Novel investigations with infrared multiphoton dissociation (IRMPD) and collisionally activated dissociation (CAD) in a quadrupole ion trap
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Infrared multiphoton dissociation (IRMPD) has been compared to collisionally activated dissociation (CAD) in a quadrupole ion trap for the structural characterization of macrolide antibiotics and for the sequencing of modified and unmodified peptides. The larger effective m/z range in IRMPD tandem mass spectrometry experiments as compared to CAD allows more effective characterization of macrolide antibiotics, particularly for identifying aminoglycoside substituents which result in fragment ions below the low-m/z cutoff in CAD experiments. For the same reason, IRMPD also allows for the acquisition of more peptide sequence information than CAD, and it is easier to implement because, unlike CAD, it is not necessary to match a supplemental AC signal to a trapped ion’s frequency of motion. In addition, the strong absorption attributed to the P-O stretch of the phosphate moiety for 10.6 µm (the wavelength used for IRMPD) allows the selective dissociation of phosphorylated peptides in complex mixtures. This technique has been used to differentiate phosphorylated and unphosphorylated peptides in direct infusion ESI-MS experiments and following HPLC separations. This has allowed the determination of the phosphorylation status of peptides in complex mixtures including tryptic digests of a phosphoprotein, α-casein; shown in Chapter 4 for a direct infusion experiment and in Chapter 5 following HPLC fractionation of the tryptic fragments. Energy-resolved CAD has been applied for the determination of gas-phase basicities of peptide basic sites via the gas-phase dissociation of [peptide+H+ +polyether]+ complexes. These results showed that not only the type of amino acid, but also its position relative to the N and C termini and any intramolecular hydrogen bonding that occurs with the peptide of interest, affect the gas-phase basicities and that these differences are observed based on threshold CAD measurements. Collisionally activated dissociation and energy-resolved dissociation were also applied to carbamoylmethylphosphine oxide (CMPO) ligands complexed with rare earth ions. These studies show that it is possible to determine the favorable binding stoichiometries for various ligands bound to trivalent lanthanides. In addition, energy-resolved CAD experiments and studies of post-dissociation solvent adduction showed that the CMPO ligands are able to more completely solvate the lanthanide ions as their ionic radii decrease.