Consecutive infrared multiphoton dissociations in a fourier transform ion cyclotron resonance mass spectrometer

Consecutive infrared multiphoton dissociations (IRMPD) may be observed in a Fourier transform ion cyclotron resonance mass spectrometer (FTICR). This is the IRMPD equivalent of previous MS(n)() experiments using CID. This work presents a versatile technique, using a bistable shutter to gate ON and OFF a continuous-wave (CW) CO(2) laser for multiple irradiation periods of 0.1-1000 s duration. Consecutive photodissociations, up to MS(4), are demonstrated for the proton-bound dimer of diethyl ether and the resulting fragment ions. The photoproducts are formed close to the center of the FTICR cell, resulting in high product ion recovery efficiency. This differs from CID products, which are formed throughout the FTICR cell causing reisolation/detection problems. The fragmentation resulting from the use of low-intensity, CW, infrared laser radiation is shown to be much more energy selective than CID. Photodissociation of C(2)H(5)OH(2)(+) ion produces the lowest energy product ion exclusively, even though the two product channels differ only by ～5 kcal/mol. Low-energy CID, however, produces a mixture of C(2)H(5)(+) and H(3)O(+) products in the ratio of 1.3:1. Hence, the higher energy pathway (C(2)H(5)(+)) is substantially favored. The current results indicate that this IRMPD MS(n)() technique may be successfully applied to large biomolecules prepared by electrospray or MALDI.     

Combined electron capture and infrared multiphoton dissociation for multistage MS/MS in a Fourier transform ion cyclotron resonance mass spectrometer

We have mounted a permanent on-axis dispenser cathode electron source inside the magnet bore of a 9.4-T Fourier transform ion cyclotron resonance mass spectrometer. This configuration allows electron capture dissociation (ECD) to be performed reliably on a millisecond time scale. We have also implemented an off-axis laser geometry that enables simultaneous access to ECD and infrared multiphoton dissociation (IRMPD). Optimum performance of both fragmentation techniques is maintained. The analytical utility of performing either ECD or IRMPD on a given precursor ion population is demonstrated by structural characterization of several posttranslationally modified peptides: IRMPD of phosphorylated peptides results in few backbone (b- and y-type) cleavages, and product ion spectra are dominated by neutral loss of H3PO4. In contrast, ECD provides significantly more backbone (c- and z*-type) cleavages without loss of H3PO4. For N-glycosylated tryptic peptides, IRMPD causes extensive cleavage of the glycosidic bonds, providing structural information about the glycans. ECD cleaves all backbone bonds (except the N-terminal side of proline) in a 3-kDa glycopeptide with no saccharide loss. However, only a charge-reduced radical species and some side chain losses are observed following ECD of a 5-kDa glycopeptide from the same protein. An MS3 experiment involving IR laser irradiation of the charge-reduced species formed by electron capture results in extensive dissociation into c- and z-type fragment ions. Mass-selective external ion accumulation is essential for the structural characterization of these low-abundance (modified) peptides.     

Identification of intact proteins in mixtures by alternated capillary liquid chromatography electrospray ionization and LC ESI infrared multiphoton dissociation Fourier transform ion cyclotron resonance mass spectrometry.

Here we propose a novel method for rapidly identifying proteins in complex mixtures. A list of candidate proteins (including provision for posttranslational modifications) is obtained by database searching, within a specified mass range about the accurately measured mass (e.g., +/- 0.1 Da at 10 kDa) of the intact protein, by capillary liquid chromatography electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (LC ESI FT-ICR MS). On alternate scans, LC ESI infrared multiphoton dissociation (IRMPD) FT-ICR MS yields mostly b and y fragment ions for each protein, from which the correct candidate is identified as the one with the highest "hit" score (i.e., most b and y fragments matching the candidate database protein amino acid sequence masses) and sequence "tag" score (based on a series of fragment sequences differing in mass by 1 or 2 amino acids). The method succeeds in uniquely identifying each of a mixture of five proteins treated as unknowns (melittin, ubiquitin, GroES, myoglobin, carbonic anhydrase II), from more than 1000 possible database candidates within a +/- 500 Da mass window. We are also able to identify posttranslational modifications of two of the proteins (mellitin and GroES). The method is simple, rapid, and definitive and is extendable to a mixture of affinity-selected proteins, to identify proteins with a common biological function.     

Distinguishing of Ile/Leu amino acid residues in the PP3 protein by (hot) electron capture dissociation in Fourier transform ion cyclotron resonance mass spectrometry

In hot electron capture dissociation (HECD), multiply protonated polypeptides fragment upon capturing approximately 11-eV electrons. The excess of energy upon the primary c, z* cleavage induces secondary fragmentation in z* fragments. The resultant w ions allow one to distinguish between the isomeric Ile and Leu residues. The analytical utility of HECD is evaluated using tryptic peptides from the bovine milk protein PP3 containing totally 135 amino acid residues. Using a formal procedure for Ile/Leu (Xle) residue assignment, the identities of 20 out of 25 Xle residues (80%) were determined. The identity of an additional two residues could be correctly guessed from the absence of the alternative w ions, and only two residues, for which neither expected nor alternative w ions were observed, remained unassigned. Reinspection of conventional ECD spectra also revealed the presence of Xle w ions, although at lower abundances, with 44% of all Xle residues distinguished. Using a dispenser cathode as an electron source, identification of four out of five Xle residues in a 2.7-kDa peptide was possible with one acquisition 2 s long, with identification of all five residues by averaging of five such acquisitions. Unlike the case of high-energy collision-induced dissociation, no d ions were observed in the HECD of tryptic peptides.