronounced with peptide ions containing basic amino acid residues (for example, tryptic peptides). While such ions can be fragmented in some instrument configurations, most FTICR instruments have comparatively little facility for high-energy fragmentation. Here, a novel MS/MS approach implemented with MALDI-FTICR-MS and specifically intended for enhanced fragmentation of singly protonated peptides is described. The method involves infrared irradiation in concert with the simultaneous application of sustained off-resonance irradiation collision-induced dissociation (SORI-CID). This form of MS/MS, described as a combination of infrared and collisional activation (CIRCA), is shown to provide a greater capacity for dissociation of singly charged model peptide ions as compared to infrared multiphoton dissociation (IRMPD) or SORI-CID alone. Overall, the CIRCA approach is demonstrated to be a feasible technique for accessing useful fragmentation pathways of singly charged peptides, including those harboring basic amino acid residues--a crucial feature in the context of proteomics.

A computational study of structure–reactivity relationships in Na-adduct oligosaccharides in collision-induced dissociation reactions

Elucidating the fragmentation mechanisms in oligosaccharides using theoretical calculations is useful in analyzing the experimentally obtained mass spectra. Semi-empirical and ab initio quantum mechanics calculations were used to study the relationship between the structure and reactivity and the chemical properties of oligosaccharides. In these calculations, sodium-cationized oligosaccharides were investigated to determine Na+ ion affinity at several binding positions; in addition, the dependence of the glycosidic bond cleavage on the Na+ position was examined. The calculated structures reported in this study are directed at interpreting experimentally observed fragment ions, resulting from the cleavage of the glycosidic bonds. The calculated results for oligosaccharides containing between three and five monosaccharide units (27 oligosaccharides) were compared with experimental data generated by matrix-assisted laser-desorption/ionization (MALDI) using a quadrupole ion trap (QIT) with a time-of-flight (TOF) mass spectrometer (MS).

Fourier transform ion cyclotron resonance mass spectrometry for the characterisation of kavalactones in the kava plant: elemental formulae confirmation by dual spray accurate mass measurement and structural confirmation by infrared multiphoton dissociation and sustained off-resonance irradiation collision induced dissociation

Roots and extracts of the kava plant have been used in herbal medicine to treat sleep disturbances, stress and anxiety, although reported cases of liver toxicity led to many countries restricting its sale. The detection of the presence of kava in many medicinal products requires the use of methods capable of identifying the kavalactones with high certainty. Here, we describe the use of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) for the characterisation of six kavalactones (kavain, dihydrokavain, methysticin, dihydromethysticin, yangonin and desmethoxyyangonin) utilising accurate mass measurement for the determination of their elemental formulae and product ion MS (both sustained off-resonance irradiation collision-induced dissociation and infrared multiphoton dissociation (SORI-CID and IRMPD) for structural confirmation. High performance liquid chromatography/FT-ICR-MS with a dual spray system for internal calibration of mass spectra was employed for accurate mass measurement and the determination of elemental formulae of the kavalactones in both standards and a root extract to confirm the presence of the kavalactones in the root powder. Mass accuracy of < 1 ppm was achieved. For structural confirmation, the IRMPD and SORI-CID spectra of the kavalactones in standards and a kava root powder extract were compared. Accurate mass measurement of the product ions was also conducted by external calibration and the elemental formula determined to aid with structural confirmation. The presence of the same fragment ions detected in the standards as in the extract further confirmed the presence of the kavalactones in the kava root powder with high certainty.